Reviews in Fish Biology and Fisheries9: 151–186, 1999.© 1999Kluwer Academic Publishers. Printed in the Netherlands.

151

Ichthyofaunal assemblages in estuaries: A South African case study

Alan K. WhitfieldJ.L.B. Smith Institute of Ichthyology, Private Bag 1015, Grahamstown 6140, South Africa (E-mail: a.whitfield@ru.ac.za)

Accepted 2 December 1998

Contents

Abstract Page 151Introduction 152Life-history patterns 152Estuary association analysis 155Early life-history stages 155Factors influencing fish assemblages 161

SalinityWater temperatureRiver flowTurbidityMouth phaseDissolved oxygenHabitat variabilityZoogeography and seasonalityCatchment and estuary sizeLarval linkagesEstuary type

Fish biomass and resource utilization 173Conservation of fishes in estuaries 177Acknowledgements 181References 181

Abstract

This review places the life-history styles of fishes associated with South African estuaries in a global contextand presents a classification system incorporating all the major life-history categories for estuary-associated fishspecies around the world. In addition, it documents the early life histories of the major fish groups in SouthAfrican estuaries, with particular emphasis on the differing modes of estuarine utilization by marine, estuarine andfreshwater taxa.

This review details factors influencing the ichthyofaunal community structure in South African estuaries. Theavailability of fish for recruitment into an estuary depends primarily upon the distributional range of euryhalinemarine and estuarine species, with tropical and temperate taxa showing marked abundance trends. Within aparticular biogeographic region, however, estuarine type and prevailing salinity regime have a major influenceon the detailed ichthyofaunal structure that develops. There is an increasing preponderance of marine fish taxawhen moving from a freshwater-dominated towards a seawater-dominated type of system, and a decline in speciesdiversity between subtropical estuaries in the north-east and cool temperate systems in the south-west. Similardeclines in fish species diversity between tropical and temperate estuaries in other parts of the world are highlighted.

Fish assemblages in estuaries adjust constantly in response to changing seasons, salinities, turbidities, etc.Despite persistent fluctuations in both the biotic and abiotic environment, the basic ichthyofaunal structure appearsto have an underlying stability and to be predictable in terms of the response of individual species to specificconditions. This stability seems to be governed by factors such as the dominance of eurytopic taxa within estuarineassemblages and the robust nature of food webs within these systems. The predictability arises from factors such

152

as the seasonality associated with estuarine spawning cycles and juvenile fish recruitment patterns. These patterns,together with a well-documented resilience to a wide range of physico-chemical and biotic perturbations, appearto be an underlying feature of fish assemblages in estuaries around the world.

In contrast to marine fish species, estuary-associated taxa have received little conservation attention. Apartfrom the designation of protected areas, the main direct means of conserving estuary-associated fish stocks includehabitat conservation and controls over fishing methods, effort, efficiency and seasonality. Of these, the conservationof fish habitats is the most important, because healthy aquatic environments invariably support healthy fish popula-tions. The use of estuarine sanctuaries for fish conservation is briefly reviewed, as well as the legislation governingthe USA National Estuarine Research Reserve System (NERRS) and the Australian Marine and Estuarine ProtectedArea (MEPA) system. It is concluded that South Africa requires an expansion of the existing Estuarine ProtectedArea (EPA) network, as well as the upgrading of selected ‘estuarine reserves’ where fishing is permitted, into‘estuarine sanctuaries’ where no exploitation of biological resources is allowed.

Key words:aquatic conservation, estuaries, fish assemblages, life histories, South Africa, zoogeography

Introduction

Estuaries are regions where marine and fresh watersmeet, where environmental gradients are steep, andwhere exceptionally high levels of primary andsecondary production are often recorded (J.W. Dayet al., 1989). These factors have a major influenceon the density, diversity and biomass of fishes thatcan be supported in these systems. In particular, theoften abrupt changes in salinity, water temperature,dissolved oxygen and turbidity place considerablephysiological demands on the fishes that utilize estu-aries. However, species that are broadly tolerant ofbiotic and abiotic variability are at a considerableadvantage over those fishes that cannot survive suchfluctuations, because the former group are able tooccupy a food-rich environment from which manypotential competitors are excluded (Whitfield, 1998).In this review, the life-history styles and attributes offishes utilizing subtropical and warm-temperate SouthAfrican estuaries (Figure 1, Table 1) are examined andcompared with ichthyofaunal assemblages elsewherein the world.

It has been postulated by several authors (Blaber,1981; Cooper et al., 1995) that South African estu-aries are ecologically important because they providethe only significant sheltered areas for the juvenilesof certain marine fish species. Potter et al. (1990)suggested that in contrast to the high-energy coastlineassociated with the African subcontinent, the south-western Australian inshore marine waters are usedas an alternative nursery habitat by many estuary-associated marine species owing to the physicalprotection afforded by fringing reefs and rocky head-lands. Similarly, the embayments associated with

many large North American and European estuariesprovide protection, not only for resident estuarine fishspecies, but also for a wide range of marine taxa(Weinstein, 1985).

A variety of factors influence the utilization ofSouth African estuaries by fishes (Whitfield, 1983;Blaber, 1985; Marais, 1988). Because no two estu-aries are identical in terms of either biotic or abioticcharacteristics, it could be postulated that the ichthy-ofaunas of each estuary will also differ. However,if the resident and marine migrant fishes respondto the environment in a consistent manner, thenthe communities occupying similar types of estu-aries in a particular region would be expected toreflect this similarity. In the following sections, thefish assemblages from various types of estuaries inthe different biogeographic regions (Figure 1) arereviewed and compared, and the importance of estu-arine protected areas in achieving fish conservationgoals is highlighted.

Life-history patterns

Apart from a few freshwater species, the fishes inhab-iting South African estuaries may be divided into twomajor groups according to their ability to breed withinthe estuarine environment (Whitfield, 1990). The firstgroup is dominant and comprises euryhaline marinespecies that spawn at sea. The second group spawnwithin the estuarine environment, although certainspecies may also breed at sea or in fresh water (Whit-field, 1998). For the purposes of this review, theformer group has been classified as marine and thelatter as estuarine. A similar division has been used

153

Figure 1. Map of southern Africa showing biogeographical regions mentioned in the text.

for estuary-associated fishes in other parts of the world(Dando, 1984).

The marine group can be subdivided into marinemigrants and marine stragglers (Table 2), the migrantsmaking extensive use of estuaries during juvenileand/or adult life stages (Figure 2a) and the strag-glers recorded in small numbers, usually in thelower reaches (Figure 2b). The estuarine spawninggroup can be subdivided into estuarine residentsand estuarine migrants, the residents possessing theability to complete their life cycle within the estuary(Figure 2c) and the migrants usually having a marinelarval phase (Figure 2d) and/or regular movementsbetween the estuary and adjacent aquatic habitats.Two other smaller groups of fishes, the freshwaterand catadromous groups, use South African estuaries.The freshwater group can be subdivided (Table 2)into freshwater migrants (Figure 2e) that are presentin a wide range of estuaries throughout the year,and freshwater stragglers (Figure 2f) which entercertain estuaries during brief periods when condi-tions are favourable. Catadromous migrants (eitherobligate or facultative) spawn at sea but use fresh-water catchment areas for the juvenile and subadult

life stages (Figure 2g). Catadromous species there-fore utilize estuaries as conduits between the fresh-water and marine environments although facultativetaxa are sometimes restricted to estuaries when thejuveniles are unable to access the adjacent catch-ment. South African estuaries contain no anadromousspecies (Table 2), a group that is also absent from mosttropical and subtropical estuaries around the world(Blaber, 1997).

The main feature of the life cycle of most marinespecies utilizing South African estuaries is a divisioninto a larval phase that takes place at sea, a juvenilephase that is predominantly estuarine and an adultperiod that is predominantly marine (Whitfield, 1998).The proportion of 0+ juveniles entering estuariesvaries according to species, with most species enteringestuaries between 10 mm and 30 mm standard length(SL; Wallace and van der Elst, 1975). After a resi-dence period of 1–3 years these fish return to the sea,with certain species (e.g.Rhabdosargus holubi) neverentering estuaries following completion of this phasein their life cycle. Although some taxa may attainsexual maturity within the estuarine environment,spawning generally occurs in the sea (Wallace, 1975a).

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Table 1. Scientific names and South African common names for fish species mentioned in this review

Family Scientific name and author(s) Common name

Anguillidae Anguilla mossambica(Peters, 1852) Longfin eel

Ariidae Galeichthys feliceps(Valenciennes, 1840) White seacatfish

Carangidae Caranx sexfasciatus(Quoy and Gaimard, 1825) Bigeye kingfish

Lichia amia(Linnaeus, 1758) Leervis

Carcharhinidae Carcharhinus leucas(Valenciennes, 1839) Bullshark

Cichlidae Oreochromis mossambicus(Peters, 1852) Mozambique tilapia

Clupeidae Gilchristella aestuaria(Gilchrist, 1914) Estuarine roundherring

Dasyatidae Dasyatis chrysonota(Linnaeus, 1758) Blue stingray

Himantura uarnak(Forsskål, 1775) Honeycomb stingray

Haemulidae Pomadasys commersonnii(Lacepede, 1801) Spotted grunter

Lutjanidae Lutjanus argentimaculatus(Forsskål, 1775) Mangrove snapper

Monodactylidae Monodactylus falciformis(Lacepede, 1800) Oval moony

Mugilidae Liza richardsonii(Smith, 1846) Southern mullet

Mugil cephalus(Linnaeus, 1758) Flathead mullet

Myxus capensis(Valenciennes, 1836) Freshwater mullet

Odontaspididae Carcharias taurus(Rafinesque, 1810) Spotted ragged-tooth

Platycephalidae Platycephalus indicus(Linnaeus, 1758) Bartail flathead

Pomatomidae Pomatomus saltatrix(Linnaeus, 1766) Elf

Pristidae Pristis zijsron(Bleeker, 1851) Green sawfish

Sciaenidae Argyrosomus japonicus(Temminck and Schlegel, 1843) Dusky kob

Sparidae Acanthopagrus berda(Forsskål, 1775) Estuarine bream

Lithognathus lithognathus(Cuvier, 1830) White steenbras

Rhabdosargus globiceps(Cuvier, 1830) White stumpnose

Rhabdosargus holubi(Steindachner, 1881) Cape stumpnose

Rhabdosargus sarba(Forsskål, 1775) Tropical stumpnose

Sarpa salpa(Linnaeus, 1758) Strepie

Syngnathidae Hippocampus capensis(Kaup, 1858) Knysna seahorse

Syngnathus acus(Linnaeus, 1758) Longsnout pipefish

Syngnathus watermeyeri(Smith, 1963) Estuarine pipefish

Torpedinidae Torpedo fuscomaculata(Peters, 1855) Blackspotted electric ray

The reliance of juveniles of migrant marine specieson estuarine nursery grounds varies considerably, andranges from marine stragglers, which are seldomfound in estuaries, to those species that are dependenton estuaries during the juvenile phase of their lifecycle (Whitfield, 1998). Some species such asLizarichardsonii and Pomatomus saltatrixappear to usefavourable estuarine conditions opportunistically, withthe juveniles also being very abundant in the sea. Othertaxa such asRhabdosargus holubiandMonodactylusfalciformis are considered to be dependent on estu-aries, and Bennett et al. (1985) have gone so far as tosuggest that certain species might even become extinctif denied access to these nursery areas.

Adult marine fish also show considerable vari-ability in their association with estuaries, with some

species making extensive use of this environmentand others never returning to their natal habitat.For example, large postspawning shoals ofPoma-dasys commersonniiregularly enter estuaries duringearly spring, whereasRhabdosargus holubiadultsare seldom recorded in these systems, despite theoverwhelming dependence of their juveniles on thisenvironment.

There are relatively few fish species that cancomplete their entire life cycle within southern Africanestuaries and these are invariably small species (Whit-field, 1998). Sexual maturity usually occurs at lessthan 70 mm SL, in contrast to marine taxa wheremost mature above 200 mm SL. Wallace (1975a) hassuggested that the small size of estuarine spawnerswould reduce their physical ability to undertake large-

155

scale migrations to and from the sea. Predation byadult piscivorous fish populations in the sea may alsodeter mass migrations by these species. Furthermore,the typically shallow microtidal estuaries of southernAfrica tend to favour occupation by small fish.

There are four obligate catadromous anguillideel species in southern African waters but, onlyAnguilla mossambicais both abundant and wide-spread. According to Bruton et al. (1987), the probablereasons for the paucity of diadromous fish on thesubcontinent are the unstable nature of the rivers, highsoil moisture deficits, and the unreliable availabilityof resources such as food and space compared withthe ocean. Because anguillid eels are not resident estu-arine species, and only use these systems as a conduitbetween the marine and freshwater environments, thelife-history styles of this family are not dealt with inthis review.

The cichlidOreochromis mossambicusis the onlyabundant freshwater species in South African estu-aries. It occurs in large numbers in coastal lakes andtemporarily closed estuaries, but is usually absentfrom the lower reaches of permanently open systems(Whitfield and Blaber, 1979). When temporarilyclosed estuaries are breached, this species usuallyretreats into the upper reaches. Widespread breedinghas been recorded in several estuaries during theclosed phase, with parental care enhancing survival ofoffspring in these environments. Young are released bymouthbrooding adults when they attain about 10 mmSL (Bruton and Boltt, 1975), by which time they canavoid being swept out to sea when the estuary opens.In small subtropical estuaries, which are seldom opento the sea, this species is often a dominant componentof the ichthyofaunal community (Begg, 1984).

Estuary association analysis

The extent to which individual fish species aredependent on estuaries has been the subject of consid-erable research and debate (Lenanton, 1982; Elliottand Dewailly, 1995; Able and Fahay, 1998) buta global synthesis is lacking, primarily because ofregional differences in life-history terminology andlevels of ichthyofaunal knowledge. A preliminaryanalysis of the degree of estuarine association bythe various categories of indigenous fishes in SouthAfrican estuaries is given in Whitfield (1998). Speciesthat are totally dependent on these systems comprise32 (21%) of the 155 estuary-associated fish taxa. Estu-

arine spawning species comprise 27% (42 species) ofthe total ichthyofaunal diversity, with the dominantcomponent being derived from the marine groupthat use estuaries as nurseries and/or foraging areas(61 species, 39% of total). Indications are that 103fish species (66% of total) are either completely orpartially dependent on estuaries for their existence.

Altogether, 53 families of fishes are associ-ated with South African estuaries, of which 50 areprimarily marine in origin, two are derived from fresh-water environments, and one is represented by obligatecatadromous species. In terms of species number, theGobiidae (23 species), Mugilidae (13 species) andSparidae (13 species) are the most diverse. Within theabove families, 43% (10 species) of the Gobiidae, 62%(8 species) of the Sparidae and 23% (3 species) ofthe Mugilidae associated with estuaries are endemicto the region. Altogether, 38 species (25% of allestuary-associated taxa) are endemic to South Africanwaters.

The dominant families associated with tropical(including subtropical) and temperate estuaries insouthern Africa and the rest of the world are givenin Table 3. This analysis shows that the numberof families that make important contributions to theoverall ichthyofauna in the different regions is small,comprising about 10% of the total number of fishfamilies described and overwhelmingly dominatedby marine taxa. This parallels the low diversity ofspecies represented in estuaries (Whitfield, 1994a)and emphasizes the fact that only relatively few kindsof fishes can live in these types of aquatic systems(Haedrich, 1983).

Early life-history stages

The immigration of marine fish larvae and postlarvaeinto large, well-flushed estuaries of the NorthernHemisphere mainly takes place using passive and/orselective tidal transport both for entry to and retentionwithin these systems (Weinstein et al., 1980; Fortierand Leggett, 1982; Boehlert and Mundy, 1988). Incontrast, however, in microtidal estuaries in SouthAfrica, New Zealand and Australia, where for muchof the year the two-layered circulation pattern is lesspronounced or absent, the larvae and juveniles of somemarine species enter these systems on the flood tideand are retained by rapidly settling along the banksor on the bottom where water movements are reduced(Beckley, 1985; Roper, 1986; Neira and Potter, 1994).

156

Figure 2. Diagrammatic representation of the life cycles of fish groups associated with South African estuaries: (a) marine migrants, e.g.Rhabdosargus holubi; (b) marine stragglers, e.g.Pomadasys olivaceum; (c) estuarine residents, e.g.Gilchristella aestuaria; (d) estuarinemigrants, e.g.Psammogobius knysnaensis; (e) freshwater migrants, e.g.Oreochromis mossambicus; (f) freshwater stragglers, e.g.Tilapiarendalli; (g) catadromous species, e.g.Anguilla mossambica.

157

Figure 2. Continued.

Motile juveniles can, however, easily enter estu-aries on the ebb tide by keeping to the margins wherecurrent speeds are attenuated. In the Zotsha Estuary,postlarval fish were recorded moving into the systemthrough the bottom of standing waves, swimmingupstream in a series of steps (Harrison and Cooper,1991).

Whitfield (1989a) has quantified marineichthyoplankton recruitment into the SwartvleiEstuary and found up to 315 000 fish larvae andpostlarvae entering the system over a 24 h period. He

also documented that there was a net loss of the larvaeof certain estuarine spawners, which subsequentlyreturned to the estuary as postlarvae. Movementsof larvae and 0+ juveniles between the estuary andsea occurred mainly during twilight and nocturnalhours when predation rates would probably be lowerthan during the day (Johannes, 1978). Nevertheless,the magnitude of the above immigration figures inrelation to juvenile and adult densities within theestuary suggests that high mortalities of these earlyrecruits occurs.

158

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159

Table 3. Listing of well represented ichthyofaunal families associated with estuaries in differentregions around the world. Criteria for inclusion include one or more of the following; (a) the ubiquit-ousness of a family in estuaries, (b) the abundance of individual species of the family and (c) thenumber of species in the family that occur within estuaries of the region

Southern African region Indo–Pacific region Atlantic region

Family Tropical Temperate Tropical Temperate Tropical Temperate

Apogonidae + +Ambassidae + +Ammodytidae +Anguillidae + + + + +Ariidae + + + +Atherinidae + + + + +Belonidae + +Bothidae + +Carangidae + + +Chandidae + +Centropomidae +Cichlidae + + +Clupeidae + + + + + +Cottidae +Cynoglossidae + +Cyprinodontidae + +Engraulidae + + + + +Eleotridae + +Elopidae + + +Embiotocidae +Gadidae + +Gasterosteidae +Gerreidae + + + +Gobiidae + + + + + +Haemulidae + + +Hemirhamphidae + + +Leiognathidae + +Lutjanidae + + +Monodactylidae + + + +Mugilidae + + + + + +Osmeridae + +Percichthyidae + +Phycidae +Pleuronectidae + +Platycephalidae + + + +Plotosidae + +Polynemidae + +Pomatomidae + + +Salmonidae + +Sciaenidae + + + + + +Serranidae + +Siganidae + +Sillaginidae + + +Soleidae + + + +Sparidae + + + + +Syngnathidae + + + + + +Teraponidae + + +Tetraodontidae + + + + +Zoarcidae +

160

Once the juveniles have entered an estuary, theyusually continue to move up the system, often inconcert with flood tidal currents. Hall et al. (1987)found that more than 99% of the juvenile fish capturedin the Serpentine channel (Wilderness Lakes System,South Africa) were moving upstream on flood tidalcurrents but very little migration took place at lowtide. There was a strong positive correlation betweenthe number of fish moving upstream per hour andmean water depth. Furthermore, the highest numbersof migrating fish were recorded during daylight hours,but several species only moved up the channel duringthe night. Altogether, 52 000 juvenile marine fishes,comprising at least seven species, were estimatedto have migrated up the Serpentine during February1984.

Several cues are available within an estuary toassist fish orientation towards specific nursery areas.Salinity gradients are perhaps the most obvious buthave been largely discounted by Blaber (1987) onthe basis of experimental and field evidence. Temper-ature is an unlikely cue, as thermal gradients withinestuaries are irregular and highly variable, dependingupon tidal regime, river flow, oceanic upwelling etc.However, the juveniles of a number of fish species areattracted to warm littoral areas, so the use of watertemperatures, possibly in conjunction with other cues,cannot be discounted. Turbidity gradients are usuallystrongly developed within estuaries, and Blaber (1987)considers that both vertical and horizontal gradi-ents may be important to certain species. Studiesby Cyrus and Blaber (1987a, b, c) have shown thatthe juvenile marine fishes of KwaZulu-Natal estuariescan be divided into five main groups according totheir occurrence in various turbidities. The above fieldand laboratory studies indicate that turbidity plays amajor role, either singly or in combination with othervariables, in determining the distribution of juvenilemarine fishes in estuaries. Future research in this fieldshould focus on olfactory cues as one such variable,because these ‘markers’ hold great potential in ‘finetuning’ ichthyonekton movements, not only withinestuaries but also between the marine and estuarineenvironments.

In contrast to the marine migrants, larvae ofspecies such asGilchristella aestuariado not haveto enter the estuary. Instead, they need to remainwithin the system during the vulnerable embryonicand larval stages. Melville-Smith et al. (1981) havedescribed howG. aestuaria larvae in the SundaysEstuary utilize tidal transport to avoid being swept out

to sea. Evidence suggests that these larvae remain inthe middle and upper reaches of the estuary wherezooplanktonic food resources are most abundant, andavoid the more marine areas near the mouth untilat least the juvenile stage has been attained. Never-theless, large numbers ofG. aestuaria larvae aresometimes flushed into the sea during river floodevents.

The migration of marine larvae and juvenilesinto KwaZulu-Natal estuaries occurs mainly duringlate winter and spring when river flow is oftenat a minimum (Wallace and van der Elst, 1975).Although all of the permanently open estuaries andbays are available for colonization, many of thesmaller systems along the KwaZulu-Natal coast areclosed during the winter and open only after springrains in October (Whitfield, 1980a). Recruitment intothese temporarily closed estuaries is therefore onlypossible when increased water flow forces open themouths of the above systems. The prolonged periodof juvenile immigration, which is a function of theextended spawning season of most species, may there-fore be regarded as a strategy against unseasonalfloods which could open blind estuaries prematurely,and droughts that would delay mouth opening untilmidsummer.

In Eastern and Western Cape estuaries (Figure 1),a similar prolonged recruitment pattern is evident,but the peak immigration phase occurs during earlysummer when most temporarily closed estuaries inthe region are already open. In addition, late winterand spring rains send pulses of fresh water into theestuaries, thereby ensuring the replenishment of nutri-ents needed to stimulate summer primary productivity.Thus, in both KwaZulu-Natal and Cape estuaries,juvenile fishes are able to exploit the abundant summerfood resources and warm temperatures to grow rapidlybefore the onset of winter. Refuges in the form ofsubmerged aquatic vegetation are also most prolificduring summer, and higher turbidities due to increasedriver flow would aid predator avoidance by juvenilefish.

Spawning by estuarine fish occurs mainly duringspring, with the larvae and juveniles being particularlyabundant during summer. Some of these fish speciesgrow very rapidly in the warm, highly productivewaters and, with the onset of winter, have alreadyattained sexual maturity. For example,Gilchristellaaestuariaconsume approximately 12% of body massper day in summer and mature within 7 monthsof hatching (Talbot and Baird, 1985). Daily food

161

consumption then declines to less than 2% of bodymass in winter, when water temperatures decline andzooplankton resources become more scarce.

Factors influencing fish assemblages

Estuaries are characterized by a relatively low ichthy-ofaunal diversity but high abundance of individualtaxa, most of which exhibit wide tolerance limits tothe fluctuating conditions found within these systems.Indeed, most South African estuaries are occupiedby fewer than 50 species, with fish population sizesprobably totalling several million individuals in someof the larger systems. Even very small estuaries (<1km2) can have single species populations ranging from10 000 to 50 000 individuals (Blaber, 1973a), andfish densities in these systems usually exceed that ofthe adjacent marine or freshwater environments (Whit-field, 1993). The above observation leads directlyto the question which has been repeatedly asked byichthyologists – why are South African estuaries soattractive to fishes? Although selected aspects of thisquestion have been directly and indirectly addressedover the years, it was only during the 1980s thatoverviews of the situation were presented (Whitfield,1983; Blaber, 1985; Marais, 1988). During the pastdecade, much additional information on the biologyand ecology of a number of fish species associatedwith estuaries has been published, and we are now in aposition to examine the issue in more detail (Whitfield,1996a).

The major biotic and abiotic factors whichdetermine the distribution and abundance of fishesin southern African estuaries are shown in Figure 3.These factors, some of which are expanded below,are not independent but interact directly and indirectlywith the fishes that inhabit estuaries. For example,river floods directly influence mouth condition, estu-arine water temperature, salinity, turbidity, dissolvedoxygen concentrations and olfactory cues, and indir-ectly affect habitat diversity, productivity, fish recruit-ment, food availability and competition. Similarly,Blaber (1997) has shown that a complex matrix ofinteracting physical factors directly and indirectlydetermine the occurrence, distribution and movementpatterns of fishes in tropical estuaries (Figure 4).

Salinity

The most essential adaptation by fish that enter estu-arine systems is an ability to adjust to changes in

salinity (Panikkar, 1960). The change may be gradual,as normally occurs in a temporarily closed estuary, orsudden, as often takes place in tidal estuaries. Themagnitude of the change in salinity depends mainlyupon the balance between freshwater inflow and thetidal regime, with evaporation playing a major role inlagoonal or lacustrine systems having a high surfacearea to volume ratio.

A characteristic of many fish species enteringSouth African estuaries is an ability to adapt to bothlow and high salinity regimes, although it is note-worthy that fewer than 20 species have their upperrecorded limits above 69‰, whereas more than 60species can survive in water with a salinity of 1‰(Whitfield, 1998). Fishes are therefore more tolerantof low rather than high salinity conditions. Thisis important because most estuaries are subject toperiods of freshwater flooding, whereas salinitiesseldom rise above sea water except in Lake St Luciaand a few temperate systems to the south. Further-more, the closure of estuaries is usually associatedwith declining salinities, and only fishes toleratingthese conditions are able to utilize the rich foodresources available within these systems.

A few southern African freshwater teleosts havedeveloped hypotonic regulation, but most species areincapable of this adaptation and are therefore excludedfrom estuaries. Of the eight species found in estuaries(Whitfield, 1998), onlyOreochromis mossambicusmay be classified as truly euryhaline.O. mossambicuswere abundant in Lake St Lucia during hypersalineconditions and were recorded in areas where thesalinity exceeded 90‰.

According to Panikkar (1960), only a few speciesof sharks and rays are known to enter estuarinewaters, because of their method of osmoregulation.Nevertheless, three species of elasmobranchs wererecorded from the St Lucia system during 1975 and1976 (Whitfield et al., 1981). The sharkCarchar-hinus leucas, stingrayHimantura uarnakand sawfishPristis zijsronall occurred in water with a salinity ofless than 3‰.C. leucaswas also regularly netted atLake St Lucia in salinities up to 47‰ by Bass et al.(1973). Large numbers of the sharkCarcharias taurusmove into the freshwater-deprived Kariega Estuaryduring coastal upwelling events. In addition, Paterson(1995) recorded juveniles and adults of the electric rayTorpedo fuscomaculataand stingrayDasyatis chryso-nota in the lower half of this estuary under euhalinesalinity conditions (30–40‰).

162

Figure 3. Biotic and abiotic factors influencing fish species in southern African estuaries (after Whitfield, 1998). The scale on the left-handside illustrates the trend from predominantly abiotic variables in the top of the diagram to biotic variables at the bottom.

Figure 4. Physical factors influencing the occurrence, distribution and movements of fish in tropical and subtropical estuaries (after Blaber,1997). Solid arrows indicate direct influences on fishes; broken arrows denote indirect influences.

163

Do salinity fluctuations within estuaries limit theuse by marine fish of estuaries? Observations fromthe Kariega and other estuaries in the Eastern CapeProvince suggest that some stenohaline marine speciesdo penetrate estuaries which have little or no fresh-water input for extended periods, but that these taxaare poorly represented and usually confined to thelower reaches (Ter Morshuizen and Whitfield, 1994).

Several euryhaline marine fish species sometimespenetrate considerable distances up the rivers ofsouthern Africa. Pooley (1975) recordedAcanthopa-grus berdaandMugil cephaluson the Phongolo flood-plain approximately 100 km from the sea, and Bok(1984) found bothMyxus capensisand Mugil ceph-alus120 km upstream from the head of the GamtoosEstuary. Similarly, in the Great Fish system, marinespecies tend to dominate the fish assemblage immedi-ately above the ebb and flow region (Ter Morshuizenet al., 1996a). However, relatively high conductivityof these and other river systems, which sometimescontain large numbers of marine fishes, suggests thatdissolved salts of terrestrial origin may be importantin reducing osmoregulatory stress for these temporaryriverine residents.

Both species composition and abundance seemto respond to salinity changes. Van der Elst et al.(1976) have shown that there is an inverse relationshipbetween salinity and numbers of fish species at Lake StLucia. Abundance is also affected, with gillnet catchrates when salinities were less than 20‰ being doublethose when salinities exceeded 50‰. Although thelower catches during the hypersaline period could havebeen due to osmoregulatory stress, forcing certain fishtaxa out of the area, the disappearance of certain foodresources (Boltt, 1975) may also have played a role inreducing fish abundance. Wallace (1975b) found thatPomadasys commersonniiand Rhabdosargus sarbacaptured in areas where the salinity was in excess of70‰ were no longer feeding on normal molluscanand crustacean prey but were consuming filamentousalgae.

Prolonged closure of an estuary, in association withdilution of lagoonal waters from catchment rivers,can cause osmoregulatory stress in many marine fishspecies. However, estuarine spawners appear to betolerant of prolonged oligohaline conditions, with allspecies recorded in salinities below 10‰ and mosthaving been found in fresh water (Whitfield, 1998). Incontrast to marine taxa, the estuarine group does notappear well adapted to hypersaline conditions, withonly Gilchristella aestuariahaving been recorded in

salinities above 60‰. Although the eggs and larvaeof estuarine species are found under both oligohalineand hypersaline conditions, it would appear that theeggs of many marine taxa cannot survive decreasedsalinities (Sylvester et al., 1975), but the fry of thesespecies are attracted to low-salinity waters (Mires etal., 1974).

Most mass mortalities of fish in southern Africanestuaries are associated with a combination oflow salinities (<3‰) and low water temperatures(<14 ◦C). More than 100 000 fish belonging to atleast 11 species were recorded dying in subtropicalLake St Lucia during June 1976 when the salinitydeclined below 3‰ and the temperature dropped to12 ◦C (Blaber and Whitfield, 1976). Examination ofdead and dying fish during the winter of 1976 revealedskin lesions and haemorrhaging over large areas of thebody. The large mortality was probably due to eithera lethal combination of low salinities and sudden lowtemperature leading to osmoregulatory failure, or tofungal infection of the skin lesions which usuallyfollow severe osmoregulatory stress. The disorienta-tion shown by dying fish in the lake was symptomaticof osmoregulatory failure (Blaber, 1974a).

A similar mass mortality was recorded in thewarm-temperate Botriviervlei during October 1981.Salinity concentrations in the lake declined to 2–3‰and the temperature was less than 18◦C when at least7000 fish belonging to nine species died in the system(Bennett, 1985). Indications that all the species killedwere avoiding the lowest salinities is provided by thefact that most dead fish were found in the southernareas of the estuary, where salinity readings werehighest (3‰). It should be emphasized that the marinefish killed in October 1981 had survived salinities lessthan 8‰ for 4 months, and 3‰ for at least 2 weeksprior to the mortality. If the duration of exposure hadbeen shorter or the water temperature higher, thesespecies might not have succumbed. Another factorwhich may have played a role was the age of the fish.All the fish that died in the Bot system were estimatedto be older than 3 years, indicating that juveniles maybe more tolerant of low salinities than subadult andadult fish (Bennett, 1985).

Mass mortalities under hypersaline conditionsappear to be less frequent than under oligohalineconditions, and such kills usually occur when fish aretrapped in an estuary that lacks freshwater inflow forprolonged periods. During April 1989, the tempor-arily closed Seekoei Estuary on the Eastern Cape coastexperienced salinities above 90‰ as a result of a

164

protracted drought and excessive freshwater abstrac-tion by farmers in the catchment. More than 6000juvenile and adult fish, belonging to at least 11 species(comprising both marine and estuarine spawningtaxa), were recorded dying in the estuary (Whit-field, 1998). In contrast to the above mass mortalities,Wallace (1975b) documented the fish fauna in the StLucia system under salinities ranging from 60‰ to110‰. He recorded very few dead fish at these salin-ities and suggested that the lower species diversity inthe most hypersaline regions was indicative of a move-ment towards lower-salinity areas in the south of thesystem.

Water temperature

As discussed above, water temperature often plays animportant role in determining the salinity extremestolerated by both marine and freshwater fish species.Under normal circumstances, fish can escape temper-ature extremes by either moving from the littoral zoneinto deeper offshore waters or vice versa. In veryshallow systems such as Lake St Lucia, with highsurface to volume ratios, this is often not possible.A fish kill involving an estimated 250 000 estuarine,marine and freshwater teleosts was noted during thewinter of 1987 at Lake St Lucia (Forbes and Cyrus,1993). A sudden cold snap during mid-June 1987 waslisted as the primary cause of the above mortality,with salinities at the time (29–33‰) being euhaline(Cyrus and McLean, 1996). The majority of the 21species recorded dying during the above event weresmall species, with taxa of tropical origin being mostaffected. Numerous species of tropical origin werealso recorded dying in the nearby Kosi Estuary duringJune 1987 (Kyle, 1989), with an overnight watertemperature decline, from 14◦C to 10 ◦C, probablythe source of this mass mortality.

High littoral water temperatures of up to 32◦Cresulted in the death of at least 3000Hippocampuscapensisand several hundredSyngnathus acusinthe marginal areas of the Swartvlei Estuary (Russell,1994). These fish were presumably trapped in over-heated macrophyte beds by receding water levels afterthe estuary mouth had opened in February 1991.More mobile species, which are also associated withaquatic plants in estuaries (e.g.Rhabdosargus holubi),presumably escaped the warmer littoral areas.R.holubi is abundant in the Swartvlei Estuary (Whit-field, 1988a) and cannot survive in water temperaturesabove 30◦C (Blaber, 1973b).

River flow

River flow into estuaries influences not only thesalinity but also the biochemical properties of thewater body, including the introduction of catchmentolfactory cues. Fishes have a highly developed senseof smell and it is possible that olfactory cues guidethe larvae of species such asAnguilla mossambicaupestuaries and into the river catchments. Olfactory cues,which can be of freshwater or estuarine origin (Stabell,1992), may also guide those marine postlarvae thatutilize estuaries as nursery areas. Permanently openEastern Cape estuaries with longitudinal salinitygradients greater than 19‰ have considerably higherdensities of 0+ juvenile marine fishes than thosesystems where salinity gradients are small or absent(Whitfield, 1994b). A similar finding was arrived atby Martin et al. (1992) who found that the dens-ities of postlarval marine migrants in the St LuciaEstuary increased markedly for virtually all speciesfollowing an episodic flushing of the system. Althoughthe inference from the above studies is that the salinitygradients are the attractant for these juvenile fishes, itis more likely that the increased amount of olfactorycues that are exported to the marine environment oneach ebb tide are vital to the recruitment process.Indications are that these juveniles remain within theselected estuary for several years, as gillnet catchesof subadults and adults in Eastern Cape estuaries arealso positively correlated with increasing longitudinalsalinity gradients (Marais, 1988).

Studies in the Gulf of México region (Yáñez-Arancibia et al., 1985a) have shown that fish produc-tion in the coastal zone is significantly correlated tofluvial inputs into the numerous estuarine lagoons inthis region (Figure 5). These authors attributed thismainly to increased primary productivity and organicmaterial input brought about by the elevated riverineinputs, as well as the high degree of ecological inter-action between the estuaries and adjacent coastal shelf.More than 75% of the dominant fish species in thesouthern Gulf of México depend on these estuarinesystems and their fluvial discharge to provide nurseryareas and elevated food production for both juvenileand adult populations (Yáñez-Arancibia et al., 1985b).Similarly, a review of the literature (Grimes andKingsford, 1996) suggests that river discharge plumesstrongly influence fish larvae in coastal waters andmay play a significant role in the recruitment of fishesinto estuaries.

165

Figure 5. Regression (ln) between commercial fish catches (tonnes)divided by estuarine surface area (km2), and river discharge (× 103

m3), in the southern Gulf of Mexico (after Yañez-Arancibia, et al.1985a).

In addition to influencing the abundance ofestuary-associated marine fish species, river pulsesalso appear to be important in determining theabundance of estuarine spawners. Whitfield andWooldridge (1994) have described the cascadingeffect freshwater pulses have on South African ecosys-tems, primarily through detrital input and the stimu-lation of the planktonic food web (Figure 6). Martinet al. (1992) recorded considerably higher abundancesof virtually all estuarine resident species followingthe flushing of the St Lucia Estuary. They found thatan order of magnitude increase in the abundance ofGilchristella aestuariacould be indirectly linked tothe phytoplankton bloom and increased zooplanktonstocks which followed the flooding. Conversely, thedecimation of the rare estuarine pipefishSyngnathuswatermeyerihas been attributed to the indirect effectfreshwater deprivation has had on the three EasternCape estuaries where this endemic species had previ-ously been recorded (Whitfield, 1995a).

Despite the positive effects of frequent fresh-water pulses on estuarine productivity, river floodingand/or prolonged large freshwater inputs into estuariescan result in a temporary depletion of marine andestuarine fish species within these systems (Marais,

1983; Ter Morshuizen et al., 1996b). River floodscarrying high suspensoid loads can be lethal to bothmarine migrants and estuarine residents. Whitfield andPaterson (1995) recorded extensive mortalities of bothgroups of fishes in the Sundays Estuary following aflash flood. Although the suspended silt resulted ina clogging of the gills of fishes which died in themass mortality, it is also plausible that reduced oxygenlevels associated with the floodwaters contributed tothe asphyxiation of fishes in this estuary.

The morphology of an estuary may also influencethe response of certain fish taxa to major floodingevents. Marais (1982) found that densities of Mugil-idae increased in the broad Swartkops Estuary afterriver floods but decreased markedly in the channel-likeSundays Estuary. He postulated that the organic richmud and silt which are deposited in the floodplain-likemiddle and lower reaches of the Swartkops Estuary actas a food source for the mullet species, whereas therich epibenthic layer in the Sundays Estuary is washedaway by heavy floods.

Turbidity

Water turbidity influences fishes associated withsouthern African estuaries, with the juveniles of mostestuary-associated marine species being attracted toturbid waters (Cyrus and Blaber, 1987c). However,excessively high water turbidities have been shown tonegatively affect fish egg survival, hatching success,feeding efficiency, growth rate and population size.In the case of those estuarine species which remainwithin these systems for their entire life cycle, all life-history stages need to be tolerant of turbid waters.Indeed, few southern African estuaries fall into theclear-water category (<10 NTU) of Cyrus (1988),with the majority either semi-turbid (10–50 NTU) orturbid (>50 NTU). Closure of an estuary mouth leadsto decreased water turbidity but when it re-opens attimes of high river inflow, turbidities increase rapidly,mainly due to suspensoids carried into the estuary byfloodwaters. This may account for the occurrence ofboth clear- and turbid-water species in these systems(Harrison and Whitfield, 1995).

Blaber (1981) suggested that many southernAfrican estuary-associated fishes are essentiallyturbid-water species that have evolved in turbid areasof the Indo–Pacific. Research conducted on juvenilemarine fishes in KwaZulu-Natal estuaries and in thelaboratory (Cyrus and Blaber, 1987a, 1987b) hasshown that 80% of the species studied were turbid-

166

Figure 6. Relationship between biomass of different biota and the magnitude of freshwater inflow over a range of permanently open estuariesalong the Eastern Cape coast, South Africa (after Schlacher and Wooldridge, 1996). The measure of riverine inflow is reflected in the longitud-inal salinity gradient which is the difference in mean salinity between the tidal head and mouth of an estuary. Each data point represents meanvalues for an estuary and the 95% confidence limits are shown by a dotted band on either side of each regression line.

water taxa, whereas only 20% could be classedas truly clear-water species. It was suggested thatthe protective isolation created by turbidity, coupledwith other factors, are advantageous to the survivaland growth of juvenile fish in estuaries. Supportingthis hypothesis, Blaber (1981) showed that wherethese conditions exist outside estuaries in the Indo–Pacific region, the same juvenile fish are also present.Shallow, turbid areas are usually found only withinestuaries along the southern African coast.

Turbidity gradients usually exist not only withinestuaries but also from the mouth region into thesea. Blaber (1987) suggested that by following anincreasing turbidity gradient, postlarval fishes in themarine environment, could ultimately reach shallowestuarine areas. If this hypothesis is true, the exportof estuarine suspensoids could affect fish recruitmentinto these systems, thereby influencing communitystructure.

Why is turbidity so important to juvenile fish?Two possible factors include the cover it affords smallfishes from predatory teleosts and birds, as well asincreased feeding success in suspensoid-rich waters.Turbidity preference of juveniles may differ from thatof the adults, e.g. Blaber and Cyrus (1983) have shownthat Caranx sexfasciatusjuveniles inhabit estuarinewaters which are more turbid than those occupied by

the adults. These authors suggested that this differ-ence in turbidity preference may reduce intraspecificpredation.

Although it is important for fishes to evade pred-ators, the ability to detect food in water wherevisibility is poor is also important. Species such asGaleichthys felicepshave long barbels which performtactile foraging functions, whereas others (e.g.Argyro-somus japonicus) can use olfactory and lateral linesense organs to detect prey (van der Elst, 1988).In Eastern Cape estuaries, visual foraging pisci-vorous fishes are adversely affected by high turbidityconditions, whereas non-visual foragers are largelyunaffected (Whitfield et al., 1994). Hecht and vander Lingen (1992) determined that the feeding rateof visual fish predators was reduced at high turbiditylevels. They also confirmed that visual pelagic pred-ators are more affected by increased turbidity thannon-visual macrobenthic feeders. However, both fishgroups do have the ability to change their foragingstrategies in order to optimize the acquisition of foodunder different turbidity conditions.

Although the zoogeographical affinities of fishesin south-east African estuaries lie with the turbidwaters of the north-east Indian Ocean zone (Blaber,1981), low turbidity estuaries in southern Africa some-times have a higher species diversity than more turbid

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systems. Approximately 100 fish species have beendocumented from the turbid St Lucia system, whereasthe clear Kosi system has more than 150 species(Whitfield, 1980b). Similarly, a much lower speciesdiversity was recorded in the turbid Great Fish Estuarywhen compared to the clearer Kowie Estuary (Whit-field et al., 1994). According to Day and Grindley(1981), estuaries characterized by physically unstableconditions often have a low biological diversity, sothe lower diversities in the St Lucia and Great Fishsystems may simply be a reflection of the moreextreme physical conditions in these water bodieswhen compared with similar sized estuaries with lowerfreshwater inputs.

Mouth phase

The magnitude of river inflow into an estuary isusually related to catchment size, which in turn has amajor influence on the condition of an estuary mouth.Although the degree of isolation of an estuary fromthe sea has long been recognized as a major factorinfluencing South African ichthyofaunal diversity, itwas only comparatively recently that studies havequantified this evaluation. Formation of a sand baracross the mouth of an estuary effectively blocks anyfurther recruitment of marine juveniles, or emigrationof subadults/adults back to the sea, thereby directlyinfluencing the composition of the fish community.Bennett (1989), working in the temporarily closedBot Estuary, calculated that marine migrants numer-ically comprised only 1% and estuarine spawners99% of the fish fauna, whereas the two groupsoccurred in approximately equal abundance in thenearby permanently open Palmiet Estuary. In contrastto the prolonged (several years) closed phase of theBot Estuary, the Swartvlei system usually opens annu-ally, so the marine migrants are well represented inthis estuary (Kok and Whitfield, 1986; Russell, 1996).

Evidence from Australian studies suggests thatthe state of an estuary mouth is probably thesingle most important factor influencing fish speciesdiversity. Lenanton and Hodgkin (1985) found thatthe number of fish species in the temporarily openBeaufort Estuary (Western Australia) more thandoubled, compared with the closed phase, dueto juvenile recruitment of marine species into thatsystem. However, larval fish assemblages in bothtemporarily closed and permanently open estuaries inAustralia and South Africa are usually dominated byspecies that spawn within these systems, with marine

migrants usually accounting for<5% of the totalichthyoplankton (Melville-Smith and Baird, 1980;Harrison and Whitfield, 1990; Neira and Potter, 1994).

Comparisons between the nearby Swartvlei andKnysna estuaries indicated that the catch per unit effortof most marine migrants were higher in the lattersystem (Whitfield and Kok, 1992). Apart from mouthcondition, most of the factors influencing fish abun-dance in the two estuaries were similar, so it appearsthat the higher densities of most species in the Knysnasystem may be attributed to the deep, permanentlyopen mouth and strong marine influence. Similarly,Margalef’s species richness index (R) of fishes associ-ated withZostera capensisbeds in the Knysna Estuarywas more than twice that of Swartvlei Estuary eelgrassbeds (Whitfield et al., 1989).

In KwaZulu-Natal, Begg (1984) found thatspecies-rich estuaries were normally open and domin-ated by a wide variety of marine teleosts, especiallymugilids and sparids. In contrast, systems that werenormally closed had relatively few species and werecommonly dominated by a few estuarine and fresh-water taxa. Harrison and Whitfield (1995) found thatthe species diversity in three temporarily open/closedKwaZulu-Natal estuaries was related to the duration ofthe open mouth phase. The Damba Estuary remainedclosed for much of the study period and had the lowestdiversity, whereas the Zotsha Estuary remained openfor most of the study period and had the highestdiversity. In addition, in both the Zotsha and Mhlangaestuaries there was a significant positive correlation(r > 0.80,P < 0.05) between the number of speciesrecorded within each system and the time that themouth remained open during a particular season.

The fact that ichthyofaunal diversity is posi-tively linked to the duration of the open mouthphase can result in an overemphasis of the impor-tance of permanently open estuaries as nursery areasfor juvenile marine fish. There are distinct bene-fits to those juvenile fish which enter estuaries thatsubsequently close, e.g. the nursery area available toforaging fishes in the Mhlanga, Bot and Swartvleiestuaries increases considerably during the lagoonalphase due to elevated water levels inundating intertidaland supratidal habitats (Whitfield, 1980c; Bennett etal., 1985; Kok and Whitfield, 1986). These shallowlittoral areas are less accessible to large predatoryfishes, thereby enhancing their refuge function (Ruizet al., 1993). Breaching of closed estuaries results in adecline in the volume and area of the aquatic environ-ment, together with a slump in aquatic plant and inver-

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tebrate food resources (Whitfield, 1980c; Branch etal., 1985). In contrast, permanently open systems suchas the Knysna Estuary offer a more predictable nurseryarea, which fluctuates in depth and area according tothe tidal regime. However, low tide conditions usuallyresult in the intertidal flats being made unavailablefor occupation by juvenile fishes, often forcing theminto channel areas where they are more vulnerable topredation by large piscivorous fishes.

Immigration of juvenile fishes into estuaries maybe either active or passive when the mouth isopen (Beckley, 1985; Harrison and Cooper, 1991).However, Begg (1984) postulated that juvenile marinefish may also gain access into temporarily closedestuaries by being washed into them at high tidewhen waves overtop the bar. This was confirmed byWhitfield (1992), who recorded postlarval mugilidsand Rhabdosargus holubientering the Haga HagaEstuary when the sand bar at the mouth was beingovertopped. Similar juvenile recruitment mechanismshave been observed in the Nhlabane and Siyaya estu-aries in KwaZulu-Natal (L. Vivier, pers. comm.)and the Kleinemonde system in the Eastern Cape(P. Cowley, pers. comm.). Overwash water depthsof as little as a centimetre are sufficient to facil-itate the above immigration process. Based on theabove evidence, large-scale overtopping events mayhave a profound effect on the ichthyofaunal struc-ture of temporarily closed estuaries, particularly thosesystems that remain closed for several years in succes-sion.

Estuary mouth state affects water level and there-fore also water connections within segmented coastallakes and lagoons. In the Wilderness system, move-ment of fishes between the lakes can only takeplace along narrow channels linking each compart-ment (Hall et al., 1987). When the estuary mouthis open, some interconnecting channels are tooshallow to permit the migration of even juvenilefishes. Only when the mouth closes and water levelsrise do these connections become viable migrationroutes for fishes. Ultimately, however, the naturalprogression for segmented estuarine lake systems istowards the isolation of sections which then becomebrackish or freshwater coastal lakes. This has alreadyhappened to Groenvlei, which was previously linkedto the Swartvlei Estuary. Although relic estuarine fishspecies are still found in Groenvlei, all the marinemigrants have disappeared from the system (Ratteand Ritchie, 1984). A similar situation exists in LakeSibaya, where only a few estuarine spawners have

survived the loss of a link to the marine environment(Bruton, 1979).

Dissolved oxygen

Oxygen depletion within the water column has thepotential to restrict the distribution and movementof fishes within estuaries. In subtropical estuariesthis may be more pronounced because of the gener-ally higher water temperatures. Additional oxygen-demanding substances added to the water in the formof domestic or industrial pollution may further limitthe distribution of fishes in an estuary, or result infish mortalities. Begg (1984) noted that organic pollu-tion played a role in suppressing species diversity insome degraded KwaZulu-Natal estuaries such as theSipingo, Tongati, Sezela and Mbokodweni. Blaberet al. (1984) suggested that low dissolved oxygenvalues contributed to the impoverished fish fauna inthe Tongati Estuary, and Russell (1994) has shownhow depleted oxygen levels resulted in fish mortalitieswithin a Western Cape estuarine lake.

Available evidence (Burton et al., 1980) suggeststhat dissolved oxygen levels below 1 mg l−1 arelethal to many estuary-associated fish species. Undernatural conditions it is probable that only a fewsouthern African estuaries would have experiencedlow oxygen levels. Examples of such systems includethe meromictic lakes Mpungwini and Swartvlei wherethe bottom waters are usually anoxic and rich inhydrogen sulphide (Allanson and Howard-Williams,1984). Periodic dissolved oxygen depletion can alsooccur during episodic river floods which, together withexcessive silt loads, may cause mass mortalities offishes in estuaries (Whitfield, 1995b).

Habitat variability

A factor frequently overlooked in the assessment ofichthyofaunal diversity is habitat variation. Estuarieswith a wide range of substrata and littoral plant growthnormally have a higher species diversity than moreuniform systems (Whitfield, 1983). Twenty per centof the fish species in the Kosi Estuary were found tobe associated with a rocky outcrop near the mouth(Blaber, 1978), and their absence from the rest of thesystem suggests that their presence in the estuary wasdependent on the reef. Branch and Grindley (1979)found that more than 60% of the fish species inthe Mngazana estuary were associated withZosterabeds. Most southern African estuaries lack submerged

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macrophyte beds, and extensive rocky intrusions areusually confined to the upper reaches.

J.H. Day et al. (1981) reviewed the ichthyofaunaldiversity of certain very turbid estuaries such as theMzimkulu, Mzimvubu and Kei, and concluded that therelatively low biodiversity was due to the heavy siltloads carried by these rivers. Suspended silt reduceslight penetration and when large quantities are depos-ited following river flooding, it smothers submergedvegetation, thus reducing habitat diversity. The lowspecies diversity of very turbid estuaries may there-fore be linked to the indirect effect of silt depositionand sediment disturbance on aquatic macrophytes andtheir associated invertebrate prey, rather than turbidity.In the Mhlanga and Swartvlei estuarine systems, theichthyofaunal diversity did not decline when watertransparency decreased as a result of river flooding(Whitfield, 1983), thus supporting Blaber’s (1981)postulate that marine teleosts occurring in Indo–Pacific estuaries are tolerant of turbid water. However,the long-term effects of persistent siltation on both thefood resources and species diversity of an estuary haveyet to be quantified. In the Swartvlei system, siltationdue to river flooding is a relatively infrequent occur-rence, whereas in the Mzimvubu Estuary, siltationis a feature even under relatively low flow condi-tions. Marais (1982, 1983) determined that river floodsin the Swartkops, Sundays and Gamtoos estuarieshave a considerable impact on the fish populations,depending on the severity of the flood, the configura-tion of the estuary and the extent to which food sourcesare affected.

Prolonged droughts can increase habitat diversityin permanently open estuaries where eelgrass (Zosteracapensis) beds expand in systems where freshwaterinflow is minimal or has ceased altogether (Adams andTalbot, 1992). Fish that are usually closely associatedwith aquatic macrophyte beds in estuaries include thesyngnathidsHippocampus capensisand Syngnathusacus, and sparidsRhabdosargus holubiand Sarpasalpa. Hanekom and Baird (1984) recorded bothR.holubi and Monodactylus falciformisin significantlyhigher numbers inZosterathan in non-Zosteraregionsof the Kromme Estuary.R. holubiandM. falciformisdeclined in abundance in the Swartvlei littoral zonewhen Potamogetonbeds entered a senescent phase(Whitfield, 1986), thus emphasizing the importanceof suitable aquatic macrophyte habitats to certain fishspecies.

Zoogeography and seasonality

According to Vieira and Musick (1993) latitude (watertemperature characteristics) affects both fish diversity(number of species) and equitability (relative abun-dance of the different taxa) in tropical and warm-temperate estuaries of the western Atlantic. Theirreview showed that studies conducted in tropicalestuaries reported more species, and more equita-bility between species, than did those from temperateareas. Similarly, Ayvazian et al. (1992) describe a50% decline in the numbers of estuary-associated fishspecies north of Cape Cod, indicating that the Capeacts as a zoogeographic boundary between warm-temperate and cold-temperate systems in the westernAtlantic. Major declines in estuary-associated fishesaround Cape Point (Figure 1; Table 4) also suggest thatthis Cape forms a zoogeographic boundary betweenwarm-temperate and cool-temperate systems in thisregion.

A preliminary comparison between the number offish species recorded in a variety of tropical and warm-temperate western Atlantic estuaries, and numbers ofspecies from equivalent biogeographic areas in thewestern Indo–Pacific, is shown in Figure 7. In boththe Atlantic and Indo–Pacific there is a decline infish species diversity between tropical and temperateregions, with the reduction appearing considerablygreater in the Indo–Pacific zone (Figure 7).

On a regional basis there is a decline in fishdiversity from KwaZulu-Natal, along the Eastern Capecoast, around the Western Cape and up the Atlanticwest coast (Wallace and van der Elst, 1975; J.H. Day etal., 1981; Whitfield, 1994c). The reason for the declineis linked to the subtraction of tropical and subtrop-ical species (Blaber, 1981), which together comprisethe bulk of the estuary-associated ichthyofauna. Onlyseven of the 24 species associated with cool-temperateestuaries have distributions which extend into tropicalwaters, the remainder being confined to areas southof 26 ◦S. As many as 16 of the 24 taxa are endemicto southern Africa (Table 4) and this group includesall the dominant species found in these systems. Theoverwhelming dominance ofLiza richardsonii andrelative scarcity of most other species from west coastestuaries is noteworthy. Marine water temperaturesprobably play a major role in the low fish speciesrepresentation described above. There is a markeddecline in the average winter and summer sea temper-atures from Durban (19–24◦C) southwards to PortElizabeth (16–22◦C) and Table Bay (13–18◦C). As

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Table 4. Biogeographical breakdown of ichthyofaunal estuary-association and endemism in South African waters

Region Estuary-associated fish species1 Species dependent on estuaries2 Southern African endemic species3

Number % Total Number % Region Number % Region

Subtropical 142 92 72 51 26 18

Warm-temperate 78 50 39 50 31 40

Cool-temperate 24 15 14 58 16 67

South Africa 155 100 77 50 38 25

1 Information from Table 13 in Whitfield (1998).2 Fish species that are either partially or totally dependent on estuaries for their existence in southern African waters.3 Estuary-associated fish species that are endemic to the African subcontinent south of 10◦S.

Figure 7. Number of fish species (mean, standard error, minimumand maximum) recorded from tropical and warm-temperate estu-aries in the western Atlantic (shaded plots – data from Vieira andMusick, 1993) and tropical and warm-temperate estuaries in thewestern Indo-Pacific (open plots – data from Potter et al., 1990;Harrison and Whitfield, 1995; Blaber, 1997).

expected, lower temperatures are also found in estu-aries as one moves in a south-westerly direction alongthe coast (Day, 1981a). The minimum winter watertemperatures of 10–12◦C recorded in cool-temperateestuaries would probably be lethal for many of thewarm water species found in subtropical and warm-temperate systems. In addition, low winter temper-atures in combination with low salinities can resultin severe physiological stress, even to estuarine fishspecies (Whitfield et al., 1981). Because low temper-atures and salinities are frequently recorded in cool-

temperate estuaries, this combination probably limitsthe colonization of these systems by certain species,e.g.Rhabdosargus holubi, if present, is represented bya few ‘stragglers’ in cool-temperate systems. Indeed,winter temperature and salinity combinations in theseestuaries are at the edge of the tolerance limits for thisspecies (Blaber, 1973b).

The warm-temperate region represents a transitionzone between subtropical estuaries to the north-eastand cool-temperate systems to the west (Figure 1).Most warm-temperate estuaries contain species whichare representative of all three biogeographic regionsincluded in this review, as well as the tropical region.Of the 78 estuary-associated species occurring inwarm-temperate systems, 31 are endemic to southernAfrican waters, with the remainder mainly having adistribution which extends into tropical areas north of26 ◦S on the African east coast. A total of 39 fishspecies may be regarded as having a strong associationwith estuaries in the region (Table 4).

Many of the tropical and subtropical species foundin warm-temperate estuaries are more common in thenorthern part of the Eastern Cape during the summermonths, and are usually rare or absent from estu-aries west of Algoa Bay (e.g.Acanthopagrus berda).In addition, a number of endemic fish species whichare particularly abundant in some Western Cape estu-aries reach the north-eastern limits of their distribu-tion in the Eastern Cape area (e.g.Rhabdosargusglobiceps). The north-eastern portion of the warm-temperate region therefore represents an importantsubtraction zone in the distribution of both tropical andtemperate fish species.

The highest ichthyofaunal diversity on the subcon-tinent occurs in the subtropical region. Altogether, 142species are associated with these systems, of which26 are endemic to southern Africa. A total of 72species may be regarded as having a strong associ-

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ation with estuaries in the subtropical region (Table 4).More than 100 of the species have a distribution whichextends into the tropics, thus illustrating the domin-ance of subtropical estuaries by species with tropicalzoogeographical affinities (Blaber, 1981).

Maximum summer temperatures in the estuaries ofsouthern Africa do not differ greatly and usually rangefrom 24–28◦C. This suggests that high temperaturesare not an important factor in limiting fish distribu-tion, since temperate, subtropical and tropical speciesall live in estuaries which attain these levels. Forexample, the maximum recorded channel tempera-tures in the Berg Estuary (Western Cape Province) is27 ◦C and that in the Morrumbene Estuary (Mozam-bique) is 28◦C (Day, 1981a). Minimum winter watertemperatures in KwaZulu-Natal estuaries are usuallyabove 14◦C (Begg, 1978) and are an important factor,together with the general absence of cool upwelledmarine waters along the coast, in accounting for therelatively high fish species diversity in subtropicalestuaries.

Seasonal temperature changes influence the fishcommunity structure of estuaries on the subcontinent,but are not nearly as marked as in boreal systems.In the Mngazana Estuary, there was an increase intemperate fish species during winter whereas manyspecies of tropical origin appeared only in the summer(Branch and Grindley, 1979). In cold-temperateNorthern Hemisphere estuaries, marine fishes (largelyjuveniles) migrate into estuaries when the surface icesheets melt and the water becomes warmer. Biolog-ical activity in these highly seasonal environmentspeaks in the summer but it declines by late fall asmany populations emigrate to the ocean as temper-atures fall (Kennish, 1990). In contrast to the harshthermal climate described above, water temperaturesin southern African estuaries seldom decline below12 ◦C in winter and are generally warmer than theadjacent ocean during summer (Day, 1981a). Indeed,during upwelling events on the Eastern and WesternCape coasts (Schumann et al., 1982), offshore marinefishes have been known to take temporary refuge inadjacent estuarine systems where warmer conditionsprevail (Hanekom et al., 1989).

Catchment and estuary size

These two factors are difficult to examine in isolationsince both have an influence on other processes andparameters such as hydrodynamics and mouth condi-tion. How does estuary size and catchment charac-

terisitics influence ichthyofaunal abundance? Clearlylarge estuaries will invariably have greater fish popu-lations than small estuaries, due primarily to increasedfood and habitat availability. As far as catchmentcharacterisitics are concerned, Marais (1988) deter-mined that in Eastern Cape Province estuaries therewas a highly significant positive correlation (r = 0.46,P < 0.001) between fish abundance and catchmentsize, as well as between fish biomass and catchmentarea (r = 0.59, P < 0.001). He suggested that itwas not the actual catchment size that influenced fishstocks, but rather the hydrological consequences ofincreased river inflow with increasing catchment area.The higher run-off from larger catchments almostinvariably leads to positive estuarine salinity gradientsand increased water turbidity, both of which have beenshown to be associated with increased fish abundance(Whitfield et al., 1994).

A factor not included in the Marais (1988) analysisis the tendency towards higher nutrient and organicmatter loading of estuaries with larger catchments,and hence the potential for elevated primary andsecondary productivity within these systems. Anotherpotential factor accounting for the positive correla-tion between fish abundance and catchment size inEastern Cape estuaries is the magnitude of olfactorycues entering the marine environment. Large peren-nial rivers are going to transmit greater volumesof land-based cues to potential marine fish recruitswhen compared with small intermittent river systems.Clearly, further research in this field is required beforethe exact cause(s) of the positive correlations betweenfish abundance/biomass and catchment size in EasternCape estuaries can be positively identified.

The number of species found in an estuary appearsto be correlated to estuary size, with larger systemsgenerally having a higher ichthyofaunal diversity(Whitfield, 1980b; Blaber, 1985). This is probablyindirectly related to the influence of estuary size onmarine interaction and habitat diversity. Firstly, smallestuaries are often closed to the sea for prolongedperiods, with a concomitant reduction in the numberof marine species that can recruit into these systems(Begg, 1984). Secondly, the range of habitats isgenerally lower in smaller systems which tend tohave a greater degree of uniformity in both phys-ical and biotic characteristics. For example, thelongitudinal salinity gradient between the upper andlower reaches of small temporarily closed estuaries isusually minimal when compared with larger, perman-ently open systems. FringingPhragmitesbeds usually

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extend into the lower reaches of temporarily closedestuaries, whereas the lower reaches of permanentlyopen estuaries are usually occupied by salt marshplants or mangroves, with reed beds often dominatingthe upper reaches. Submerged aquatic macrophytesare also more diverse in large permanently open estu-aries when compared with small, temporarily closedsystems (Day, 1981b).

Maximum water depths are also related to estuarysize, with large estuaries usually having a wider rangein water depths than small estuaries. Juvenile fish inestuaries tend to prefer littoral areas, especially wherenursery habitats such as aquatic macrophytes, emer-gent reed beds or mangroves are present (Wallace andvan der Elst, 1975). Shallow waters provide thesesmall fishes with a refuge from larger piscivorousteleosts, which are usually restricted to deeper areas(Whitfield and Blaber, 1978a). Harrison and Whitfield(1995) have shown that in temporarily closed estuariespiscivorous species such asArgyrosomus japonicus,Caranx sexfasciatus, Lichia amia, Lutjanus argen-timaculatusand Platycephalus indicuswere presentin the deeper Mhlanga and Zotsha systems (chan-nels 1.5–3 m depth), while in the relatively shallowDamba Estuary (channel<1.5 m depth) these pred-ators were usually absent. A similar trend pertainsto other types of estuaries, where large piscivorousspecies are abundant only in those systems which havechannel depths greater than 1.5 m (Coetzee, 1982;Marais, 1984).

Larval linkages

The ichthyoplankton recorded in estuaries around theworld is usually dominated by larvae spawned withinthese systems (Able, 1978; Olney, 1983; Neira et al.,1992). The low representation of marine and fresh-water taxa has been attributed to a variety of reasons,ranging from river flushing events to a lack of tidalexchange (Neira and Potter, 1992; Potter et al., 1993).Within southern Africa, the juveniles of a numberof marine fish species are dependent on estuaries asnursery areas but their preflexion and flexion larvaeare usually absent from these systems. The avail-able evidence suggests that abiotic constraints to thesurvival of egg and larval stages, under fluctuatingsalinity, temperature and dissolved oxygen regimes,could be problematical (Whitfield, 1998). In addi-tion, tidal fluctuations could easily transport embryosand non-motile larvae into unfavourable reaches of anestuary. The unpredictable abiotic characteristics of

estuaries outlined above contrast to conditions in themore predictable marine environment.

The biotic environment within estuaries mayalso present problems for the survival of largenumbers of ichthyoplanktonic organisms. Larvalfishes feed mainly on microzooplankton (Houde andSchekter, 1980), whereas postlarval fishes utilizeboth micro- and macrozooplankton (Whitfield, 1985).Zooplankton are the dominant primary consumers inoceanic waters, so fish larvae are more likely to finda suitably sized and reliable planktonic food supply inthe sea. Carter (1978) found that zooplankton biomassoff the KwaZulu-Natal coast was higher over thecontinental shelf than in offshore areas. The highestbiomass values were recorded within 5 km of thecoast, a region usually occupied by the fish larvaeof estuary-associated marine species (Heydorn et al.,1978).

Zooplankton abundance in southern African estu-aries is spatially highly variable, with east coastsystems tending to have a higher biomass (up to 1200mg m−3) than south coast estuaries (up to 120 mgm−3) (Grindley, 1981). In addition, the unpredict-able nature of estuarine zooplankton on a temporalbasis, even in those systems with characteristicallyabundant stocks, militates against the use of thisenvironment by the larvae of marine species. However,the clupeidGilchristella aestuariais dependent uponthe zooplankton stocks of these systems. The seasonalabundance ofG. aestuariain Eastern Cape estuaries ispositively correlated with copepod densities (Harrisonand Whitfield, 1990), thereby increasing the poten-tial growth and survival of the larvae. The spring andsummer peak in zooplanktonic productivity is particu-larly important to those estuarine fish species whoselarvae do not develop within the marine environ-ment. Available evidence (Whitfield and Harrison,1996) tends to support the expectation that estuarieswith large zooplankton stocks also have high densi-ties of resident planktivores such asG. aestuaria. InSwartvlei, an estuarine lake that has poor zooplanktonresources (Coetzee, 1981),G. aestuarialarvae aver-aged 26 individuals per 100 m3 during 1986/87 (Whit-field, 1989b). In contrast, the Sundays Estuary has arich zooplanktonic resource (Wooldridge and Bailey,1982) andG. aestuaria larvae averaged 204 indi-viduals per 100 m3 during 1986/87 (Harrison andWhitfield, 1990). In addition, larvalG. aestuariafrom the Swartvlei system are narrower bodied thanthose from the Sundays Estuary (Haigh and Whitfield,1993), thus reinforcing the suggestion by Blaber et al.

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(1981) that zooplanktonic prey abundance may influ-ence the morphometrics of this species. The thinnerlarvae from Swartvlei would presumably be moresusceptible to starvation-associated mortalities thanthose in the Sundays Estuary.

Estuary type

Based upon the information already presented, itbecomes apparent that biogeography and estuarymouth condition are two major factors influencingsouthern African ichthyofaunal structure. Even inthose tidal inlets that are permanently open (e.g. estu-arine bays and river mouths) there are considerabledifferences in mouth depth, width, substratum, tidalprism etc., all of which can influence fish move-ments into and out of an estuary. At the otherend of the spectrum are temporarily closed estuarieswhere fish migrations may be severely constrained forextended periods, thus having a considerable influenceon ichthyofaunal structure. Indeed, Schlacher andWooldridge (1996) have shown that calculated indicesof southern African fish biodiversity are markedlyhigher in estuaries having a permanent connectionto the sea than those which have a temporarilyclosed phase. This trend of declining biodiversity withincreasing isolation from the marine environment isreflected in the low number of fish species present incoastal lakes (e.g. Sibaya and Groenvlei) that have losttheir estuarine links with the sea.

As has been mentioned, the geographical positionof an estuary on the coastline has a major influenceon ichthyofaunal composition. Although factors suchas salinity have been shown to influence the fish in aparticular system, the availability of juvenile recruitsfrom the sea is primarily dependent on the distribu-tional range of that species in the marine environment.Within a particular biogeographic region, the type ofestuary will have a major influence on the detailedichthyofaunal structure that develops. A conceptualdiagram of how estuary type can influence the fishassemblage in a particular system is given in Figure8. What this illustration shows is a gradual declinein biodiversity from left to right along the figure axisas the number of marine migrants decreases. Theproportions of the different fish groups within theoverall community also changes between the differenttypes of systems, with an increasing preponderance ofmarine species being recorded when moving from afreshwater-dominated towards a seawater-dominatedsystem. The paucity of freshwater fish taxa used in

Figure 8 is indicative of the exceptionally low speciesdiversity of this component. Even in river mouths,where freshwater conditions often extend almost to thesea, only a few riverine species are usually recorded(Whitfield, 1998).

Despite the influence of biogeography and estuarytype on the composition of fishes in an estuary,the basic trophic structure within these systems isusually very similar. For example, in terms of thenumbers of species recorded in a wide variety oftropical and temperate Indo–Pacific estuaries, manysystems appear to have similar proportions of speciesbelonging to each broad trophic category (Table 5;Whitfield, 1998). This applies particularly to the SouthAfrican estuaries, where little variation was apparentbetween the documented systems. The differencesin trophic structures between the Australasian andsouthern African estuaries centres mainly around theplanktivorous and detritivorous fish groups (Table 5),probably reflecting whether or not a particular systemis phytoplankton or detritus driven (Blaber, 1997).

Fish biomass and resource utilization

Fish biomass in estuaries is highly variable in botha spatial and temporal context (Table 6). Indeed, ithas been shown that the biomass of fishes in estu-aries do not always exceed those of adjacent marine orfreshwater habitats (Whitfield, 1993; Blaber, 1997).Nevertheless, estuaries are highly productive ecosys-tems ranking with coral reefs and mangrove swampsin terms of carbon production. According to Correll(1978), the elevated primary productivity of estuariesis maintained because of high nutrient levels in boththe sediments and water column. This view is elab-orated upon by Knox (1986) who describes estuariesas nutrient traps or sinks, with the essential elementsbeing recycled over and over within the system. Someestuaries in the Eastern Cape Province have largeinputs of nutrient-rich river water (Emmerson, 1989),often leading to elevated phytoplankton levels (Hilmerand Bate, 1991). The abundant fish stocks associ-ated with estuaries such as the Great Fish are prob-ably linked to the exceptional primary and secondaryproductivity of these systems (Whitfield et al., 1994;Allanson and Read, 1995).

Although the high productivity of estuaries iswidely accepted, the considerable autochthonousproduction would have little effect on the biota ifexported to the sea. A key to the attractiveness of estu-

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Table 5. Percentage contribution of each trophic level to the total number of species (n) in tropical and temperate Indo-Pacific estuarinesystems. Abbreviations: PO = permanently open estuary, EL = estuarine lake, TC = temporarily closed (blind) estuary

Estuary Type Country Herbi- Detri- Plankti- Benthi- Pisci- n Reference

vorous1 vorous vorous vorous2 vorous

Ponggol PO Singapore 5 3 33 40 19 78 Chua (1973)

Morrumbene PO Mozambique 5 4 22 50 19 113 Day (1974)

Embley PO Australia 5 4 15 50 26 197 Blaber et al. (1989)

Trinity PO Australia 5 9 27 36 15 55 Blaber (1980)

Palmiet PO South Africa 5 26 21 37 11 19 Branch and Day (1984)

Nhlange EL South Africa 3 20 24 33 20 30 Blaber (1978)

St Lucia EL South Africa 3 20 16 39 21 61 Whitfield (1977)

Swartvlei EL South Africa 4 19 19 41 19 27 Whitfield et al. (1983)

Wilderness EL South Africa 7 21 18 39 14 28 Hall et al. (1987)

Botriviervlei EL South Africa 9 16 19 38 19 32 Bennett et al. (1985)

Tongati TC South Africa 6 25 16 34 19 32 Blaber et al. (1984)

Mhlanga TC South Africa 2 28 11 48 11 46 Harrison and Whitfield (1995)

Zotsha TC South Africa 2 23 13 46 15 48 Harrison and Whitfield (1995)

1 More than 50% of diet consists of plants.2 Macrobenthic invertebrate feeders.

Table 6. Fish biomass (g m−2) estimates from various tropical (including subtropical) and temperate estuariesaround the world

Region and country Estuary type Fish biomass Reference

or habitat (g m−2)

Tropical Indo–Pacific

Australia Estuarine creek 2.5–29.0 Robertson and Duke (1990)

Australia Estuarine bay 2.9–25.3 Morton (1990)

Australia Large estuary 5.0–16.1 Blaber et al. (1989)

Solomon Islands Small estuaries 11.6 Blaber and Milton (1990)

Tropical West Atlantic

USA Mangrove estuary 15.0 Thayer et al. (1987)

Mexico Estuarine lagoon 1.0–11.3 Yañez-Arancibia et al. (1980)

USA Estuarine bay 1.8–3.8 Naughton and Saloman (1978)

Mexico Estuarine lagoon 0.4–3.4 Yañez-Arancibia et al. (1988)

Tropical East Pacific

Mexico Mangrove estuary 8.0–12.5 Warburton (1979)

Temperate Indo–Pacific

South Africa Estuarine lake 12.4 Whitfield (1993)

Australia Mangrove creek 6.4 Bell et al. (1984)

South Africa Salt marsh creek 2.4 Paterson and Whitfield (1996)

Australia Estuarine littoral 0.1–4.2 Loneragan et al. (1986)

Temperate West Atlantic

USA Estuarine bay 4.8–104.0 Lubbers et al. (1990)

USA Estuarine bay 0.1–11.2 Lubbers et al. (1990)

USA Estuarine bay 6.0 Adams (1976)

USA Salt marsh 1.9 Hettler (1989)

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Figure 8. Diagrammatic representation of trends in fish group composition between one estuarine type and another (after Whitfield, 1998).

arine systems to fishes therefore lies in the fact thatthey act as detritus traps for both authochthonous andallochthonous production, thus providing abundantfood resources for filter- and deposit-feeding inverte-brate prey, as well as detritivorous fish species. Howdo the different types of estuaries act as detritus traps?Temporarily closed systems automatically accumulateall detritus that enters them during the closed phase.Estuarine lakes usually have only narrow channelslinking them to the sea, which effectively reduces theloss of organic products from these systems. Indeed, itcould be argued that because of the narrow exit chan-nels and the limited scouring action within the lakes,only a relatively small proportion of the accumulateddetrital material within these systems is exported tothe sea. In permanently open estuaries, however, riverfloods cause large-scale exports of both macrodetritusand finer particulate organic matter into the sea. Never-theless, during the non-flood phase, which is thedominant condition in permanently open estuaries,seawater wedges and tidal action aid in the reten-tion and accumulation of detrital material (Whitfield,1988b).

Does the efficient utilization of estuarine foodresources depend on the species diversity of the fishcommunity? There were fewer than 10 fish taxafound to be abundant in the Mhlanga Estuary, yet astudy by Whitfield (1980c) indicated that all potentialfood categories, with the possible exception of phyto-

plankton (which was not consumed by any of the fishspecies), were utilized. Furthermore, the compositionof the ichthyofaunal community was related to theproportions, based on standing stock energy values,of the different food types. In the Swartvlei Estuary,there were also fewer than 10 abundant fish speciescaptured, with the available food resources beingexploited but not always in proportion to their avail-ability (Whitfield, 1988a). Using Ivlev’s electivityindex, which compares the availability of a particularfood resource to its utilization by the fish community,it was shown that there was a strong positive selec-tion for epifaunal invertebrates but poor utilization ofinfauna and aquatic plants.

Plant consumption by herbivorous fishes in theSwartvlei Estuary indicated poor utilization of theavailable resources, and mainly centred around fila-mentous algae and diatoms growing onZosteracapensis, rather than eelgrass leaf material (Whitfield,1988a). BothRhabdosargus holubiand Sarpa salpadigest algae but appear unable to assimilate aquaticmacrophyte material (Blaber, 1974b; Gerking, 1984).The low abundance of predominantly herbivorous fishspecies in estuaries is not simply a reflection of theabsence of a cellulase in the digestive tract of tele-osts (Kapoor et al., 1975) but is more likely due tothe fluctuating nature (or absence) of submerged plantcommunities within these systems.

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Detritus in estuaries consists of a mixtureof plant debris and amorphous organic matter,together with associated heterotrophic and autotrophicmicro-organisms (Bowen, 1976). Detritus is animportant food source for many estuarine invertebrates(Whitfield, 1989c) which are in turn consumed bycarnivorous fishes (Bennett and Branch, 1990). Detrit-ivorous fish taxa are well represented in southernAfrican estuaries (Blaber, 1977) and, despite the oftenhigher species diversity of carnivorous fishes, theformer group are often dominant in terms of biomass(Branch and Grindley, 1979; Harrison and Whitfield,1995).

In most estuaries, the supply of energy to fishesdepends primarily on the detrital food web (Naimanand Sibert, 1979; Yáñez-Arancibia, 1985) and studiesin southern African systems reinforce this concept(Blaber, 1985; Whitfield, 1988a). Indeed, food websin estuaries on the subcontinent seem to show asurprisingly small number of energy pathways andtherefore a high energy flow per pathway (Whitfield,1980c, Heymans and Baird, 1995).

Zooplanktonic invertebrates, which feed extens-ively on phytoplankton and particulate organicmaterial (Jerling and Wooldridge, 1995), are some-times present in large numbers in estuaries and arean important food source for planktivorous fishes(Blaber, 1979). However, phytoplankton biomassvaries both spatially and seasonally (Campbell et al.,1991), whereas an abundance of detritus is availableto estuarine consumers, including planktivorous anddetritivorous fish species, throughout the year (Whit-field, 1980d). It would appear, therefore, that detritusconfers stability to estuarine ecosystems by extendingthe availability of seasonally fixed energy.

Microphytobenthos is an important component inthe detrital food web. Most invertebrate and fishdetritivores ingest diatoms and other benthic algaealong with particulate organic material. The highincidence of unicellular algae in the diet of mugilidsfrom the Eastern Cape (Masson and Marais, 1975)and KwaZulu-Natal (Blaber, 1976) suggests thatbenthic microalgae (especially diatoms) are a favouredfood item. The senescence of SwartvleiPotamogetonpectinatusbeds, and their replacement by benthicmicroalgae and filamentous algal mats, resulted in anincrease in the abundance of mugilids in the littoralzone of this lake (Whitfield, 1986).

Very little quantitative information is availableon competition for food resources by fishes withinSouth African estuaries. Most carnivorous species

feed on a wide range of prey and can adjust their dietaccording to environmental conditions and food avail-ability (Marais, 1984). In the Bot Estuary, resident fishconsumed only 30% of the secondary production byprey species (Bennett and Branch, 1990), suggestingthat food was not in short supply. Dietary overlap wascommon, with the most successful fish species in thisestuary being those with the broadest niche.

There appears to be an even greater overlapin the diets of detritivorous fish species in estu-aries, with competition being reduced by differentfeeding mechanisms which result in the available fooditems being consumed in differing quantities (Whit-field and Blaber, 1978b). However, among mugilidsthere appears to be little feeding segregation betweenspecies (Blaber, 1976, 1977), with the large biomass,abundance and diversity of this family indicating apossible superabundance of detritus within estuaries.

Phytoplankton production in all types of estuariesis usually much lower than that of macrophytes andbenthic microalgae, with detritivory greatly exceedingherbivory (Baird and Ulanowicz, 1993). Most detrit-ivory occurs in association with the benthos, withmacro-invertebrates being particularly active in theprocess. Estuaries, in contrast to certain othertypes of coastal ecosystems (e.g. upwelling regions),show a strong coupling between the benthic andpelagic components. This coupling is, to a largeextent, facilitated by the ichthyofauna which convertsbenthic carbon sources into a highly mobile fishresource. Organic carbon transfer occurs both vertic-ally and laterally depending on fish movements. Themagnitude of lateral carbon transfer, especially bymarine fish species utilizing South African estuarineresources, is an area of research that should yield inter-esting results. Preliminary work by Deegan (1993)has shown thatBrevoortia patronus(Clupeidae) in theLouisiana Estuary export, on average, 38 g biomass(930 kJ) per square metre to the nearshore Gulf ofMexico. This is about 5–10% of the total primaryproduction of the Louisiana Estuary.

The high productivity of estuaries has often beenidentified as a primary reason why fish are attractedto these areas in such large numbers. Food, espe-cially detritus and benthic invertebrates, is abundantin most systems. However, the availability of aparticular food type is likely to show marked fluctu-ations over time and space, especially in response toenvironmental changes which characterize all typesof systems on the subcontinent. For example, priorto 1979 Swartvlei was characterized by extensive

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submergedPotamogeton pectinatusand charophytebeds (Howard-Williams and Liptrot, 1980). Theseplants, together with associated epiphytic algaeand periphyton, accounted for 74% of the totalprimary production in the lake (Howard-Williamsand Allanson, 1981). In the spring of 1979 thiscommunity underwent a senescence that was to lastmore than a decade, and resulted in a 60% decline inprimary production (Taylor, 1983), a 74% slump inlittoral invertebrate biomass (Davies, 1982) and a 54%decline in the abundance of macrophyte-associatedfish species (Whitfield, 1984). These changes didnot lead to the disappearance of fish species butdid result in alterations to the energy flow pathways(Figure 9) and a restructuring of the Swartvlei fishcommunity (Whitfield, 1986). In particular there wasan increase in the abundance and biomass of all themugilid species and a decline in the dominant vegeta-tion associated taxa such asMonodactylus falciformisand Rhabdosargus holubi. Invertebrate consumptionby the latter two fish species declined from 33 mgm−2 day−1 during thePotamogetoncanopy phase to8 mg m−2 day−1 during the senescent phase (Whit-field, 1984). As a consequence of the above fluc-tuating energy pathways, fishes in the lake showedconsiderable flexibility in their diet. An exampleof this flexibility can be seen in the collapse ofisopodExosphaeroma hylecoetesstocks, a principalfood source ofM. falciformis. This fish species thenbecame piscivorous and preyed extensively on thesmall shoaling clupeidGilchristella aestuaria(Figure9). The dietary flexibility revealed byM. falciformisisnot unique, but a characteristic of many fishes on thesubcontinent, thus conferring stability to the estuarinefood web.

Despite years of research on the feeding ecology ofmany estuary-associated fish species, the exact iden-tity of the plants which form the base of food websthat support abundant ichthyofaunal populations islargely unknown. This situation has arisen because ofthe multiple potential organic matter sources, spatialcomplexity of estuaries and variations in organicmatter exchange. Indeed, a model developed byDeegan et al. (1994) has shown that differencesin organic matter inputs among estuaries can haveconsiderable effects on the amount and kind of fishproduced. More recent stable isotope research in boththe USA and South Africa (Peterson et al., 1994;Deegan and Garritt, 1997; Paterson and Whitfield,1997) has also shown spatial heterogeneity in organicmatter sources, with fish consumers tending to utilize

organic matter sources from the same region of theestuary in which they reside.

Conservation of fishes in estuaries

Factors that have caused fishes to become threatenedare varied and often differ from one biogeographicarea to another (Bruton, 1995). These factors caninclude habitat degradation, disruption of essen-tial ecological processes, hydrological manipula-tions, environmental pollution, overexploitation dueto fishing activities, global effects, genetic contamin-ation, and impacts of introduced aquatic animals. InSouth African estuaries, habitat degradation throughchanges in land use, and hydrological manipula-tions through excessive freshwater abstraction areimportant factors. Less important at present, butshowing increasing signs towards becoming a majorproblem is environmental pollution, especially organicand inorganic wastes from industrial, agricultural anddomestic sources.

Cyrus (1991) lists 20 ‘problems’ arising fromanthropogenic activities “which could lead to the peri-odic or permanent elimination of estuarine-dependentfish species from individual systems”. These includeincreased siltation, loss of certain habitat types, hyper-saline conditions, fish mortalities due to pollutionand prolonged mouth closure. According to Cyrus(1991), virtually every ‘pressure’ that estuaries facecan be considered under the heading habitat destruc-tion, which can be divided into three subcategories,viz. physical, chemical or biological in origin. Hegoes on to suggest that the mounting pressures facingSouth African estuaries and the consequent problemsthat arise, indicate that the fish fauna is under threat.In this context, 17% of the estuaries in the subtrop-ical province, 11% in the warm-temperate province,and 50% in the cool-temperate province were recentlyclassified as being in a poor condition (Whitfield,1998).

Although major threats to fishes are usuallylinked to environmental degradation, there is alsoevidence to suggest that certain estuary-associated fishspecies are declining in abundance primarily as aresult of overfishing. In the USA, total fish catcheshave increased over the last two decades but manyestuarine-dependent fisheries have declined (Houdeand Rutherford, 1993). Similarly, the sparidLitho-gnathus lithognathusformed an important componentof anglers catches in the Swartkops Estuary during the

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Figure 9. Simplified food web in Swartvlei during (a) the macrophyte phase, and (b) the macrophyte senescent phase (after Whitfield, 1998).

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early part of this century (Gilchrist, 1918). However,monitoring of angling in the Swartkops Estuary duringthe 1970s (Marais and Baird, 1980) suggested adramatic decline in the abundance of this species andits replacement byPomadasys commersonniias theprime angling species. More recent sampling infor-mation shows a complete absence ofL. lithognathusfrom gillnet catches in the Swartkops Estuary (Bairdet al., 1996), with angling pressures in estuaries andalong the coast being identified as a primary cause ofthe depleted stocks.

Bruton (1995) has suggested that fish conservationneeds to be approached from a different perspectiveto the conservation of mammals, mainly becauseof the high mobility of fishes and the continuousnature of many aquatic habitats. With few exceptions,fish species associated with South African estuariesare highly mobile and have wide distributions, oftenencompassing both the marine and estuarine environ-ment. Fish cannot be confined to specific protectedestuaries by the erection of barriers, but this does notmean that estuarine reserves would be of little or novalue. On the contrary, well conserved estuaries wouldfacilitate the rapid recolonization of adjacent systemsif the latter areas were depleted of fish stocks throughpollution or catchment mismanagement (Whitfield,1997).

Apart from the designation of protected areas,the main direct means of conserving fish speciesand stocks include habitat conservation, controls overfishing methods, effort, efficiency and seasonality,other forms of legislation, construction of fish passes,pollution control, benign translocations and captivebreeding (Bruton, 1995). Of these, the conservation offish habitats is by far the most important, since healthyaquatic environments invariably support healthy fishpopulations. A local example of the importance of theabove statement can be seen in the rapid recovery ofthe Sezela Estuary fish community following habitatrestoration (Ramm et al., 1987). Another exampleof how habitat conservation in conjunction with wisefisheries management has led to sustainable humanexploitation of fish stocks, can be seen in the Kosiestuarine system (Kyle, 1993). Both traditional andmodern methods of fishing by local residents havebeen examined, and destructive methods such asjigging have been discouraged.

The basis for any effective fish conservationprogramme must also include appropriate legislation,such as the Endangered Species Act in the UnitedStates of America (Angermeier and Williams, 1994).

Many countries, including South Africa, have onlylists of threatened and extinct fishes, which have nolegal standing. Conservation legislation or lists areuseless if enforcement and restoration programmes arenot effectively carried out (Bruton, 1995). Conserva-tion actions need to be based on a sound knowledge ofthe biology and ecology of threatened species, theircommunities and their habitats. This view is rein-forced by Skelton (1987) who stated “Research isan essential component of any conservation exercise.Conservation authorities need to know what speciesare threatened, why they are threatened and what thepriority requirements are for the effective conservationof those species”.

Endemic fishes are those species which areconfined to a particular region and are found nowhereelse in the world. In southern Africa, approximately13% of the 2200 marine species are endemic to thesubcontinent (Smith and Heemstra, 1986), comparedto a freshwater ichthyofaunal endemicity of approxi-mately 61% (Skelton, 1993). Estuaries lie betweenthese two extremes, with 38 fish species (25% of allestuary-associated taxa) being endemic to southernAfrican waters. However, the percentage endemi-city in the region increases from 18% (25 species)in subtropical estuaries to 67% (16 species) in cool-temperate estuaries (Table 4). By focusing on theconservation of endemic fish species, it is probablethat other (usually more widespread) taxa will alsobenefit from such actions.

The concept of marine reserves or marine protectedareas (MPAs) is well established in South Africaand elsewhere (Bennett and Attwood, 1993). Marinereserves are usually designed as a viable alterna-tive to classical marine fisheries management tech-niques, or at least as an additional tool in themanagement of fishes (Buxton, 1993). Reasons forthis include (a) protection of the spawner stock, (b)providing a recruitment source for surrounding areas,(c) restocking of adjacent areas through adult emigra-tion, (d) maintenance of natural population age struc-ture, (e) conservation of biodiversity, undisturbedhabitat and natural life support processes, (f) insuranceagainst failure of other management techniques and(g) simplified law enforcement. Most, if not all, of theabove advantages apply equally to estuarine reserves.

In recent years the benefits of marine reserves inreplenishing depleted fish stocks and ‘seeding’ adja-cent unprotected areas have become apparent (Bennettand Attwood, 1991; Buxton, 1993). For severaldecades estuaries have been recognized as nursery

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areas for a number of recreationally important fishspecies, yet these systems have not been targeted forprotection or formed an integral component in theselection of marine reserves. Surely the dependenceof many marine fishes on estuarine habitats as nurseryareas necessitates the inclusion of these ecosystems inany marine reserve planning exercise?

Perhaps the answer for South African estuarineprotection lies with a modified version of the UnitedStates of America system, one of the most developedin the world. The USA National Estuarine ResearchReserve System (NERRS) is distinct from the NationalMarine Sanctuary Program, and is a system whichrecognizes that estuaries are important and havespecial management requirements. The NERRS wasauthorized under the Coastal Management Act, inresponse to concerns about threats to estuaries. Estu-aries require integrated management of the entireriver catchment, the coastal zone and the adjacentocean. The NERRS provides this, not by imposingrestrictive legislation in estuaries, but rather by facilit-ating federal, state and local partnerships which serveto promote informed estuarine management. Publicstewardship, education and scientific monitoring areimportant initiatives of the NERRS. Potential estu-arine sites are designated by NERRS for NERR-status,which elevates the estuary to a higher level of manage-ment, with access to financial, administrative andtechnical support.

Estuarine sanctuaries in Australia are not treatedseparately but are included in the general category,Marine and Estuarine Protected Area (MEPA), ofwhich 228 have been established (Rigney, 1990).MEPAs are based on a sustainable use principleand most activities are allowed in certain zones orunder reasonable control. Less than 2% of AustralianMEPAs are closed to all forms of fishing (Attwood etal., 1997).

Major marine protected areas along the southernAfrican coast include only a limited number of minorestuaries within their borders. The Tsitsikamma andSt Lucia Marine Reserves are each approximately 80km in length, with the former reserve incorporatingonly six short river mouths and the latter one smallestuary. Indeed, from an estuarine perspective it wouldbe difficult to select two worse 80 km sections ofcoastline as multi-purpose aquatic reserves. Fortu-nately the creation of the St Lucia Game Reserve,Kosi Bay Nature Reserve and Wilderness NationalPark have provided protection for some of southernAfrica’s largest and more important estuarine systems.

Altogether there are 33 functional estuaries or portionsof these systems that are conserved within SouthAfrica, few of which protect fish species from exploi-tation. This is surprising when it is considered thatestuaries are important nursery areas for severalimportant angling species on the subcontinent. Whatis needed is an expansion of the existing Estu-arine Protected Area (EPA) network, as well asthe upgrading of selected ‘estuarine reserves’ wherefishing is permitted, into ‘estuarine sanctuaries’ whereno exploitation of biological resources is allowed.

Although it is generally recognized that the pathto effective species conservation is through soundecosystem conservation, threatened species some-times demand urgent individual attention if they areto survive at all. In other words, each threatenedspecies has its own set of circumstances which callsfor different solutions (Whitfield, 1997). Even ifthere is little or no room for a strategy aimed atthe species level in South Africa, the species them-selves can be most useful indicators as to what arethreatened ecosystems. A comparison of the propor-tion of threatened fish taxa in freshwater environ-ments (Skelton, 1987) to those found in estuarieson the subcontinent, suggests that the latter habitatsare generally in better condition than their inflowingrivers. This coincides with the views of Moyle andLeidy (1992) who conservatively estimated that atleast 20% of the freshwater fish species of the worldare already extinct or in serious decline.

There are approximately 250 functional estuariesalong southern Africa’s coastline and each systemrequires careful management and protection if it isto be maintained as a vital national resource (Whit-field, 1996b). A wide range of government authoritiesand interest groups form opinions and make decisionswhich have major impacts on estuaries. Only withaccess to the best possible scientific information canthese decisions result in the wise management ofestuarine resources, now and into the next century.The future health of estuaries and associated ichthy-ofauna depends on imaginative resource managementand on the improved implementation of conserva-tion measures in partnership with decision-makers,managers, scientists and estuarine users.

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Acknowledgements

I thank Elaine Grant for drawing the illustrations usedin Figure 2 and Angus Paterson for his comments onan earlier draft of this paper.

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