chapter 2 habitat use by fishes in estuaries and other ... · 1 2 fishes in estuaries 2.2.1.2...

Chapter 2 Habitat Use by Fishes in Estuaries and Other Brackish Areas

L. Pihl, A. Cattrijsse, I. Codling S. Mathieson, D.S. McLusky and C. Roberts

2.1 Introduction

In assessing the importance of estuaries for fish and macrocrustaceans it is of note that estuaries consist of a complex mixture of many distinctive habitat types and that these habitats do not exist in isolation. Rather, there are physical, chemical and biological links between them, for example in their hydrology, in sediment transport, in the transfer of nutrients and in the way that mobile animals move between them both seasonally and during single tidal cycles (Davidson etal., 1991). The value of any habitat ‘patch’ in an estuary as a fish habitat may depend both on its proximity and/or degree of connectivity to other habitat patches.

The definition of an estuary as adopted by the Habitats Committee on 25 April 1996 under the Habitats &Species Directive (European Council Directive, 1992 (92/43/EEC)) isgiven as ‘ Downstream part o fa river vallej subject to the tide and extending from thelimit ofbrack- ish waters. River estuaries are coastalinlets where, unlike “large shallow inlets and bays” there isgenerally a substantial fresh water influence. The mixing of fresh water and sea water and the reduced current flo wsin the shelter of the estuarylead to deposition o f h e sediments, often forming extensive intertidal sand and mud flats. Where the tidal currents are faster than flood tides, most sediments deposit to form a delta at the mouth of the estuary’ (Romao , 1996).

It is considered here that nine habitats of importance for estuarine fish can be defined and described, and the selected habitats encompass all major environmental categories/zones found within European estuarine waters. These classifications provide a holistic view of habitat variation together with detailed information on fish/macrocrustacean associations, both within and between estuarine locations. Definitions of estuarine habitats have previously not been attempted on the basis of their importance for, and use by, fish and macrocrustaceans, although the principal definition of natural habitats attempted at a European level is the CORINE classification (Commission of the European Communities, 1991). This has been used as the basis for defining habitats for the implementation of the European Habitats & Spe- cies Directive (European Council Directive, 1992 (92/43/EEC)). This assessment is also of relevance for the European Water Framework Directive (European Council Directive, 2000 (2000/60/EC)), in which coastal and transitional water bodies will require to be monitored and managed. In this context, the habitats included here are contained mostly in the term ‘transitional waters’, but also in ‘coastalwaters’. Although the present assessment is based on

Fishes in EstuariesEdited by Michael Elliott, Krystal HemingwayCopyright © 2002 by Blackwell Publishing Ltd

Habitat Use byFishes in Estuaries 1 1

European estuarine habitats and areas, it is considered that the approach and the conclusions will apply to other temperate areas and some tropical ones.

The definitions of marine and estuarine habitats in the CORINE list are generally inap- propriate for consideration of the use of individual estuarine habitats by fish and mobile macrocrustaceans. This is partly due to the range of geographical scales (e.g. the use of ‘Estuar- ies’, a relatively large physiographic unit, as a CORINE habitat) and the lack of coverage in the classification of some estuarine habitats such as seagrass beds, limited in the CORINE classification to ‘ Posidonia beds’, which are restricted to southern Europe. Classification of estuarine habitats for general purposes must also consider terrestrial habitats (e.g. sand dunes, coastalgrasslands, etc.) and the maritimekerrestrial marginwhich have no direct significance for fish and thus are not considered further here. Following studies throughout Europe, the agreed list of fish habitats to be considered is:

Tidal freshwater; Reed beds; Saltmarsh (intertidal vegetated habitats) ; Intertidal soft substratum; Intertidal hard substratum; Subtidal soft substratum; Subtidal hard substratum; Subtidal sea grass beds (subtidal vegetated habitats) ; Biogenic reefs.

A further habitat ~ the pelagic part of the water column ~ can be considered, but it is not included here as a separate habitat but rather as a component part of each habitat listed.

In considering the above habitat types, it is necessary to assess their value for fish and shellfish as described by associated faunal guilds within both spatial and temporal scales. The recognition of such associations will contribute to establishing a set of criteria that may be required for future environmental and commercial management purposes.

2.2 Habitat definitions and descriptions (including subhabitats)

The definitions described below have been created here and are only used for estuaries. Salin- ity values are given throughout as psu (practical salinity units).

2.2.1 Tidal freshwater

2.2.1.1 Habitat definition

In tidal estuarine areas, this is the zone upstream of saline influence and salinity values are typically less than 0.5 psu. Water movements in this habitat are highly dynamic, with tidal rise and fall, and tidal reversals in the direction of river water flow, resulting from river flow backing up against saltwater incursion downstream. Subtidal and intertidal habitats here are included together in the following analysis.

1 2 Fishes in Estuaries

2.2.1.2 Habitat description

This habitat is described as the tidal freshwater of rivers, downstream of the tidal limit but upstream of the effects of saline water incursion. The extent of the zone is dependent on relative volumes of freshwater and saline water inflows and thus climatic condition, tidal range, and geological or physiographic characteristics. Channels are typically steeper-sided and narrower than in the downstream estuary, leading to a narrow intertidal zone. Subtidal and intertidal habitats are typically composed of mobile substrata, from fine silts to coarse gravels. Hard substrata may be more significant in areas of limited sediment supply andor scoured areas.

2.2.1.3 Subhabitats

These include intertidal soft substrata (e.g. sediment banks and shoals), subtidal soft substrata, creeks, backwaters and gravel riffles. The soft substrata habitats are regarded as subhabitats in tidal fresh water (although also described as a habitat below) since the most significant habitat feature is the tidal nature of the freshwater. Additionally, reed beds may form a significant intertidal subhabitat component in these regions. However, due to the potential significance of reed beds in some estuarine areas of limited tidal movement, they are described here as a separate habitat.

2.2.2 Reed beds


Reed beds are dense stands of tall herbaceous plants in low salinity zones, upstream of the tur- bidity maximum (typically 0- 5 p s ~ ) . The typical dominant flora includes Phragmites communities and, in tidal fresh water, Phalaris arundinacea.


In tidal estuarine areas, reed beds form in the upper intertidal zone, while in non-tidal estuarine areas, such as parts of the Baltic, they may form in shallow permanently subtidal areas. The habitat may be extensive, although the linear extent on tidal estuaries is linked to the length of the tidal freshwater zone. While subtidal reed beds in non-tidal estuarine areas are permanently flooded and continuously accessible to fish, intertidal reed beds in tidal estuarine areas are accessible to fish only when flooded, either periodically during the spring cycle of high tides, or during periods of river floods.

2.2.2.3 Subhabitats

These include intertidal and subtidal reed beds. Intertidal reed beds may have open water pools and creeks which are unvegetated.

Habitat Use byFishes in Estuaries 13

2.2.3 Sal-arsh


Saltmarshes are intertidal, sediment-based, macrophyte-dominated, saline-influenced habitats. Europeansaltmarshes may develop from the mean high water level (MHWL) (Beeftink, 1977), to the upper shore where, in undisturbed systems, they may undergo transition to brackish, freshwater or terrestrial habitats.


Saltmarsh develops where tidal waters are sufficiently quiescent to allow sediment to settle out from suspension, andwhere conditions are suitable for settlement and growth of a limited number of halophytic plant species in assemblages which undergo species successions as the saltmarsh matures. Areas of saltmarsh often form a complex mosaic in estuaries with other intertidal habitats such as mudflats. A wide range of typical saltmarsh types have been described, such as estuarine fringing marsh, beach-head marsh and barrier beach marsh. The development of linear or dendritic creeks systems is additionally a typical feature of maturing marshes. These creeks may be of great significance to fish as the principal means of entry to the marsh environment. Marsh surface pools are often a distinctive feature of saltmarsh habitats.

2.2.3.3 Subhabitats

A variety of subhabitats may be present in saltmarshes, ranging from permanently subtidal areas of creeks or pools, intertidal creeks which are largely drained at low tide, mud walls, mud banks and mud boulders (created by undercutting or slumping of walls), to the upper marsh surface (with permanent pools or flooded during higher tides).

2.2.4 Interfidal SOB substratum


This includes areas of unvegetated intertidal habitats in tidal estuarine areas, lying between the highest and lowest tides, and composed predominantly of sediments ranging from fine silt to coarse sands or shingles.


These areas are sediment-based habitats between the high and low water mark, largely with- out higher plants. They are defined in the CORINE biotopes manual (Commission of the European Communities, 1991) as ‘sands and muds, submerged for part of the tide, devoid of vascular plants, but usually coated by blue-green algae and diatoms’. In many mesotidal or macrotidal estuaries, these habitats are extensive and relatively gently sloping in nature. The sediment composition of any particular area of intertidal soft substratum depends on a


number of factors, including the nature and quantity of sediment supply from freshwater and marine sources, degree of exposure to water currents or wave action, and salinity regime. Intertidal soft substrata usually contain a high density and large biomass of macrobenthos, which provides abundant food for estuarine fish and macrocrustaceans when the intertidal flats are covered by the tide and may be of recognised international importance for wading birds and waterfowl during low water periods (McLusky, 1989).

2.2.4.3 Subhabitats

In many European estuaries, intertidal soft substratum habitats form an intertidal habitat mosaic with other habitats, such as saltmarsh. The degree to which habitats should be divided may not be clear-cut, such as that between pioneer saltmarsh and open mud flat. The extent to which intertidal subhabitats are subdivided here may, therefore, be dependent on the extent and nature of habitat patches, and their use by fish, in an individual estuary. Dispersed small patches of saltmarshvegetation on a mudflat, for example, might be considered in some cases to be insignificant as a fish habitat, although this might not be true in other sites.

In comparison with most other habitats here, intertidal soft substrata are superficially relatively homogeneous. In areas of reduced wave energy and current speeds (such as in more sheltered inner estuarine areas), finer sediments may be deposited to form mudflats (Elliott etal., 1998). Where wave energies or current speeds are too high to allow settlement of finer sediments (such as outer estuarine areas), sandflats may develop. Subhabitats may include creeks and banks, pools of standing water (during low tide periods), and intertidal patches of sea grasses (although not a significant structural feature by comparison with subtidal sea grass beds).

Biogenic structures may also be a component subhabitat of intertidal soft substrata (e.g. extensive beds of the bivalve genus Mytilus) but are considered here as a separate habitat in creating biogenic reefs (Section 2.2.9). In estuaries, suffering from the eutrophic effects of excessive inorganic nutrients from anthropogenic sources, dense beds of ephemeral macroalgae (e.g. from the genus Ulva) may constitute a significant seasonal subhabitat (Scott et al., 1999).

2.2.5 Intertidal hard substratum


These are areas of unvegetated or vegetated intertidal habitats in tidal estuarine areas, lying between the highest and lowest tides, and composed predominantly of hard substrata ranging from gravels to bedrock.


Although European estuaries are predominantly characterised by their soft sediment habitats, many also have areas of intertidal hard substrata, particularly where wave action or current speeds are sufficiently high to prevent long-term settlement of sediments. Intertidal hard substrata may be dominated to a considerable degree by macrophytic algae, although domination


by animal crusts (barnacles, mussels) may occur on intertidal hard substrata subjected to a greater degree of water movement due to wave action or currents.

2.2.5.3 Subhabitats

Intertidal hard substrata may be composed of an extremely heterogeneous mixture of subhabitats, depending on a complex interaction of physical and environmental factors such as underlying geology, degree of exposure to wave action and hydrodynamic characteristics. Subhabitats vary from hard but potentially mobile gravel, cobble or boulder habitats, to immobile rock habitats, such as ridges and bedrock features. Boulder habitats may provide many cryptic under-boulder niches. Subhabitats in rock-based intertidal habitats which may have significance for fish include caves, overhangs or permanent pools. Subhabitats may derive from biological features, such as beds of macroalgae or biogenic reefs (see Section 2.2.9).

Artificial hard substrata may also form an important component of the hard substrata (both intertidal and subtidal) of many developed estuaries. Such subhabitats, while unlikely to support populations of estuarine species in wholly natural circumstances, may provide important habitat heterogeneity in an otherwise degraded estuarine environment (see Sections 2.2.5 and 2.2.7).

2.2.6 Subtidal SOB substratum


These are areas of permanently subtidal unvegetated habitats in estuarine areas, composed predominantly of sediments ranging from fine silts to coarse sands. In tidal estuarine areas, this habitat lies below the level of lowest tides.


In estuarine areas of very limited tidal range, the majority of soft substratum habitats are likely to be permanently subtidal. In tidal areas, most estuaries have permanently subtidal soft substratum habitats. Some estuarine basins may, however, drain almost completely of water during the low water period and the extent of this habitat may be very limited in such circumstances. In tidal estuaries, subtidal soft substrata typically support lower densities and biomass of benthic invertebrates than adjacent intertidal areas of soft substrata (Elliott et al., 1998). The Habitats Directive (92/43/EEC) has a habitat category ‘sandbanks which are slightly covered by sea water all the time’ which is defined as ‘Subtidal sandbanks, permanently submerged. Water depth is seldom more than 20 m below Chart Datum’.

2.2.6.3 Subhabitats

Like intertidal soft substrata, subtidal soft substrata ranges in composition from fine silts to coarse sands, dependent on sediment supply, hydrodynamic regime and salinity characteristics. Permanently subtidal soft substrata are found in a wide range of estuarine areas, includ-


ing creeks, main channel areas, deeper water areas, backwaters and lagoons. Clearly, other habitats described here have, or may have, subtidal soft substrata as a component (e.g. tidal freshwater, saltmarsh, subtidal seagrass beds), although this feature is not the main defining component. Subhabitats may be defined according to sediment composition, whether mud or sand-based or some intermediate composition. Coastal lagoons may have limited intertidal soft substrata but the majority of soft substrata are likely to be subtidal.

2.2.7 Subtidal hard substratum

2.2. Z 1 Habitat definition

Permanently subtidal areas of unvegetated or vegetated habitats composed predominantly of hard substrata ranging from gravels to bedrock. In tidal estuarine areas, these habitats lie below the level of lowest tides.

2.2. Z 2 Habitat description

Like hard substrata in intertidal habitats, the nature of subtidal hard substrata depends on the underlying geology and hydrodynamic regime. Hard substrata may range from potentially mobile hard substrata such as gravels, cobbles and boulders, to rock features. Macroalgae may also form a significant feature of subtidal hard substrata, and may add considerable structural heterogeneity to otherwise relatively featureless rock habitats.

2.2.Z3 Subhabitats

Subtidal hard substrata may be composed of a wide range of subhabitats. These may vary from hard but potentially mobile gravel, cobble or boulder habitats, to immobile rock habitats, such as ridges and bedrock features. Boulder habitats may provide many cryptic under- boulder niches. Many subhabitats in rock-based subtidal habitats may have significance for fish, including ridges, crevices, caves, overhangs and vertical rock faces. Subhabitats may derive from biological features, such as beds of macro-algae or biogenic reefs (see Section 2.2.9). Beds of maerl, accumulations of twig-like or nodular unattached calcareous red algae, may be a significant subhabitat in some northern estuarine environments, such as semi- isolated saline lagoons.

Artificial hardsubstrata may also form an important component of the hardsubstrata (both intertidal and subtidal) of many developed estuaries. Such subhabitats, while unlikely to support populations of estuarine fish species in wholly natural circumstances, may provide important habitat heterogeneity in an otherwise degraded estuarine environment (see Sections 2.2.5 and 2.2.7).

2.2.8 Subtidal seagrass beds


These are subtidal vegetated habitats, based on soft substrata, and are dominated by


halophytic macrophytes adapted to complete and continuous submergence in water of low to high salinity.


Seagrass beds may be extensive in size and relatively stable over many years. They add considerable vertical structure (up to 1 m in height for some species of seagrass) to the soft substratum environments on which they develop. Typical genera which comprise European sea grass beds include Zostera, Posidonia (in the Mediterranean) and Ruppia. Due to their structural significance in some lagoonal and other brackish water habitats, beds of stoneworts or charophytes (complex algae which are often encrusted with calcium carbonate deposits) are also included.

2.2.8.3 Subhabitats

Newly developed or regenerating patches of seagrass beds (e.g. following physical distur- bance from natural or anthropogenic sources) may have a different topographical nature to that of mature seagrass beds.

2.2.9 Biogenic reefs


This is an elevated structure or extensive epibenthic bed, either intertidal or subtidal, which is built from calcareous or other concretion-forming organisms, or is formed from surface- dwelling bivalve molluscs.


Extensive beds of bivalve molluscs (e.g. from the genera Mytilus, Ostrea, Modiolus) may add considerable heterogeneity to the hard or soft substrata on which they develop. Reef structures may be built by sabellid or serpulid polychaete worms. These may have considerable structural complexity, including vertical development, and may be based on hard substrata or on small hard features based in largely soft substrata.

2.2.9.3 Subhabitats

The unique features of each type of biogenic reef may constitute a subhabitat within this habitat. Large bivalve beds in intertidal areas may have pools of standingwater as a subhabitat during low-water periods. Mature bivalve reefs may have a different topography to reefs, or parts of reefs, composed of young individuals. Serpulid and sabellarian reefs may have cryptic niches as a sub-habitat (Holt etal., 1998).


2.3 Quantification of fish habitats in selected European estuarine systems

2.3.1 Introduction

This section quantifies the extent of the nine previously identified fish habitats in selected Eu- ropean estuarine systems by gathering information from 26 estuarine systems in 10 European countries. These estuarine systems were selected principally on the basis of readily available information on their fish communities.

2.3.2 European context of the selected estuarine systems

The 26 estuarine systems referred to in this chapter are distributed among three biogeographical regions within European coastal waters: BoreaVAtlantic (Atlantic and North Sea coasts) ; BaltidSkagerrak and Mediterranean. Figure 2.1 shows the primary countries from which data for the volume have been derived, together with the main water bodies referred to throughout the chapters. Figures 2.2- 2.14 show selected estuaries in further detail.

2.3.2.1 Boreal/A tlantic region

The BoreaVAtlantic region covers the region from Denmark to Gibraltar, including the Brit- ish Isles, and represents all the estuarine systems possessing predictable and pronounced influence from semi-diurnal tides. The estuaries of this region thus possess substantial intertidal habitats. The total estuarine habitat of the region is approximately 18 600 km2; details of the distribution of this total among countries is shown in Table 2.1.

The estuarine resource of most countries has not been quantified in detail. However, within Great Britain (England, Scotland and Wales), 155 separate estuaries have been identified, selected on the criteria of having either a tidal channel of >2 km or soft sediment shores of >500 m at low water. These 155 estuaries have a total area of 5293 km2, incorporating 3079 km2 of intertidal area and 2214 km2 of subtidal area. The 155 estuaries have a total shorelineof9231 kmanda totaltidalchannellengthof 2451 km (Davidson etal., 1991).The

Table 2.1 The total estuarine habitat of the BorealLAtlantic region of Europe. (Modified from Davidson et al., 1991.)

Country

Britain Denmark wadden Sea) Germany (Wadden Sea) Netherlands Dutch Delta (Ems-Dollard) Belgium France Spain Portugal Ireland Total BorealiAtlantic

Area (km’)

5293 1004 3790 2691

498 19

2729 1061 625 87 1

18581

Percentage

28.5 5.4

20.4 14.5 2.6 0.1

14.7 5.7 3.4 4.7

100


u 5 5 '5


N

A

Fig. 2.2 The Mersey estuary, UK.

average size of a UK estuary is therefore 34 km2, with an intertidal area of 19.8 km2, a subtidal area of 14.2 km2, and a shoreline of 59 km.

The estuaries included here have been selected on the basis of available records of fish populations and represent only a small proportion of the total number of estuaries. It should be recognised that they are generally larger than the mean size of UK estuaries, and indeed represent some of the largest estuaries in Britain. Thus they represent a greater proportion of the total estuarine resource than is apparent by simply counting estuaries.

Among the five British estuaries in the present study, four (Forth, Humber, Thames and Mersey) were identified as estuaries for the purposes of the assessment of the extent of the UK estuarine resource (Davidson et al., 1991), but one site (Loch Etive) was not. The four estuaries represent 2.6% of the total number of identified estuaries, but 10.9% of the total British estuarine area, and 9.7% of the estuarine intertidal habitat and 12.5% of the subtidal habitat. As some of the largest British estuaries are included, the diversity and abundance of fish assemblages in these may be greater than for a British estuary of average size.

A similar assessment of estuarine habitats on a national basis is not available for the other countries in the BoreaVAtlantic region. Accordingly, the contributions of the selected estuarine systems, in terms of total surface area, have been approximated (Table 2.2) using the areas given in Table 2.1. The estuarine systems included in the present study comprise 11- 64% of the estuarine resource in the representative countries, and the role of the estuaries with regard to commercial species should, therefore, be adequately described.

2.3.2.2 BaltidSkagerrak region

The BaltidSkagerrak region covers the area east from the interface with the North Sea


N

Mattersey

Gainsborough

10 km

Dunham Trent

Winthorpe Bridge

Fig. 2.3 The Humber estuary, UK. (Modified from National Rivers Authority, 1993.)

between Norway and Denmark and includes all of the Baltic Sea. Estuarine systems in this region are not influenced by significant tidal movement, and both salinity and temperature may be significantly reduced in comparison with the BoreaVAtlantic area. The seasonal occurrence of significant ice cover may modify habitats physically and may, thereby, affect fish communities.

There has been no overall quantification of the estuarine systems of this region. Accord- ingly, it is not possible to describe the context of the selected estuarine systems in quantitative terms. The five selected estuarine systems are located in three different locations within the region: two systems in the Skagerrak (Gota River and Gullmarsfjord insweden) ; two systems on the Baltic coast of Germany and Poland (Darss-Zingster Boden and OderhaffBtettin La- goon) and one on NW &and, off the coast of Finland.


Table 2.2 resources in participating countries in the BorealiAtlantic region.

Total surface area

Approximate total surface areas of the selected 26 estuarine systems in relation to the total area national

Total surface area of Participating country of estuaries' (km2) selected estuarine systems (km2) Percentage of national resource

France2 2729 409 Germany3 3790 1713 Netherlands4 2691 1144.4 Portugal5 625 402.7 Spain6 1061 149.7 UK' 5293 576.3

15 45 42 64 14 11

' Adapted from Davidson et al. (199 1). 2Loire, Seine, Bay of Somme. 3Elbe, Weser. 4Ems-Dollard, Westerschelde, Oosterschelde 5Ria de Aveiro, Tagus, Mira, Obidos. 6Bay of Cadiz, Guadalquivir. 'Forth, Humber, Thames, Mersey.

N

Beckton STW

10 km

Fig. 2.4 The Thames estuary, UK.

2.3.2.3 Mediterranean region

The Mediterranean region covers the area east from the Strait of Gibraltar, and includes all of the Mediterranean Sea. As with the BaltidSkagerrak region, estuarine systems in the Mediterranean are not influenced by significant tides. In contrast, however, salinity is not significantly reduced and average temperatures are higher. Estuaries in the Mediterranean are typically coastal lagoons with a freshwater influence. The total coastal lagoon area in the Mediterranean has been assessed at 6500 km2 (Crivelli etal., 1995), and the distribution between countries is shown in Table 2.3. The two estuarine systems selected in this volume

Habitat Use by Fishes in Estuaries 2 3

Fig. 2.5 The Elbe estuary, Germany.

N

A

Fig. 2.6 The Waddensea, IJsselmeer and Eems estuary, Netherlands.


N

GREVELINGENMEER

10 km

Fig. 2.7 The Oosterschelde, Westerschelde and Scheldt estuary, Netherlands.

represent examples of a river delta (Ebro delta, Spain) and a coastal lagoon (Messolonghi Lagoon, Greece).

2.3.3 Diskibution and extent of fish habitats

The information gathered from the selected areas summarises the distribution and extent of fish habitats in the selected estuarine systems in each of the three European maritime regions (Table 2.4). This information has been taken from published survey information, grey litera- ture and local (unpublished) knowledge of the estuarine systems. In some cases, this is the first opportunity to attempt to quantify the extent of particular habitats within an estuarine


Table 2.3 Distribution between countries of the total area of coastal lagoons in the Mediterranean region

Country

Spain France Italy Greece Turkey

Algeria Tunisia Morocco Total

Egypt

Area (h2)

280 319

1151 618 351

24 18 72

1209 85

6503

Percentage

4.3 4.9

17.7 9.5 5.4

37.2 1.1

18.6 1.3

100

N

Baie a’e la Seine

Caen 10 km -

Fig. 2.8 The Seine estuary, France

system. The information for this latter subset represents, therefore, best estimates by work- ers with a knowledge of their particular system rather than definitive estimates derived from a rigorous and consistent survey methodology. For the majority of the 26 selected estuarine systems, it has been possible to estimate the total surface area, total intertidal area and area of each of the nine defined fish habitats such that, in most estuarine systems, in excess of 95% of the total estuarine area could be allocated to the nine fish habitats (Table 2.4).

The number, and hence diversity of fish habitats within an estuarine systemvaried between 1 (Guadalquivir river in Spain) and 8 (Ems-Dollard on the Dutch German border). In general, selected estuarine systems in the BoreaVAtlantic region contained a greater diversity of fish habitats (mean of five habitat types) than selected systems in the other regions (means of 4 in the Skagerrak/Baltic region and 2.5 in the Mediterranean region). This is partly explained by the influence of semi-diurnal tides in the BoreaVAtlantic region resulting in three fish habitats (tidal freshwater, intertidal hard and soft substrata) which are restricted to the selected

Tab

le 2

.4

Salt

mar

sh; 4

, Int

ertid

al s

oft s

ubst

ratu

m; 5

, Int

ertid

al h

ard

subs

trat

um; 6

, Sub

tidal

sof

t sub

stra

tum

; 7, S

ubtid

al h

ard

subs

trat

um; 8

, Sub

tidal

seag

rass

bed

s; 9

, Bio

geni

c re

efs.

Q

uant

itativ

e as

sess

men

t of

the

exte

nt a

nd d

istr

ibut

ion

of n

ine

fish

hab

itats

in 2

6 se

lect

ed e

stua

rine

sys

tem

s in

Eur

ope.

Hab

itat t

ypes

: 1,

Tid

al fr

eshw

ater

; 2, R

eed

beds

; 3,

Geo

grap

hic

Lat

itude

re

gion

e N)

Bal

tic/

60

Skag

gera

k 58

58

54

54

Est

uari

ne

Tot

al s

urfa

ce

syst

em

area

(km

2)

NW

han

d

184

Got

a R

iver

60

G

ullm

arsf

jord

19

D

arss

-Zin

gste

r 19

6.8

Ode

rhaf

fiSt

ettin

92

.5

Inte

rtid

al

area

(h2)

ns 0 0 0 0

NW

Atla

ntic

/ 56

L

och

Etiv

e 29

.5

1.8

Bor

eal

56

Fort

h 84

48

54

H

umbe

r 30

3.6

135.

2

53

Mer

sey

89

56.1

53

Wes

er

900

611.

1

53

Elb

e 81

3 32

6.2

53

Em

s-D

olla

rd

498.

7 26

1

51

Wes

ters

chel

de

355.

3 14

9.3

Hab

itat t

ype (h2)

1 2

3 4

5 6

7 8

9 ha

bita

t %

know

n

0 1.

9 0

0 0

161

6.2

15

0 10

0 0

00

00

50

4

5 1

10

0

0 0

0 0

0 0.

7 0.

6 0.

5 0.

1 10

0 0

15.2

0

0 0

156

5.9

19.7

0

100

0 74

0

0 0

708.

1 27

.8 9

2.5

22.7

10

0

0 0

0 1.

5 0.

3 26

.6

1.2

0.2

ns

100

1 nd

1.

7 43

.3 n

s 36

ns

0

0 98

nd

ns

ns

12

7 0

168.

4 0

0 0

97

ns

ns

8.47

47

0.1

33

ns

0 0.

1 10

0

36.8

10.

8 0

600

0.3

252.

1 0

0 0

100

38.7

19

0 30

6.8

0.4

448.

1 0

0 0

100

16.5

1.

2 14

.1 2

54.7

0

220.

6 0.

1 0

0.4

100

30

5.2

31.7

112

.4

0 17

6 0

0 0

100

No.

of

habi

tats

pr

esen

t R

efer

ence

s

4 E

. Bon

sdor

ff (

pers

. com

m.)

4 L

. Pih

l (p

ers.

com

m.)

4 L

. Pih

l (p

ers.

com

m.)

4 H

. Win

kler

(pe

rs. c

omm

.) 5

H. W

inkl

er (

pers

. com

m.)

Hol

t (19

91);

R. G

ibso

n (p

ers.

Dav

idso

n et

al. (

199 1

) D

avid

son

etal

. (19

91);

M.

Elli

ott

(per

s. co

mm

.) D

avid

son

etal

. (19

91);

I.

Cod

ling

(per

s. c

omm

.) B

aum

ert &

Zab

ansk

i (1

996)

; B

unde

sans

talt

fur

Gew

asse

rkun

de (

1994

); K

ies

etal

. (19

92);

Pre

isin

ger

(199

1) ; S

chir

mer

(19

94) ;

U

mw

eltb

ehor

de H

ambu

rg

(199

6)

As

for W

eser

(abo

ve)

Dijk

ema

(198

9)

A. C

attr

ijsse

, per

s. co

mm

.; V

an S

chai

k et

al. (

1988

);

deJo

ng &

de

Jong

e (1

995)

; So

etae

rt &

Her

man

(199

5)

com

m.)

51

Oos

ters

chel

de

350

51

49

51

47

41

41

39

39

36

36

Med

iterr

anea

n 4 1

38

Tham

es

Sein

e So

mm

e Lo

ire

Ria

de A

veiro

M

ira

Obi

dos

Tagu

s B

ay o

f Cad

iz

Gua

dalq

uivi

r

Ebro

M

esso

long

hi

84.1

99.7

60

.1

86.8

86

.8

140

41.9

18

2.2

44.9

47

4 3.

7 0.

4 7

1.1

344.

5 16

0 99

.7

27.2

50

0

74.1

0

160

0

0 0

6.45

75.

2 2.

5 22

3.1

0.9

1 40

.8

100

ns

ns

4.73

56

0.2

38.8

0

0 ns

10

0

ns

ns

21.7

64

.3

0.8

0 0

0 0

100

17.5

9.

9 0.

3 30

.1

1.6

80.6

ns

0 ns

10

0 16

.1

3.3

0.1

34.6

4.

1 12

2.5

nd

0 1.

4 10

0

0 0.

8 1.

6 1.

2 0.

4 25

.8

0 12

.9

4.3

100

0.1

0.5

0.6

0.4

ns

2.1

ns

0 0.

2 10

0 ns

ns

0.

4 0.

6 0

4.5

1.4

ns

ns

99

24.5

nd

20

130

10

140

ns

ns

20

100

nd

ns

8.2

19.1

nd

37.8

ns

14.2

nd

80

00

0 0

0 5

00

00

10

0

0 0

20.8

0

0 53

.3

0 0

0 10

0 0

0 0

0 0

112

40

8 0

100

7 4 3 6 7 7 7 5 6 4 1 2 3

A. C

attri

jsse

(pe

rs. c

omm

.);

de Jo

ng &

Meu

lste

e (1

989)

; de

Jong

etal

. (19

94);

de

Jong

&va

n de

r Plu

ijm

(199

4); M

eije

r &

Waa

rden

- bu

rg (

1994

); va

n St

rale

n &

D

ijkem

a (1

994)

D

avid

son

etal

. (19

91);

I. C

olcl

ough

(pe

rs. c

omm

.) (E

A, T

ham

es)

S. D

uham

el (

pers

. com

m.)

S. D

uham

el (

pers

. com

m.)

J. M

arch

and

(per

s. co

mm

.);

Ano

nym

ous

(198

9); M

igni

ot

& L

e H

ir (1

994)

J.

Reb

elo

(per

s. co

mm

.) P.

Alm

ada

(per

s. co

mm

.) P.

Alm

ada

(per

s. co

mm

.) P.

Alm

ada

(per

s. co

mm

.) P.

Dra

ke (

pers

. com

m.)

C. F

erna

ndez

-Del

gado

(pe

rs.

com

m.)

0, ha

bita

t not

ava

ilabl

e.

ns, h

abita

t not

sign

ifica

nt

nd. n

o da

ta.


N

A

'b

7 Gudrande

Nantes

Pornic -r.i 10 km

Fig. 2.9 The Loire estuary, France.

systems in this region. The diversity of fish habitats, and thus the number of niches, within an estuarine system is one of the factors likely to determine the biodiversity of the fish community. A second factor is the area of habitat available (Wootton, 1990).

The most extensive and widely distributed of the nine identified fish habitats was habitat 6 (subtidal soft substratum) in all three biogeographic regions, accounting for over 50% of the total surface area of the selected estuarine systems (Table 2.4). The contribution of habitat 6 in the Skagerrak/Baltic and Mediterranean regions increased to over 70% of the total because of the reduced influence of tides and thus the relatively limited extent of intertidal areas. The estuarine systems with the greatest areas of subtidal habitat were also the largest systems in terms of total surface area (Weser, Elbe, Ems-Dollard, Oosterschelde, Westerschelde in the BoreaVAtlantic; OderhaffBtettin lagoon in the Baltic and Messolonghi Lagoon in the Mediterranean).

Habitat 4 (intertidal soft substratum) was the second most extensive habitat, accounting for almost 30% of the total surface area of the selected estuarine systems in the Boreal/ Atlantic region. This habitat was absent from the other biogeographical regions due to the reduced influence of tides. The greatest extent of intertidal soft sediment was found in the large estuaries, in particular the Elbe, Weser and Ems-Dollard.

Habitat 3 (saltmarsh) accounted for more than 10% of the total surface area of the selected systems in the Mediterranean as a result of the wide expanse of saltmarsh in the Ebro delta, Spain. This habitat was also represented in the BoreaVAtlantic region with greatest areas in the Oosterschelde, Westerschelde, Tagus and Ems-Dollard. Saltmarsh was not found in any of the selected systems in the Skagerrak/Baltic region.

Habitat 7 (subtidal hard substrata) accountedfor more than 10% of the totalsurface area of the selected systems in the Mediterranean due to the large area (40 km2) in the Messolonghi


N

A

B a y of Biscay

GmMes la Tremblade

St-Laurent-Medoc 10 km

St-Andrk-de-Cubzac

Bordeaux

Fig. 2.10 The Gironde estuary, France.

lagoon, Greece. This habitat was also present in all selected systems in the Skagerrak/Baltic region with the greatest area (27.8 km2) in the OderhaffBtettin lagoon. Subtidal hard substrata were present at some of the selected sites in the BoreaVAtlantic region, but the areas were very small or not significant reflecting the soft sedimentary nature of the estuaries.

Habitat 8 (subtidal seagrass) accounted for more than 10% of the total surface area of the selected systems in the Skagerrak/Baltic region because of the large area (92.5 km2) in the OderhaffBtettin lagoon. Subtidal seagrass was also present in selected systems in the other regions with significant areas in the Bay of Cadiz, Spain and the Ria de Aveiro in Portugal (BoreaVAtlantic region).

The remaining four habitat types (1, Tidal freshwater; 2, Reed beds; 5, Intertidal hard substrata; and 9, Biogenic reefs) contributed less than 5% of the total surface area of the selected systems in all regions.

30 Fishes in Estuaries

N

pI U G A

Fig. 2.1 1 The Ria de Aveiro, Portugal (highlighting sampling stations as cited within text)


I i Ericeira

ATLANTIC OCEAN

10 kin

Fig. 2.12 The Tagus estuary, Portugal

Habitat 1 (Tidal freshwater) was present only in the BoreaVAtlantic region as estuarine systems in this region are significantly influenced by semi-diurnal tides. Within this region, the area of tidal freshwater habitat was greatest in the Weser, Elbe, Ems-Dollard, Wester- schelde, Loire and Tagus estuarine systems.

Habitat 2 (Reed beds) was present in both the BoreaVAtlantic and Skagerrak/Baltic regions. In the BoreaVAtlantic region, this habitat was present in eight of the estuarine systems as a small area although, in the Weser and Elbe systems, a significant area of reed bed (10.8 and 19 km2, respectively) was recorded. In the Skagerrak/Baltic region, significant areas were recorded in the Darss-Zingster Boden (15.2 km2) and the OderhaffEtettin lagoon (19 km2). The often fringing nature of this habitat dictated that it is difficult accurately to quantify its area.

Habitat 5 (Intertidal hard substrata) was restricted to the BoreaVAtlantic region since only estuarine systems in this region had a significant intertidal zone. This habitat was found in 12 estuarine systems, but the areas were very small.


N

Sanldcar de Barrameda i / 0 Jer6z de la Frontera

0 El Puerto de Santa Marid Bay of CBdiz

SanFernando \

10 km

Fig. 2.13 The Bay of Cadiz, Spain

Habitat 9 (Biogenic reefs) was found in the BorealIAtlantic and SkagerrakIBaltic regions. In the BorealIAtlantic region, this habitat was found in seven of the selected systems with the greatest areas in the Oosterschelde and in the Ria de Aveiro, Tagus and Mira systems in Portugal. In the Oosterschelde, the biogenic reefs comprise oyster and mussel beds. In the SkagerrakIBaltic region, this habitat was found in three of the five selected systems, with the greatest area (22.7 km2) in the OderhaffIStettin estuary.

Habitat Use by Fishes in Estuaries 33

N

A 0

LosPalacios

Province of Seville

Brazo de la Torre 4 n 4 Province of Huelva 1 //

0 Trebujena

Province of Ccfdiz

Sanlhcar de Barrameda

ATLANTIC OCEAN

20km

0 Leinija

Fig. 2.14 The Guadalquivir estuary, Spain

2.4 Use of habitats by fish in selected European estuarine systems

2.4.1 Fish species habitat use

2.4.1.1 Data treatment

When evaluating fish habitat use in estuaries, consideration has been given to four different functions that estuarine habitats may fulfil for fish. Habitats can serve as spawning grounds,


nursery areas, feeding grounds, and as pathways in diadromous migrations. A further use of habitats as refugia can be included here, although this is less straightforward (see below). To deposit or release their eggs and to mate, fish may select specific areas because of the environmental characteristics that optimise the survival of the eggs or of the early larval stages. Estuarine resident species use estuarine habitats to spawn, while catadromous and many marine species return to the sea or to polyhaline waters and anadromous species move into freshwater habitats for spawning. In some areas, a large proportion of the freshwater species uses the estuary for spawning.

Sediment characteristics, currents, water depth and vegetation may all have an impact on the survival or retention of the earliest life stages in an estuary and thus determine the likeli- hood that a fish species will spawn in a habitat. Juveniles migrate either passively or actively (some may use selective tidal stream transport) to estuarine nursery grounds and concentrate in specific habitats to spend their early lives. Nurseries are defined as areas where juveniles aggregate, are spatially or temporally separated from the adults, and where their survival is enhanced through better feeding conditions, optimal growth andor refuge opportunities. Recruitment to the adult or subadult populations follows the emigration from these nurseries after attaining a well-defined length class.

It is very difficult to define a refuge as a habitat usage, even though its understanding here is straightforward and mostly seen as a refugium from predation. It is similarly not possible to quantify the refuge value in the field especially where the fish are also feeding in a habitat. Adult fish may also seek predation refuge, yet adult andjuvenile fish occupy different habitats in most cases. To distinguish between these interpretations, a refuge function is defined as an integral part of the nursery function of a habitat here and is not considered as a separate usage.

Adult fish often perform feeding migrations into habitats where they preferably or exclusively forage. These feeding migrations can be based on a tidal, a diurnal or on a seasonal basis. Diadromy obliges fish to migrate between marine waters and brackish or freshwater areas for spawning (McDowall, 1988). The only path that diadromous fish can follow to reach their spawning grounds is through the estuary, and in doing so they may use some habitats but not others ~ thus, the terms used may merely imply occurrence in a habitat.

The definitions used here when analysing the information on these four habitat functions within the selected 26 estuaries, are:

spawning: the presence of ripe adults and the production of eggs; nursery: concentration ofjuvenile stages which are feeding and growing;

0 feeding: habitats used by adults as a feeding ground; and 0 diadromy: use of the estuary or a habitat as a migration route for spawning.

In order to consider the importance of estuaries for fish but also in terms of human use, the occurrence of commercial species is considered here. A commercial species is here defined as one that is subject to a contemporary local or regional fishery at some point in its life, whether as a target species or a bycatch which is landed.

For each estuary, a speciedhabitat list has been constructed, indicating habitat utilisation and function (spawning, nursery, feeding and diadromy) for each fish species being recorded in that estuary. So, for each estuary the occurrence of all species in habitats 1 to 9 is given,


Table 2.5 CA = Catadromous species: 1- 9 = Habitat number.)

Examples of speciesihabitat classifications. (FW = freshwater species: ER = Estuarine resident species:

Species Ecological guild Habitat use

Spawning Nursery Feeding Diadromy

Abrarnis brarna FW 1 Agonus cataphractus ER 4 , 6 (4, 6) (4, 6) Anguilla anguilla CA 1 , 3 , 4 , 6 1,(6) (1 ,3 ,4 ,6 )

together with information on function for each habitat. Table 2.5 illustrates how this was car- ried out (Appendices la - x provide habitat use data for each estuary), and the guild description is based on local or published knowledge of a species (e.g. Elliott & Dewailly, 1995).

An illustration of this is to consider a hypothetical estuary where only five habitats exist: a tidal freshwater area; salt marshes; intertidal soft substratum; subtidal soft substratum; and reed beds. The latter habitat has never been investigated for fish, so no data or information are available on the occurrence of fish there. For the remaining habitats, the table was completed as follows: Abramis brama feeds in habitat 1, the tidal freshwater part of that estuary, Agonus cataphractusspawns in habitats 4 and 6 (intertidal and subtidal soft substratum), uses these habitats as a nursery and the adults also feed there, and Anguillaanguillauses the salt marshes (habitat 3) and the other habitats as pathways in the catadromous migrations, feeds in habitats 1 and 6 , and has its nursery in those four habitats. The information collected was mainly based upon general knowledge of the ecology of the different species or on the experience of the scientists studying the estuarine populations. However, some of the information was reinforced by scientific data of occurrence of the different life stages of fish species in the specified habitats, on data of the feeding ecology of the species, and on data of egg and larval surveys. In order to distinguish between these two qualitatively different types of information, the habitat use information that was supported by hard data is given in brackets in the table. For example, Agonus cataphractus is known to be an estuarine resident species, its juveniles occur in the estuary and thus the species most likely spawns in this hypothetical estuary. Despite the latter, it is possible that no data can prove the occurrence of their eggs or of the ripe adults. For this species, spawning is assumed to occur in habitats 4 and 6. Otherwise, stomach analyses have indicated that adult fish feed and that it is likely thatjuvenile individuals grow and feed in those habitats. The proportion of ' known data' versus the total amount of available information was used to identify the gaps in knowledge. Table 2.6 summarises the information of all speciedhabitat lists and the total number of species found in each habitat is given, together with the contribution of commercial species for each estuary. The percentage of known data for each of the selected estuarine systems is also presented. The number of commercial species for each of the selected estuaries is shown in Table 2.7.

2.4.1.2 Proportion of kno wn data

In general, there was a north- south gradient in research effort or knowledge about habitat use of fish for the selected sites (Table 2.6). With a regional average of 73% of ' known data', the estuarine sites studied in the Baltic and Skagerrak seem to be the best studied areas. On average, about 33% of the information of the Boreal/NW Atlantic region is supported by studies

Tab

le 2

.6

Num

ber

of f

ish

spec

ies

in e

ach

habi

tat t

ype,

tota

l num

ber

of s

peci

es a

nd c

omm

erci

al s

peci

es, a

nd th

e pe

rcen

tage

of

know

n da

ta f

or th

e se

lect

ed e

stua

ries

. Hab

itat t

ypes

: 1,

T

idal

fres

hwat

er: 2

, Ree

d be

ds: 3

, Sal

tmar

sh: 4

, Int

ertid

al s

oft s

ubst

ratu

m: 5

, Int

ertid

al h

ard

subs

trat

um: 6

, Sub

tidal

sof

t sub

stra

tum

: 7, S

ubtid

al h

ard

subs

trat

um: 8

, Sub

tidal

seag

rass

bed

s:

9, B

ioge

nicr

eefs

.

No.

of f

ish

spec

ies

foun

d in

eac

h ha

bita

t typ

e G

eogr

aphi

c E

stua

rine

T

otal

no.

T

otal

no.

of

% c

omm

erci

al

regi

on

Lat

itude

sy

stem

1

2 3

4 5

6 7

8 9

ofsp

ecie

s co

mm

erci

alsp

ecie

s sp

ecie

s %

kno

wn

data

Bal

tic/

60

Skag

erra

k 58

58

54

54

NW

Atla

ntic

/ 53

B

orea

l 52

52

53

56

56

54

51

53

50

NW

ha

nd

G

ota

Riv

er

Gul

lmar

sfjo

rd

Dar

ss Z

ings

ter

Ode

rhaf

fiSt

ettin

Wes

er &

Elb

e W

este

rsch

elde

O

oste

rsch

elde

E

ms-

Dol

lard

L

och

Etiv

e Fo

rth

Hum

ber

Tha

mes

M

erse

y S

omm

e

9 10

13

40

24

18

?

6?

8?

27

24

5

10 3

? ~~

~~

~~

~~

~~

32

4

? 57

?

? 15

3

19

7

14

24

13

11

11

21

5 33

?

? 26

?

5 28

?

41

16

21

28

29

30

28

27

27

34

14

28

39

19

32

61

43

65

? ?

39

8 47

12

2

42

63

108 57

~~

4 4 3 3 60 ? ? ? ?

44

19

53

19

50

20

44

19

50

25

78

15

56

19

74

20

53

15

50

0 45

19

85

13

11

0 16

69

8

28

10

43

36

40

43

50

19

34

27

28 0 42

15

15

12

36

90

96

96

45

40

50

38

13

25

65

41

67

11

74

30

49

47

36

36

41

39

39

41

Med

iterr

anea

n 38

41

Mea

n

Ove

rall

mea

n

Occ

urre

nce

of h

abit

at

Sein

e L

oire

B

ay o

f Cad

iz

Gua

dalq

uivi

r R

ia d

e A

veir

o O

bido

s T

agus

M

ira

10

3 4

28

2

49

?-

?

79

20

9 4

?2

3?

3

1?

~

? 46

25

?

? 37

39

?

40

? 29

?

53

36

30

32

18

4 8

13

15

15

26

~ 18

16

55

26

2

~ ?

18

2 45

26

45

29

12

2

? 11

?

73

30

4 10

84

51

8

1 2

13

~ 44

~

37

10

61

5

~~

Mes

solo

nghi

1

6 7

-

1 45

47

36

7

62

46

Ebr

o ?

-

12

24

16

Bal

tic

10.7

~

~ 34

.0 2

1.0

27.6

3.

5 48

.2

20.4

B

orea

liN

WA

tlan

tic

13.0

6.

7 11

.5

24.3

5.

5 50

.8 1

9.0

18.0

24

.0

61.3

19

.1

Med

iterr

anea

n 1.

0 6.

0 7.

0 1.

0 28

.5 4

7.0

36.0

7.

0 43

.0

31.0

12.1

7.

7 11

.1

25.0

4.

8 45

.4 2

2.8

24.0

13

.0

57.2

20

.4

16

15

16

17

16

25

13

12

15

25

54

68

56

47

64

61 8 74

67

42.4

33

.9

70.5

38.6

20 9 25 0 11

41

24

36 0 27

73.4

32

.2

13.5

39.0

, hab

itat n

ot a

vaila

ble.

?,

hab

itat n

ot in

vest

igat

ed


Table 2.7 Number of commercial fish speciesrecorded in each habitat type for each selected estuary. Habitat types: 1, Tidal freshwater: 2, Reedbeds: 3, Saltmarsh: 4, Intertidal softsubstratum: 5, Intertidal hardsubstratum: 6, Subtidal soft substratum: 7, Subtidal hard substratum: 8, Subtidal seagrass beds: 9 , Biogenic reefs.

No. of commercial fish species found in each habitat type

Geographic Estuarine Total no. region Latitude system 1 2 3 4 5 6 7 8 9 ofspecies

Baltic/ 60 Skagerrak 58

58 54 54

Atlantic/ 53 Boreal 52

52 53 56 56 54 51 53 50 49 47 36 36 41 39 39 41

Mediterranean 38 41

Mean

Overall mean

19 10 7 ~ 19 18 9 9 0 19 19 9 8 0 20 15 6 11 3 19

OderhaffiStettin ~ 5 ~ ~ ~ 20 12 14 3 25

NWAland - 3 ~~~

Gota River Gullmarsfjord ~~ ~ ~~

Darss Zingster - 5 ~~~

~~ ~ ~~

Weser & Elbe Westerschelde Oosterschelde Ems- Dollard Loch Etive Forth Humber Thames Mersey Somme Seine Loire Bay of Cadiz Guadalquivir Ria de Aveiro Obidos Tagus Mira

7 2 ~ 7 1 15 ~ ~ ~ 15 3 ? 5 1 0 - 1 8 - ~ ~ 19

~~ ? 20 ? 20 ? ? 20 20 3 ? ? 8 2 14 1 ~ ? 15

~~ 0 0 0 0 0 - 0 4 ? 8 8 - 19 ~ ~ ? 19 2 7 - 13 ~ ~ ~ 13 3 5 5 1 6 ? 16 ~ ~ ? 16 1 - ? 7 ? 8 - - ? 8 l ? 2 1 0 ? ~ ~ ~ ~ 10 1 0 2 1 1 ? 2 0 ? ~ ? 20 2 0 ? 1 5 ? 2 4 ? ~ ? 25 ? ? 21 22 ? 24 ? 14 ? 27

18 ~ ~ ~ 18 6 8 1 0 7 18 ~ 9 9 26

2 - ? 1 3 0 2 9 1 6 ~ ~ 29 5 2 ~ 6 ? 4 9 1 9 2 5 51 1 0 0 1 - 5 - 0 0 5

~ ~~ 13

Messolonghi 1 3 5 - 1 4 6 4 0 3 0 6 46 Ebro ~~ ? ~~ 1 6 - - - 16

Baltic 4.3 ~ ~ ~ 18.2 9.2 9.8 1.5 20.4 BorealDJWAtlantic 3.4 2.1 6.4 10.0 2.0 17.9 9.0 5.0 8.5 18.3 Mediterranean 1.0 3.0 5.0 ~ 1.0 31.0 40.0 30.0 6.0 31.0

3.3 2.8 6.2 10.1 1.8 19.3 12.2 9.5 5.1 20.0

performed in the respective sites. There also exists a clear difference between the northern and southern sites in the BoreaVNWAtlantic region. The systems of the Iberian peninsular are supported by data collected in the respective sites in about 23%, on average. In contrast, the northern sites are on average confirmed in about 40%. The Mediterranean sites have the lowest (0- 27%) amount of ' known data' such that in southern Europe, further effort is required to study the importance of estuarine areas for fish. This consideration of effort is important particularly with regard to the number of species encountered. In an ecological study, further sampling effort will take further species, especially the increasingly rare species (see below).


2.4.2 Species richness and distribution between habitats

2.4.2.1 All fish species

The species richness and distribution of fish over the nine habitats, throughout each region and estuary is displayed in Table 2.6 and Fig. 2.15. All fish recorded have been included for each estuary. The number of fish species found within each estuarine system ranges from the relatively poor Ebro (n= 24) to the extremely richThames estuary (n= 110).

The average number of species using estuarine habitats during one or more lifestages is 57 for the 26 estuarine systems under consideration. As was also shown by Elliott and Dewailly (1995), there was no consistent latitudinal gradient of species richness. With a regional average of 48.2 species, the BaltidSkagerrak estuarine systems appear to be less rich in species, whilst the estuarine systems of the Boreal/NW Atlantic region hold on average 61.3 fish species. A considerable difference was seen to exist for the two Mediterranean sites. In the Ebro delta only 24 species have been recorded, whereas 62 species were observed in the Mes- solonghi lagoon. However, this latter site might be more representative of Mediterranean estuarine systems, as only one habitat was represented in the Ebro estuary. The difference in intensity with which both sites were studied is, however, unknown. It is suggested that any latitudinal or other trend is obscured by the influence of the intensity of study on species richness. It must be noted that during the categorisation of regional areas (AtlantidBoreal, Mediterranean and BaltidSkagerrak) , relatively broad generalisations and assumptions are made when not considering differences in effort among investigations. For the selected sites, the number of commercial species per estuary tends to increase towards the south (Table 2.7). The contribution of commercial species was generally more than 50% for southern

50 LI

l0lLhh+ 0

n

7 8 9

Fig. 2.15 Average number of fish species per habitat in selected estuaries in three European regions.


Europe, while the commercial component of the northern areas was less than 50% of the total estuarine fish fauna. This feature may reflect the more diverse fish-based diets of southern Europe.

There was no relationship between habitat diversity and the number of species recorded (Fig. 2.16). The number of species recorded for any one estuary is more likely to be related to the efforts made and the methods used to study the fauna, and also to the degree of pollution or habitat degradation of the individual estuarine systems. Habitat diversity is only one factor determining the species richness of an estuary. The complexity of the individual habitats (which is not accounted for here) also plays an important role in determining the number of species occurring in a habitat. Similarly, the complexity of adjoining marine and freshwater areas is not considered here, but will have an influence on an estuarine habitat’s diversity.

Habitat 1, the tidal freshwater habitat, occurs exclusively in the tidal estuaries of Boreal/ NW Atlantic region. For the German estuaries, the Elbe and the Weser, this habitat is extremely important, being the second most species-rich habitat type (Table 2.6). More than 50% of all species within these two estuaries use this habitat type during at least one life stage. The remaining areas possessing tidal freshwater are generally used by 10- 30% of the estuarine fish populations. In Messolonghi lagoon, habitats 1, 2 and 3 are virtually lacking, but some parts of the lagoon exhibit characteristics of these habitats (e.g. freshwater springs). Some species tend to be strongly associated with these habitats and are therefore included in the analysis.

Reed beds (habitat 2) and saltmarsh (habitat 3) will in some cases be associated with one another, or in other cases be subparts of habitat 1. The very nature of these habitats makes sampling difficult (which means they are difficult to study), as is reflected in the amount of information available. In the reed beds of the selected Baltic areas, only about 20% of the fish

120

100

LI 8 P g 40 z

20

0 0 1 2 3 4 5 6 7 8 9 10

Number of habitats

Fig. 2.16 estuaries.

Relationship between the number of fish species and the number of habitats in selected European


fauna could be found (Table 2.6), but a higher proportion of the fish fauna occurred in reed beds of the Weser and Elbe. Where data are available from other sites it can be seen that reed beds are relatively poor in fish species. In the BoreaVNW Atlantic region, there are regions of tidal salt marshes, and these are important as a habitat for a number of estuarine fish (Drake & Arias, 1991; Cattrijsse et al., 1994). The proportion of fish occurring in these intertidal vegetated habitats was on average 18% of the total number of species, though in the Bay of Cadiz the proportion increased to almost 60%. In the Mediterranean, a non-tidal type of saltmarsh exists, but little information was available on its importance as a fish habitat.

As for the tidal freshwater habitat, the intertidal soft substratum (habitat 4) occurs exclusively in the tidally dominated BoreaVNW Atlantic region. Information on its use by fish has been collected in all sites, and it displayed a high overall mean of 38.6% of the fish species using the habitat (Table 2.6).

The subtidal soft substratum (habitat 6) had a high and constant number of fish throughout all regions (Table 2.6). In this habitat, the overall average of 45 species was the highest in any habitat and in all estuaries at least half of the fish fauna used the subtidal soft substratum during at least one life stage. Due to the frequency with which this habitat type is sampled, the probability of data bias must be considered. Indeed, the intertidal and subtidal soft substratum are the only habitats to contain data from all sites. Whilst the intertidal soft substratum was exclusive for the Boreal/NW Atlantic region, the subtidal equivalent of this habitat was present in all 26 estuarine systems. All other habitats occurred in only about half of the sites.

The intertidal (habitat 5) and the subtidal hard substrata (habitat 7) are habitats that im- pose many constraints on sampling. Based on the data presented here, the intertidal hard substratum had the lowest species richness, with an average of only five species (Table 2.6). In contrast, the subtidal hard substratum was one of the most species-rich habitat types with a overall mean of 23 species that comprised approximately 38% of the present fish fauna.

Subtidal sea grass beds (habitat 8) are also species-rich environments, but are generally absent from the selected estuaries along the North Sea and Atlantic coasts. Only in the Oost- erschelde, Bay of Cadiz and Ria de Aveiro has this habitat been investigated, though subtidal sea grass beds have largely disappeared and are still declining there. The BaltidSkagerrak and the Mediterranean region still have extensive seagrass beds that hold many fish; in fact, the average species richness for the two regions was estimated at 32 species, making this habitat the second most important estuarine habitat in terms of fish diversity.

Habitat 9 (Biogenic reefs) is the least studied of the nine habitats yet, as shown by the data of the Oosterschelde and the Ria de Aveiro, a large part of the present ichthyofauna may use this habitat in some estuaries.

2.4.2.2 Commercial species

The proportion of commercial fish species utilising estuarine systems at some stage of their life cycle accounted for approximately 38.6% (overall mean) of the total present (Table 2.6). The average proportions of commercial species found in the BaltidSkagerrak was 42%, while in the BoreaVNW Atlantic region it was 33%. A higher percentage of commercial fish was recorded in the southern estuaries averaging, on the Iberian peninsular and in the Medi- terranean estuaries, 48% and 70%, respectively. Among the northern systems of the BoreaV NW Atlantic region, the Forth and the Loire estuaries had a high proportion of commercial


species, although these estuaries had a relatively low total number of species. A similar pattern was observed for the Ebro in the Mediterranean, where few species have been recorded but a high proportion was commercially fished.

The distribution of the commercial species over the nine habitats revealed a very similar pattern as the distribution of the total fish communities. Table 2.7 shows that nearly all commercial fish species in all selected estuarine areas use habitat 6 during their life, while on average more than half of the commercial species use habitat 4 during at least one life stage.

2.4.3 Habitat utilisation

The four habitat uses are ranked by average number of fish species as follows: Feeding 44.8 species >Nursery 33.6 species > Spawning 13.6 species > Diadromy 4.5 species (Table 2.8). This ranking is evident in the three regions considered, within each individual estuarine system, and even within each of the nine habitats with only slight deviations from the above ranking (Fig. 2.17). In the southern Boreal/NW Atlantic estuarine areas (Loire to Cadiz) , the ranking of feeding and nursery is reversed. When considering the percentage of the total number of species recorded as undertaking each of the four habitat uses, the ranking order was: Feeding 75.8 >Nursery 63.1 >Spawning 24.3 > Diadromy 8.9% of the species, respectively (Table 2.8). Since only a few species exhibit diadromy, it is not surprising that this habitat use ranks the lowest. Also, the low percentage of species using estuaries for spawning is not unexpected, given that only a few marine species utilise estuaries for this purpose. It is, therefore, mainly true estuarine resident species that will spawn inside estuarine areas, although in some areas the contribution from fresh water species could be considerable.

40

30

LI 8 P

0 g 20

%

L 10

.- u 8

8 OD m

d

0 1 2 3 4 5

Habitat 6

L 7 c a i 9

Fig. 2.17 Habitat utilisation of fish among nine habitats in 26 selected estuaries.


Table 2.8 N = Nursery: F = Feeding: D = Diadromy.

Number and percentage of species by habitat usage per selected estuary. Key to habitats: S = Spawning:

No. of species recorded per estuary for each habitat usage habitat usage

Percentage of species recorded per estuary for each

S N F D Total S N F D

NWAland 35 30 40 0 44 N W h a n d 79.5 68.2 90.9 0.0 Gota River 25 39 51 3 53 GotaRiver 47.2 73.6 96.2 5.7 Gullmarsfjord 24 39 48 3 50 Gullmarsfjord 48.0 78.0 96.0 6.0 Darss-Zingster 24 28 41 3 44 Darss-Zingster 54.5 63.6 93.2 6.8 OderhaffiStettin 23 27 44 10 50 OderhaffiStettin 46.0 54.0 88.0 20.0

Weser & Elbe Westerschelde Oosterschelde Ems-Dollard Loch Etive Forth Humber Thames Mersey Somme Seine Loire Bay of Cadiz Guadalquivir Ria de Aveiro Obidos Tagus Mira

39 14 10 9

11 9 7 7 2 3 3 7 1 6

11 11 12 20

40 28 34 19 27 20 27 21 18 26 41 39 52 27 24 37 76 58

76 56 70 32 47 43 84

107 55 16 50 15 31 17 29 26 28 33

7 6 0 5 3 4

12 5 4 4

11 10 0 6

11 2 3 0

78 56 74 53 50 45 85

110 69 28 79 46 53 32 55 45 84 61


50.0 51.3 97.4 9.0 25.0 50.0 100 10.7 13.5 45.9 94.6 0.0 17.0 35.8 60.4 9.4 22.0 54.0 94.0 6.0 20.0 44.0 95.6 8.9 8.2 31.8 98.8 14.1 6.4 19.1 97.3 4.5 2.9 26.1 79.7 5.8

10.7 92.8 57.1 14.3 3.8 51.9 63.3 13.9

15.2 84.8 32.6 21.7 1.9 98.1 58.5 0.0

18.8 84.4 53.1 18.8 20.0 43.6 52.7 20.0 24.4 82.2 57.8 4.4 14.3 90.5 33.3 3.6 32.8 95.1 54.1 0.0

Messolonghi 11 52 58 1 62 Messolonghi 17.7 83.9 93.5 1.6 Ebro 1 10 4 3 12 Ebro 8.3 83.3 33.3 25.0

Mean Mean Baltic 26.2 32.6 44.8 3.8 Baltic 55.1 67.5 92.9 7.7 NorthernEurope 10.5 26.3 57.8 5.5 NorthernEurope 16.3 45.7 85.3 8.8 SouthernEurope 9.7 44.7 25.6 4.6 SouthernEurope 18.2 82.7 48.9 9.8 BorealDJWAtlantic 11.0 34.2 46.8 4.9 BorealDJWAtlantic 18.3 58.6 72.5 8.6 Mediterranean 6.0 31.0 31.0 2.0 Mediterranean 13.0 83.6 63.4 13.3

Overall mean 13.6 33.6 44.8 4.5 Overall mean 24.3 63.1 75.8 8.9

Since estuaries are generally less species-rich than the adjacent marine or freshwater systems, the number of species utilising estuarine areas as spawning sites will be limited. In general, more fish species are found using estuaries as adult feeding grounds than as nursery areas. This trend is clearly reversed for the southern BorealLVWAtlantic estuaries where more species are found using the available habitats as nurseries than as adult feeding grounds (Table 2.8).

2.4.4 Habitat importance

In an attempt to classify the nine habitats according to their relative importance, it is clear that


the relative distribution of fish species between all estuarine habitat types is quite similar. This is shown in Table 2.9, which shows the average species richness per habitat type for the three regions, and also the overall average species number for each of the four habitat uses in each of the nine habitats. The average species richness within habitat types throughout the 26 selected European estuarine systems ranks as follows: Subtidal soft substratum (6) > Intertidal soft substratum (4), Subtidal seagrass beds (8), Subtidal hard substratum (7) >Tidal freshwater (1), Biogenic reefs (9), Saltmarsh (3) > Reed beds (2) >Intertidal hard substratum (5). The average percentage of the total species present is similar: Subtidal soft substratum (6) > Subtidal seagrass beds (8), Subtidal hard substratum (7), Intertidal soft substratum (4), > Saltmarsh (3), Biogenic reefs (9), Tidal freshwater (1) > Reed beds (2) > Intertidal hard substratum (5). Only small deviations appear when making this ranking for the three regions.

The main difference exists between the micro-tidal estuarine systems of the Baltic, Sk- agerrak and the Mediterranean regions and the tidal estuaries of the BoreaVAtlantic region. In the former the intertidal areas, habitats 1 , 3 and 4 are largely or even completely absent. These habitats are important in the latter region, however, especially the intertidal soft substratum (habitat 4). Similarly, seagrass beds and subtidal hard substratum, which are less common in the estuarine systems of the BoreaVAtlantic region, are of higher importance in the Baltic and Mediterranean. Habitat 9 also appears to be more important in the Boreal/NW Atlantic region than in the BaltidSkagerrak.

There is large individual variation of habitat utilisation between the individual estuarine systems, mainly due to the extent of each habitat although lack of data for certain habitat types and regions may seriously undermine the general conclusions on habitat use. For example, within the Forth estuary (UK) extensive reed beds (habitat 2) exist. However, a complete absence of data concerning the species using this habitat reduces the value of generalisations based wholly on fish species number.

In order to evaluate further the importance of estuarine biotopes as habitats for fish, a Habitat Utilisation Index (HUI) was developed here as a summation of the various life stages of fishutilising a single habitat, divided by the amount of available data (sites) for that habitat (Table 2.10). This index approximates the overlap between fish life stages and the utilisation of each habitat type. Here, the degree of total habitat utilisation throughout the European estuarine systems, and thus the relative importance and diverse usage of each habitat type, can be identified. In the previous evaluation of habitat importance, merely the average number of fish present in a particular habitat was considered. Since a fish species may use a single habitat for several reasons (e.g. as nursery, as an adult feeding ground and in diadromous migrations) the utilisation of the nine habitats and thus the importance may, however, differ from the ranking based upon the average number of fish species. The HUI evaluates a habitat on basis of an average number of uses made by all fish species.

Using this index, the ranking of the nine habitats does not differ from that given above: Subtidal soft substratum (6) > Subtidal seagrass beds (8), Subtidal hard substratum (7), In- tertidal soft substratum (4) > Tidal freshwater (1), Biogenic reefs (9), Saltmarsh (3) > Reed beds (2) > Intertidal hard substratum (5).

To develop further the idea of habitat importance, the distribution of the commercial species over the different habitats (Table 2.7) and the utilisation of these habitats by the com- mercialspecies are also considered (Table 2.11). The average number of commercial species per habitat type changes the ranking of the nine habitats: Subtidalsoft substratum (6) > Subti-

Tab

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Table 2.10 Habitat utilisation index (HUI)

Habitadnumber

Tidal freshwater (1) Reed beds (2) Saltmarsh (3) Intertidal soft substratum (4) Intertidal hard substratum (5) Subtidal soft substratum (6) Subtidal hard substratum (7) Subtidal seagrass beds (8) Biogenic reefs (9)

HUI

23.1 15.5 19.3 37.6 9.0

69.7 43.3 46.5 20.7

~

dal hard substratum (7) > Intertidal soft substratum (4), Subtidal seagrass beds (8), Saltmarsh (3) > Biogenic reefs (9), Tidal freshwater (1), Reed beds (2) > Intertidal hard substratum (5). However, when considering the percentage of the total commercial species per habitat type the original ranking is almost restored: Subtidal soft substratum (6) > Intertidal soft substratum (4) > Subtidal hard substratum (7), Subtidal seagrass beds (8), Saltmarsh (3) > Biogenic reefs (9), Tidal freshwater (1), > Reed beds (2) > Intertidal hard substratum (5). The HUI using the commercial species instead of the total species number, gives the same ranking: Subtidal soft substratum (6) > Subtidal hard substratum (7), Subtidal seagrass beds (8), Intertidal soft substratum (4), > Saltmarsh (3), Biogenic reefs (9), Tidal freshwater (1), Reed beds (2) > Intertidal hard substratum (5).

These analyses illustrate the fact that habitat 6 is the most widely used habitat within the selected estuarine sites. Habitats 4 , 7 and 8 take a second place in each of the rankings, then habitats 1, 2 , 3 and 9 with habitat 5 always being ranked lowest. The deviations from this ranking are minor.

2.4.5 Ecological guilds

The occurrence of six ecological guilds in and among the selected estuarine sites is represented in Table 2.12 and Figs 2.18 (a) and (b). The six ecological guilds are: diadromous species (CA), freshwater species (FW), estuarine residents (ER), marine adventitious species (MA), marine juveniles migrants (MJ) and marine seasonal migrants (MS) (Elliott & De- wailly, 1995). No clear patterns exist amongst the three regions. Within the selected estuarine systems, great regional variation exists in the ecological guilds making up the estuarine fish assemblages. For example, in the Mediterranean Messolonghi lagoon, the fish populations are substantially influenced by an unusually high proportion (61%) of marine adventitious species. Similarly, the fish fauna of the three Baltic systems are characterised by a strong freshwater component. The more strongly tidal estuaries, Weser-Elbe, Ems-Dollard, Wester- schelde, Humber, Thames, Mersey, Loire and Tagus all possess a relatively large tidal freshwater influence, and therefore contain a clear freshwater component. In general, the marine seasonal migrants and the diadromous species are less well presented in the fish fauna of the selected areas. The guilds contributing most to the fish fauna of the sites considered are the true estuarine residents and the marine adventitious species. However, it is emphasised that these findings relate to number of species and that abundance of each guild would show

Tab

le 2.

1 1

F =

Fee

ding

: D =

Dia

drom

y.

Num

ber o

f com

mer

cial

fish

spec

ies u

sing

eac

h of

the

nine

hab

itats

for t

he fo

ur c

ateg

orie

s of

hab

itat u

tilis

atio

n pe

r sel

ecte

d es

tuar

y. K

ey to

hab

itats

: S

= S

paw

ning

: N =

Nur

sery

:

Hab

itat 1

H

abita

t 2

Hab

itat

3

Hab

itat

4 H

abit

at 5

Hab

itat

6

Hab

itat 7

H

abit

at 8

H

abit

at 9

N

o. o

f E

stua

ry

S N

F

D

S N

F

D

S N

F

D

S N

F

D

S N

F

D

S N

F

D

S N

F

D

S N

F

D

S N

F

D

spec

ies

Wes

er &

Elb

e W

este

rsch

elde

O

oste

rsch

elde

E

ms-

Dol

lard

L

och

Etiv

e Fo

rth

Est

uaIy

H

umbe

r T

ham

es

Mer

sey

Som

me

Sein

e L

oire

B

ay o

f Cad

iz

Gua

dalq

uivi

r R

ia d

e A

veir

o O

bido

s T

agus

M

ira

44

67

11

10

--

--

14

73

00

10

48

0

2 3

1 ?

? ?

? 0

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1 1

10

6

2 ~

~ ~

~ 2

12

~~~~

~~~~

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? 1

12

19

0?

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11

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40

01

10

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0

00

00

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43

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00

80

00

80

--

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1

11

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00

60

00

60

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18

0

1 3

3 0

4 5

1 0

4 5

1 2

10

16

3

? ?

? ?

3 9

00

01

??

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07

71

??

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07

0

10

1?

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?0

20

00

94

1?

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~ 0

10

10

0 0

0 0

2 0

0 0

11

3

l?

??

? 0

18

11

0 6

0 0

0 O

? ?

?

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14

3

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03

32

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61

05

71

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0

31

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2.5

2.8

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7.7

8.8

0.7

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6.9

0.4

0.4

1.4

4.4

0.4

Bal

tic

0 0

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4.3

2.0

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2.6

6.8

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13.2

2.8

0

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8 1.

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0

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0 0

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0


Table 2.12 Number of fish species in each of six ecological guilds per selected estuary. Key: CA = Diadromous species: FW = Freshwater species: ER= Estuarineresident species: MA= Marine adventitious species: MJ = Marine juvenile migrant species: MS = Marine seasonal migrant species.

No. of species in each ecological guild Total no.

Estuary CA FW ER MA MJ MS ofspecies

NW h a n d Gota River Gullmarsfjord Darss-Zingster OderhaffiStettin


Messolonghi Ebro

Mean BalticiSkagerrak BorealiN. Atlantic Borealis. Atlantic BorealiNW Atlantic

3 4 4 6 9

9 7 9 7 4 7 9 8 6 4

11 10

1 7 7 4 5 5

1 3

5.2 7.4 5.6 6.7

17 3 1

20 29

33 13

1 3 1 0

18 21 13 0 9 8 0 2 4 0 4 1

3 2

14

16 15 13 10 8

12 12 16 13 14 10 15 19 12 7

15 4

12 9

15 9

12 17

10 3

3 17 19 3 1

8 8

25 9

22 19 27 40 19 6

24 6

18 4

13 11 34 19

38 0

3 9 9 3 2

10 11 14 13 7 7

12 13 12 7

14 13 22

6 10 16 20 14

5 2

12.4 8.6 5.2 10.2 13.1 18.8 10.9 2.7 11.1 15.0 14.4 7.3 12.4 17.3 12.7

2 5 4 2 1

6 5 9 8 2 2 4 9 7 4 6 5 0 4 6 5 9 5

5 2

2.8 5.6 4.9 5.9

Mediterranean 2.0 2.5 6.5 19.0 3.5 3.5

44 53 50 44 50

78 56 74 53 50 45 85

110 69 28 79 46 53 32 55 45 84 61

62 12

different patterns. For example, the abundance of marine adventitious species is likely to be very low whereas that for estuarine residents will be high.

2.5 Discussion

The analysis presented here, which may be regarded as a case study for studies of estuarine habitats in other geographical areas, indicates the value of adequate habitat information. It has been possible to obtain best estimates of the extent and distribution of nine fish habitats in 26 selected estuarine systems in Europe such that, in most estuarine systems, more than 95% of the total surface area was allocated to one of the nine fish habitats. The 26 selected estuarine systems have been categorised into three biogeographic regions (BoreaVAtlantic;


(b)

Fig. 2

100%

80%

60% u M

c)

40% L

k 20%

~

18

WCA BFW OER NMA WMJ HMS

100%

90%

80%

70%

u 60% M

c, 50%

40%

30%

20%

10%

0%

k

Baltic BoreaUNW Atlantic Mediterranean

Regions

1 WCA HFW OER NMA WMJ HMSI

(a) Representation of the ecological guilds in each of the selected estuaries. (b) Representation ofthe six ecological guilds for the three geographical regions. CA, diadromous species: FW, freshwater species: ER, estuarine residents: MA, marine adventitious species: MJ, marine juveniles migrants: MS, marine seasonal migrants.

BalticBkagerrak; Mediterranean) based on the influence of semi-diurnal tides and variations in salinity and temperature regimes.

The diversity of the nine fish habitats within the selected estuarine systems varied between one and eight and was on average, greater in the BoreaVAtlantic region due to the influence


of semi-diurnal tides. Subtidal soft substratum (habitat 6) was the most extensive and widely distributed habitat in the selected estuarine systems, accounting for more than 50% of the total surface area. Intertidal soft substratum (habitat 4) was the next most extensive habitat, accounting for almost 30% of the total surface area in the BoreaVAtlantic region, although this habitat was absent from the Skagerrak/Baltic and Mediterranean regions due to the restricted tidal ranges of these regions.

It is, however, apparent that the 26 European estuaries included here all differ in the extent and composition of their constituent habitats, with great variation both within and between the three regions, in the extent to which the nine fish habitats have been identified and their areas quantified. Similarly, the availability of data on the fish assemblages of these estuaries, and the use made of individual habitats by each fish species within the estuary, varies greatly between sites. Of the three European biogeographical regions for which estuaries are included here, only the BoreaVAtlantic has had any major quantification of the estuarine resource.

The number of fish habitats were generally more diverse in the selected estuaries from the BoreaVAtlantic region where, as already mentioned, they are typically subject to greater tidal action than those of the other regions (leading generally to a greater incidence of intertidal habitats in the estuaries of this region). The relative insignificance of tides in the BaltidSkagerrak and the Mediterranean influenced the extent to which to a single habitat, subtidal soft substratum, became the dominant in terms of surface area in these regions, and also meant that the second-most extensive habitat, intertidalsoft substratum, was restricted to the selected estuaries of the BoreaVAtlantic region. Only three other habitats, i.e. saltmarsh (habitat 3), subtidal hard substratum (habitat 7) and subtidal seagrass beds (habitat 8) accounted for more than 10% of the total surface area in any individual biogeographic region. Tidal freshwater (habitat 1), reed beds (habitat 2), intertidal hard substratum (habitat 5) and biogenic reefs (habitat 9) were minor habitats, accounting for less than 5% of the totalsurface area in all biogeographic regions.

The number of fish species recorded in a single estuary varied between 24 (Ebro) and 110 (Thames), with an average of 57 species across the 26 systems. Such values are easily biased by differences in the sampling effort and timescale of the study, making the distinction of differences between estuaries on the basis of differences in habitat diversity from the current datasets extremely problematic. There was great variability between regions in terms of research effort directed towards habitat use by fish in the selected estuarine systems. Similarly, the inclusion of freshwater areas, and thus fish species varied between estuarine systems. Such freshwater species will elevate the richness recorded, a feature shown, for example, by the Thames information. In general, however, the BaltidSkagerrak systems were most species-poor. Data from too few Mediterranean systems were available to permit further meaningful comparison of total species numbers between the three regions. In addition, in general, the proportion of commercial species per estuary increases from Northern to South- ern Europe, with approximately 38.6% of species (overall mean) recorded as commercial in Europe (Table 2.6).

In considering the use of the nine habitats by fish in the selected estuaries, four habitat- use functions were considered: (i) as spawning grounds; (ii) as nursery areas; (iii) as feeding grounds; and (iv) as pathways in diadromous (catadromous or anadromous) migrations. The selected BaltidSkagerrak estuaries have been subject to the most detailed investigations, and


.

the Mediterranean least. Of the four uses, feeding was the most frequent use overall, followed by use as a nursery and use for spawning. Only a few species were recorded as diadromous. This ranking also holds for the three regions individually and generally, even within the individual nine habitats. A further use of an area ~ as a refuge ~ has not been separated but is included as a component of the above four uses.

In terms of assessing the relative importance of the nine habitats, a HUI combined species richness and number of use functions for each species. The HUI ranked subtidal soft substratum as the most important habitat overall, followed by subtidalseagrass beds, subtidal hard substratum and intertidal soft substratum, to give a similar ranking, of these four habitats as the most important, to that based on species richness. The remaining five habitats scored as relatively unimportant, with intertidal hard substratum, recorded as only a minor constituent habitat of most of the selected 26 estuaries, scoring lowest in terms of habitat utilisation.

These analyses could be improved by obtaining more consistent data between sites, or by the inclusion of comparisons of abundance of each species. Furthermore, account should be taken of the total or relative areas of each habitat available at each site. In an attempt to compare the number and extent of habitats at each site with the number of species present, Fig. 2.19 shows the number of fish species recordedversus the percentage occurrence of each of the nine habitat types, for the AtlantidBoreal and BaltidSkagerrak regions. Too few data were available to include the Mediterranean region in the analysis. From this figure it is clear that the subtidal soft substrata habitat (number 6) is dominant in terms both of area and species diversity for bothregions. For theAtlantic/Boreal, the number of species decreases as the percentage occurrence of the habitat type diminishes, but for the BaltidSkagerrak other areas such as subtidal seagrass beds (habitat 8) or subtidal hard substrata (habitat 7), which may be present in relatively small areas, may have an equally high species diversity as the subtidal soft substrata habitat.

0 9 .6 .8

250 Q2

.E 2 200

4 9

*

c

$ 150

e .- m .- g 100 R *-

L 50

z 0

0 4

0 20 40 60 Percentage occurrence of the 9 habitat types

80

Fig. 2.19 each of the nine habitat types in the AtlanticiBoreal and BalticiSkagerrak.

Number of fish species recorded in each of the nine habitat types versus the percentage of occurrence of


2.6 References

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chapter 2 habitat use by fishes in estuaries and other ... · 1 2 fishes in estuaries 2.2.1.2...

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