relationships between recruitment and postrecruitment processes in lagoonal populations of two coral...

LJournal of Experimental Marine Biology and Ecology,213 (1997) 231–246

Relationships between recruitment and postrecruitmentprocesses in lagoonal populations of two coral reef fishes

*G.P. JonesDepartment of Marine Biology, James Cook University, Townsville 4811, Queensland, Australia

Received 6 March 1996; revised 9 October 1996; accepted 11 October 1996

Abstract

Pomacentrus amboinensis Bleeker and Dascyllus aruanus (L.) exhibit similar large scalepatterns in recruitment to One Tree lagoon, southern Great Barrier Reef. They recruit in greatestnumbers near the windward perimeter compared with the centre of the lagoon, and at theperimeter, recruit preferentially to deeper sites (4–8 m depth). This pattern was observed inrecruitment to standard coral heads of Pocillopora damicornis (L.), a preferred substratumtransplanted to a range of locations and depths. As substratum was held constant, spatial variationin recruitment was not determined by the availability of suitable habitat. Patterns in growth andmortality of the two fish species were monitored for juveniles transplanted to another set ofstandard coral heads, in order to determine whether post-recruitment processes would reinforce ordisrupt patterns established at the time of recruitment. The two species exhibited contrastingpatterns. Pomacentrus amboinensis exhibited faster growth and better survivorship at deeper sites,regardless of the location in the lagoon, reinforcing the initial pattern. Dascyllus aruanus exhibitedfaster growth at perimeter sites and significantly better survivorship at central sites, patterns thatwould tend to disrupt the initial pattern. Within locations, individuals exhibiting slower growthrates (Pomacentrus amboinensis) or smaller initial size (Dascyllus aruanus) had reduced lifeexpectancy. The results suggest that location selection by settlers of Pomacentrus amboinensisresults in improved growth and/or survival. This was not the case for Dascyllus aruanus, which isprobably more influenced by the presence of conspecifics. However, even for Pomacentrusamboinensis, large-scale patterns probably reflect constraints on habitat selection. Recruitment inPomacentrus amboinensis was consistently lower to sites immediately down current of other reefs,regardless of their position. I suggest that the absence of recruits of both species to central sitesindicates that this region is a recruitment-shadow. Juveniles settle in preferred habitat at the firstopportunity after entering the lagoon, leaving much of the preferred habitat in the centre of thelagoon unoccupied. Patterns are initially recruitment-determined, but either reinforcement ordisruption mean that the numerical abundance of adults cannot be predicted on the basis ofrecruitment data alone. 1997 Elsevier Science B.V.

*Corresponding author: Tel.: ( 1 61-77) 814-559; fax: ( 1 61-77) 251-570; e-mail: [email protected]

0022-0981/97/$17.00 1997 Elsevier Science B.V. All rights reservedPII S0022-0981( 96 )02763-3

232 G.P. Jones / J. Exp. Mar. Biol. Ecol. 213 (1997) 231 –246

Keywords: Coral reef; Damselfish; Dascyllus aruanus; Disruption; Growth; Mortality; Pomacen-trus amboinensis; Recruitment; Reinforcement

1. Introduction

Most organisms exhibit predictable spatial distributions that are associated withchanges in the biotic and physical structure of their habitat (Wiens and Rotenberry,1981; Bell et al., 1990; Sebens, 1990). This generalisation is particularly true for fishesassociated with complex reef habitats (Sale, 1980; Jones, 1988a; Holbrook et al., 1990,1994; Williams, 1991). On coral reefs, distinctive fish assemblages may be observed notonly over scales encompassing broad reef zones (e.g., reef slopes, crests and lagoons)(Russ, 1984; Galzin, 1987), but also in relation to patchiness in habitat compositionwithin these broader zones (Choat and Bellwood, 1985). Changes in abundance alongdepth gradients (Bouchon-Navaro and Bouchon, 1989), between exposed and shelteredreef slopes (Choat and Bellwood, 1985) or between different regions within coral reeflagoons (Bell and Galzin, 1984; Fowler, 1990) are well known, but the demographic andbehavioural factors responsible for these patterns are poorly understood.

Spatial distributions in marine communities may be generated initially by patterns inrecruitment, but these may also be modified by post-recruitment events (Gaines andRoughgarden, 1985; Gaines et al., 1985; Olafsson et al., 1994; Caley et al., 1996). Manyreef fishes appear to exhibit strong habitat selection at the time larvae first come incontact with the reef (Williams and Sale, 1981; Sale et al., 1984; Eckert, 1985; Carr,1989, 1991; Levin, 1991). This generates distinctive recruitment patterns that may play akey role in determining adult distributions (Jones, 1988a; Williams, 1991). I use the termrecruitment in the sense of the addition of new individuals to the adult habitat (Caley etal., 1996). However, predictable spatial differences in post-recruitment growth (Jones,1986; Pitcher, 1992), mortality (Aldenhoven, 1986; Booth and Beretta, 1994; Levin,1994) and habitat selection (McFarland et al., 1985; Shulman and Ogden, 1987;Robertson, 1988; Planes et al., 1993) may act to either reinforce or disrupt patternsestablished at the time of recruitment. Reinforcement would occur where growth orsurvival were better in habitats attracting high settlement, or if there was net movementinto such habitats. This might be expected if there is an immediate advantage to makingparticular habitat choices (Jones, 1986; Wellington, 1992; Tolimieri, 1995) or benefits toliving at higher density (Booth, 1995). Where growth and survival are inversely (Jones,1987a; Forrester, 1990 Forrester, 1995; Hunt von Herbing and Hunte, 1991; Tupper andHunte, 1994; Booth, 1995) or unrelated (Doherty, 1983; Jones, 1987b; Connell andJones, 1991; Levin, 1994) to recruitment patterns these variables may be sufficientlyimportant to cause disruption of the initial pattern. Intense competition or site-specificeffects of predation are examples of the processes that might be involved. Growth andmortality in fishes may be inextricably linked if slower growing individuals are moresusceptible to predators (van der Veer and Bergman, 1987; Bailey and Houde, 1988;Mapstone and Fowler, 1988; Forrester, 1990) which may compound their separateeffects. The relative importance of recruitment determination and reinforcement ordisruption in determining spatial distributions of coral reef fishes has rarely beenassessed (but see Wellington, 1992; Levin, 1994).

G.P. Jones / J. Exp. Mar. Biol. Ecol. 213 (1997) 231 –246 233

Experiments in which habitat characteristics such as coral substratum and topographiccomplexity have been manipulated, have implicated these factors as explanations ofspatial variation in settlement (Eckert, 1985), growth and mortality (Jones, 1988b;Connell and Jones, 1991). An important question that arises is: to what extent is thepotential of a location for recruitment, growth and survival independent of thesubstratum itself? Spatial patterns in recruitment may arise due to differences in larvalsupply to different areas (Grosberg, 1982; Gaines et al., 1985). For example, highrecruitment may occur in areas where eddies concentrate larvae that are ready to settle,and low recruitment may occur in recruitment shadows, i.e., areas down-current ofexpanses of preferred habitat (cf. Bell and Westoby, 1986). Likewise, growth may varyin response to variation in the supply of planktonic food, and mortality in response to thedistribution of predators (Jones, 1991). The potential for different sites to attract andsupport recruits can be tested by monitoring identical substrata, or transplanting identicalsubstrata and/or juveniles to different sites and examining their response.

Lagoonal fish species inhabiting patch reefs at a variety of depths and locationsrepresent ideal species to examine these issues. Recruits entering the lagoon over thewindward reef crest may settle at the first opportunity or continue down-current untilthey encounter the preferred habitat. Consistently low densities of certain species in thecentre of lagoons (e.g., Fowler, 1990) may arise because they are in recruitmentshadows, or because fish fail to find suitable settlement sites or because, if they do, theyfail to survive and grow. The potential contribution of all of these factors has not beenpreviously tested.

The aim of this study was to examine the effects of location within the lagoon and thedepth of reefs on the demography of juveniles of the planktivorous reef fishesPomacentrus amboinensis and Dascyllus aruanus (Pomacentridae) at One Tree Reef(Great Barrier Reef). In particular, I was interested in determining the potential forgrowth, mortality or the interaction of these two processes, to reinforce or disruptpatterns of recruitment, and if so, determine whether this effect was independent of thereef substratum. In addition, I set out to test the recruitment-shadow hypothesis, that is,recruitment onto preferred habitat would be lower when it is down current of preferredhabitat. To achieve these goals it was necessary to:(1) Examine spatial variation inrecruitment (to preferred coral substrata) between two different depths and between twolagoonal locations (perimeter, centre).(2) Examine growth and survival of translocatedjuveniles on identical coral substrata at the same locations. By using a standard coralsubstratum and using isolated individuals, the intrinsic relationship between growth andmortality could also be examined.(3) Compare recruitment to natural patch reefs in openareas with those on the down-current side of large reef structures, and to comparerecruitment to different positions on large experimental grids made of preferredsubstrata.

2. Methods

2.1. Study area and species

This study was carried out within the lagoon of One Tree Reef, an up-lifted platform


reef in the Capricorn Group at the southern extremity of the Great Barrier Reef(238309S, 1528069E). It focused on two common damselfishes inhabiting isolated coralheads and patch reefs, Pomacentrus amboinensis and Dascyllus aruanus. Previousexperimental work on these two species has concentrated on biological interactionswithin and among species (Williams, 1980; Sweatman, 1983, 1985; Jones, 1987a,bJones, 1990) or between fish and the biological substratum (Jones, 1988b). Williams(1979) described spatial patterns in the distribution and abundance of these species inOne Tree Lagoon, showing juvenile and adult densities to be greater near the southernperimeter, compared with the centre of the lagoon. The structure of the reef is verydifferent in these two regions, with higher coral cover and greater numbers of smallpatch reefs near the perimeter.

2.2. Spatial pattern in recruitment success

Experimental standard coral units for monitoring recruitment success at differentdepths and locations were constructed during November 1983 (just prior to therecruitment season). Each unit was made of a dead coral base (0.5 3 0.5 m) and a single

3large coral head of Pocillopora damicornis (L.) (approx. 0.05 m ), similar to thestandard coral units of Sweatman (1985). Pocillopora damicornis is a preferredsettlement site for new recruits of both fish species (Eckert, 1985; Jones, 1987b). Tenreplicate coral units, placed approximately 15 m apart and 15 m from any natural reef,were constructed at each of four sites on the exposed south-eastern perimeter and foursites in the centre of the lagoon. The centre of the lagoon is approximately 1.5 km fromthe perimeter. Placement of the individual coral units was in an irregular spatial array.At each lagoonal location (perimeter and centre), two sites were shallow ( , 2 m) andtwo were deep ( . 4–8 m). At the perimeter, shallow locations were on the shallow sandflat, while in the centre, they were within the shallow waters of large, sandy floored,micro-atolls. Deep sites at both regions were on sandy channels adjacent to patches ofreef. Sites within locations were on average 200 m apart. Recruitment success wasmeasured as the total number of new recruits collected from the reefs on three occasionsbetween December 1983 and April 1984.

2.3. Spatial pattern in growth and survival

Two identical sets of ten standard coral units were also constructed at each location,depth and site to examine growth and survival in Pomacentrus amboinensis andDascyllus aruanus. Individuals collected from near the perimeter shortly after settlementwere collected with quinaldine and transplanted to the experimental reefs. Transplantedindividuals of these species readily adopt new coral heads, a procedure that has beenused extensively and successfully in previous studies (Jones, 1986, 1987b, 1988b;Forrester, 1990). All individuals were measured (standard length) and transplanted atdensities of one individual per coral unit, with ten replicate reefs within eachcombination of site, location and depth. Densities of one fish per coral unit were belownatural recruitment densities for high recruitment sites, but enabled me to monitor thegrowth and life expectancy of juveniles without the necessity of tagging, and eliminated


any effects due to interactions among conspecifics or other species. The experiment wasinitiated during April 1984, after the recruitment season. All survivors were remeasuredduring September 1984, January 1985 and May 1985, after which the experiment wasterminated. All other individuals recruiting to the coral units were removed at eachsampling occasion, including migrants of the more mobile species.

2.4. Test for a recruitment-shadow effect

Recruitment of Pomacentrus amboinensis was measured over two recruitment seasonsto natural patch reefs in two different situations. The first type of patch reef was locatedon the down-current side of contiguous tracts of reticulated reef (i.e., the contiguous reefwas between the focal reefs and the windward edge of the lagoon). The second type ofpatch reef was located in the same general position in the lagoon, but was not behindreefs. That is, they were patch reefs that had essentially open sand between them and theedge of the reef. Ten reefs of each type, interspersed over a distance of 1 km along thesouthern perimeter of One Tree Reef, were monitored during two recruitment seasons,1983–84 and 1984–85. All recruits were collected from these reefs on three occasionsduring December and January of each season.

To test directly for a recruitment-shadow effect, grids of coral units were constructedat two locations (Shark Alley and South Channel) along the southern perimeter at siteswhere there were few natural reefs between the grid and the edge of the lagoon. AtShark Alley, five rows of coral units (with five coral units spaced at 10 m per row) wereestablished, with the rows perpendicular to the main current on the incoming tide. Coralunits were assembled with a dead coral rubble base and an apex of live Poritescylindrica Dana, each one measuring approximately 1.5 m in diameter and 0.75 m inheight. The arrangement at the second site, South Channel, was essentially the same,except that here there were eight coral units per row, and each one was constructed froma single, large (0.5 m diameter) knob of Porites cylindrica. Recruits of Pomacentrusamboinensis were collected from all coral units in December 1985, and February andApril 1986.

3. Results

3.1. Spatial pattern in recruitment success

The two damselfishes exhibited broadly similar spatial patterns in recruitment, withhigh recruitment to some sites at the perimeter and negligible or low recruitment to coralheads at sites in the centre (Fig. 1 Table 1). At the perimeter, deeper sites recordedsignificantly greater recruitment of Pomacentrus amboinensis than shallow sites. Over90% of the recruitment of Pomacentrus amboinensis was to deep perimeter sites.Recruitment of Dascyllus aruanus was generally greater than that for Pomacentrusamboinensis. Over 80% of the recruitment of Dascyllus aruanus occurred at perimetersites, but there was no significant difference between the two depth strata. There was


Fig. 1. Mean number of recruits of Pomacentrus amboinensis and Dascyllus aruanus collected from tenreplicate standard coral units at each of two shallow ( , 2 m) and two deep locations (4–8 m), both at theperimeter and centre of One Tree lagoon. Error bars 5 95% confidence limits.

also significant site to site variation within depths and locations, with densities reachinga maximum at Site 4 for both species (Fig. 1 Table 1).

3.2. Spatial pattern in growth

Patterns of growth in translocated juveniles differed between the two species, but eachexhibited a distinctive pattern with respect to depth and location (Fig. 2). Over the first 6months, Pomacentrus amboinensis exhibited significantly faster growth at the deep sitesthan at the shallow sites, after which rates were similar between depths (Fig. 2a). DuringSeptember, there was a significant difference between depth strata in mean size(F 5 18.72, P , 0.05), but no significant effect of location (Perimeter vs Central[1,4]

locations, F 5 0.47, P . 0.05). At this time, juveniles of Pomacentrus amboinensis[1,4]

were on average 4–5 mm larger in deeper water, a difference which was maintainedover time. No interaction between Depth and Location was detected (F 5 0.23,[1,4]


Table 1Analyses of variance for the effects of Location (Perimeter vs. Centre), Depth (Shallow vs. Deep) and Site onthe recruitment of Pomacentrus amboinensis and Dascyllus aruanus to experimental coral units

df MS df MS F p-levelEffect Effect Error Error

Pomacentrus amboinensisLocation 1 35.11 4 2.46 14.26 0.02Depth 1 37.81 4 2.46 15.36 0.02L 3 D 1 32.51 4 2.46 13.20 0.02Site(L 3 D) 4 2.46 72 0.79 3.11 0.02Residual 72 0.79Total 79Cochran’s C 5 0.60, v 5 9, k 5 8, variances heterogeneous

Dascyllus aruanusLocation 1 186.05 4 17.93 10.38 0.03Depth 1 39.20 4 17.93 2.19 0.21L 3 D 1 22.05 4 17.93 1.23 0.33Site(L 3 D) 4 17.93 72 2.86 6.27 0.00Residual 72 2.86Total 79Cochran’s C 5 0.40, v 5 9, k 5 8, variances heterogeneous.

Raw data are analysed as transformations could not correct for heterogeneous variances.

P . 0.05), indicating that the effect of depth was similar at each location. Site-specificdifferences in mean size after 6 months growth were detected (F 5 4.90, P , 0.05).[4,56]

In contrast, Dascyllus aruanus grew at progressively slower rates at the centrallocations compared with the edge and no overall effects of depth were apparent (Fig.2b). Juveniles were on average 4 mm smaller in centre, compared with the perimetersites. At the end of the experiment the remaining juveniles were significantly larger atperimeter sites (F 5 22.82, P , 0.05), but there was no effect of depth (F 5 3.06,[1,4] [1,4]

P . 0.05). No effects of either the interaction between depth and location (F 5 6.84,[1,4]

P . 0.05) or between different sites (F 5 0.27, P . 0.05) were detected.[4,37]

3.3. Spatial pattern in survival

Patterns of survival also differed for the two fish species (Fig. 3). Pomacentrusamboinensis exhibited significantly higher survival at deep locations (F 5 32.11,[1,4]

P , 0.05), with almost 100% mortality at shallow sites by the end of the experiment(Fig. 3a). This compares with approximately 50% mortality over the year at deep sites.There was no significant difference between the perimeter and central locations (F 5[1,4]

2.78, P . 11.05) and no interaction between location and depth (F 5 2.78, P . 0.05).[1,4]

Dascyllus aruanus exhibited marginally better survivorship at the central sitescompared with the perimeter (F 5 8.0, P , 0.05) (Fig. 3b). That is, approximately[1,4]

70% survival at the centre as compared with about 50% survival at the perimeter afterone year. In contrast to Pomacentrus amboinensis, patterns of survivorship did not differamong the two depth strata (F 5 0.067, P . 0.05) and the difference between central[1,4]


Fig. 2. Patterns of growth of juvenile Pomacentrus amboinensis and Dascyllus aruanus translocated tostandard coral units in shallow ( , 2 m) and deep (4–8 m) locations, both at the centre and perimeter of OneTree Lagoon, and monitored over 1 yr (Mean size 5 Mean standard length). Error bars 5 95% confidencelimits.

and perimeter locations was consistent among depths (F 5 0.067, P . 0.05). The[1,4]

survivorship of Dascyllus aruanus was generally better than that of Pomacentrusamboinensis at all sites examined.

3.4. Relationship between mortality, initial size and growth trajectory

Patterns of mortality within strata were in part explained by variation in initial sizeand growth rate over the first 6 months (Table 2). Table 2 compares the initial size offish divided into two groups; those which survived to the time closest to which 50%mortality was observed, and those which did not survive this period. There was nosignificant difference in the initial mean sizes for surviving Pomacentrus amboinensis


Fig. 3. Percent survival over 1 yr of juvenile Pomacentrus amboinensis and Dascyllus aruanus translocated tostandard coral units in shallow ( , 2 m) and deep (4–8 m) locations, both at the centre and perimeter of OneTree Lagoon. Error bars 5 95% confidence limits.

and those not surviving, but surviving Dascyllus aruanus had a greater mean size at thebeginning of the experiment.

The rates of growth of survivors and non-survivors (at the end of the experiment)were also calculated, based on the increase in standard length of both groups betweenApril and September, i.e., the first growth increment for fish surviving this early period.This was only compared within location /depth strata exhibiting similar growth rates(based on Fig. 2). At deep sites, surviving Pomacentrus amboinensis were growing morerapidly during their first 6 months than those which ultimately died (Table 2). Survivorsof Dascyllus aruanus were also growing measurably faster than those which later died atcentral sites, although this effect was not apparent at the perimeter.

3.5. Test for recruitment-shadow effects

Natural patch reefs on the down current side of large patch reefs recorded significantlyhigher recruitment of Pomacentrus amboinensis than similar reefs at the same distance


Table 2Effect of initial size (S.L.) and growth rate (growth increment over first 6 months) on survivorship ofPomacentrus amboinensis and Dascyllus aruanus in the translocation experiment

Survivors Non-survivors t-test

Pomacentrus amboinensisInitial Size 13.2060.20 13.1160.20 t 5 0.32, NS[61]

(n 5 23) (n 5 40)Growth Increment 14.0760.43 12.2060.34 t 5 3.41, **[33]

Deep only (n 5 15) (n 5 20)

Dascyllus aruanusInitial Size 12.2060.27 10.8960.41 t 5 2.67, **[78]

(n 5 47) (n 5 33)Growth Increment 7.2560.48 6.1360.50 t 5 1.61, NS[35]

Perimeter (n 5 18) (n 5 19)Growth Increment 7.0660.37 5.1860.44 t 5 3.24, **[35]

Centre (n 5 26) (n 5 11)

Survivors are those individuals that survived to the time interval nearest to which 50% mortality had occurred,and non-survivors are those lost during this period. Locations exhibiting differences in growth rates areexamined separately. The effect of growth on survivorship could only be examined for Pomacentrusamboinensis at deep sites, since mortality was too high at shallow sites. Sample sizes vary for growth ratetests, as individuals not surviving the first 6 months could not be included in this analysis.

from the edge of the lagoon that were not behind other reefs (F 5 22.12, P , 0.05)[1,36]

(Fig. 4). The same pattern was observed over consecutive years, particularly during the1993/94 summer, when recruitment was significantly higher (F 5 5.53, P , 0.05).[1,36]

Recruitment was 2–3 times higher to reefs that were not in the immediate shadow of alarge reef.

Parallel experimental rows of identical coral units at two locations also supported theidea that reefs nearer the edge of the lagoon reduce the availability of recruits todown-current reefs (Fig. 5). Planned comparisons showed that the first (outer row) ofcoral units received significantly higher recruitment than all other rows combined, both

Fig. 4. Recruitment of Pomacentrus amboinensis to small natural patch reefs in areas between (exposed to theprevailing current) and behind large reef areas within One Tree lagoon over two recruitment seasons. Meanrecruits 5 the total number of recruits from December and January collections, averaged over ten replicatereefs. Error bars 5 95% confidence limits.


Fig. 5. Recruitment of Pomacentrus amboinensis to standard coral units in different positions on two grids (oneat Shark Alley and one at South Channel), in which reefs were laid out in rows that were orientedperpendicular to the prevailing current. Error bars 5 95% confidence limits.

at Shark Alley (F 5 7.32, P , 0.05) and at South (F 5 5.44, P , 0.05). There[1,15] [1,35]

were no obvious differences among the other rows, except that recruitment was alsounexpectedly high in the most down-current row at Shark Alley.

4. Discussion

The literature indicates that recruitment patterns are often one of the primarydeterminants of spatial and temporal patterns in adult numbers in marine organisms(Gaines and Roughgarden, 1985; Gaines et al., 1985; Doherty and Williams, 1988;Jones, 1988a; Roughgarden et al., 1988; Williams, 1991; Booth and Brosnan, 1994;Doherty and Fowler, 1994; Caley et al., 1996). The correlation between recruitmentlevels and adult numbers appears to hold over a variety of spatial scales, from patternswithin individual reefs to patterns across entire reef systems. At progressively largerscales, the distinction between cause (recruitment?) and effect (adult density?) becomesincreasingly more difficult to make, as it becomes more likely that habitats are notconnected by larval supply. At very large scales the question of whether there are moreadults because there are more recruits, or more recruits because there are more adults isanalogous to the question of which came first, the chicken or the egg? There may be nosatisfactory answer.

At smaller within reef scales, however, the degree to which spatial pattern inrecruitment determines adult numbers raises many biologically important questions. Forexample, what is the relative importance of habitat selection and passive effects indetermining recruitment patterns? To what extent do post-recruitment processes act as aselective agent which may explain the evolution and maintenance of patterns in habitatselection? To what extent do post-recruitment processes reinforce or disrupt patterns inrecruitment, and thus take a more proximate role in determining the absolute size of theadult population? Answers to these sorts of questions are only beginning to emerge.

It is becoming evident that post-recruitment processes can act to reinforce patterns


established at the time of settlement. For example, in Stegastes leucostictus andStegastes variabilis, differential depth distributions are initially set at the time ofsettlement. However, translocation experiments show a four times greater mortality rateof juveniles outside their normal settlement range (Wellington, 1992). Such rein-forcement appears to apply to the depth distribution of Pomacentrus amboinensis, whichrecruits in higher numbers, grows faster and survives better in deeper water. Patterns ofhabitat selection in settlers may be an evolutionary response to differential survival. InPomacentrus amboinensis, depth differences in growth and survival would providestrong selection pressure for settlement into deeper water. On ecological time scales,clearly, both recruitment and reinforcement contribute to spatial differences in theabsolute population densities of juveniles (see also Tolimieri, 1995). Most ecologicalstudies that have found a positive correlation between recruitment and adult numbershave ignored the contribution of reinforcement. It is likely to be a particularly importantfeature of species in which juveniles preferentially settle with conspecifics (Sweatman,1983, 1985) where, through mechanisms of facilitation, there may be greater chances ofreaching sexual maturity at high local density (Booth, 1995).

There is an increasing number of studies that have found that spatial patterns inrecruitment of reef fishes are modified by post-recruitment processes (Doherty, 1980;Shapiro, 1987; Fowler, 1990; Connell and Jones, 1991; Levin, 1994). Higher surviv-orship or growth in areas receiving lower recruitment may partially compensate for thelower input, and if extreme, disrupt the patterns established at the time of settlement(Hunte and Cote, 1989; Jones, 1988b; Hunt von Herbing and Hunte, 1991; Booth andBeretta, 1994; Tupper and Hunte, 1994; Forrester, 1995). Spatially variable growth andsurvivorship, when independent of recruitment, will also disrupt recruitment patterns(Jones, 1987a; Connell and Jones, 1991; Levin, 1994). For example, in the blennioidfish, Forsterygion varium, there is a relatively even settlement of juveniles, but there isalso differential survival in areas of greater topographic complexity, which determinesadult density patterns (Connell and Jones, 1991). In the present study, differences inmortality of Dascyllus aruanus among locations would act to modify patterns estab-lished at settlement, as they recruit in higher numbers at the perimeter of the lagoon, butsurvive better at the centre. The central areas of One Tree lagoon support relatively highdensities of adult butterflyfishes, despite low recruitment relative to windward perimetersites, which also suggests that there is better survivorship in this area (Fowler, 1990).

It is possible that the apparent choice to settle near the perimeter in Dascyllus aruanusleads to increased chances of surviving to maturity, despite reduced life expectancy,because of the faster growth rates observed here. However, it is equally plausible thatlarvae cannot always choose the best habitat for future survival and reproduction(Shapiro, 1987; Connell and Jones, 1991). Differences in recruitment rates between thecentre and perimeter of the lagoon are better explained by recruitment-shadow effectsthan broad-scale habitat selection. Recruitment of both Pomacentrus amboinensis andDascyllus aruanus to the centre of the lagoon is low, even when preferred coralsubstrata are transplanted to the centre. If habitat selection was paramount I would haveexpected greater recruitment of Dascyllus aruanus to the centre and greater recruitmentof Pomacentrus ambionensis to deep, central sites. Data presented here show that theobserved recruitment of Pomacentrus amboinensis to preferred habitat is consistently


lower in areas that are down current of reefs, whether in natural or experimentalsituations. It is unlikely that these recruitment patterns are due to differences in earlypost-settlement survival, as Pomacentrus amboinensis exhibits quite low mortality ratesin the first few weeks after settlement at One Tree reef (Sale and Ferrell, 1988). The dataof Fowler (1990) on butterflyfishes is also consistent with this recruitment-shadowhypothesis. He found high recruitment to windward, perimeter sites and a poorcorrelation between fish abundance and the availability of preferred habitats on a reefwide scale. Recruitment-shadow effects may be particularly important in lagoons such asat One Tree Reef that have low flushing rates over normal tidal cycles. It may also beimportant in extremely large lagoons and barrier reef systems that have extensive backreef areas.

These data confirm previous suggestions that there may be a strong interactionbetween growth and mortality rates in juvenile coral reef fishes (Jones, 1987a; Mapstoneand Fowler, 1988; Forrester, 1990). At a within-site level, juveniles of greater initial sizeor faster growth over the first 6 months, exhibited greater chances of surviving their firstyear on the reef. On a broader scale, juvenile Pomacentrus amboinensis both survivedand grew better in deeper water habitats. Hence, processes that are known to reducegrowth rates, such as competition at increased densities (e.g., Jones, 1987a, 1990;Forrester, 1990), may increase susceptibility to predation. On a large scale, however,Dascyllus aruanus shows the opposite pattern. Juveniles survived better in the centre ofthe lagoon, but grew better near the edge.

The reasons for the differences between the two species are unclear. Both areplanktivores, but Pomacentrus amboinensis consumes greater amounts of algae and mayderive much of this by feeding on the sandy substratum adjacent to patch reefs. Thus theobserved growth differences at the standardised densities on experimental reefs mayrelate to the abundance of their particular food resources. Certainly, the availability ofzooplankton is expected to be greatest at the windward edge of the lagoon (which mayfavour Dascyllus aruanus) and the biomass of algae growing on the sand is often high inthe deeper water of lagoons (which may favour Pomacentrus amboinensis). Studies byConnell (1996) suggest that predation pressure is lower in the centre of the lagoon,compared with the perimeter, which may explain the patterns in mortality observed forDascyllus aruanus.

In conclusion, spatial variation in recruitment, growth and mortality rates clearlyinteract to explain the distribution of adult fishes within reef systems. Horizontal patternsin recruitment probably represent an interaction between larval supply to up-current sitesand recruitment-shadow effects in down-current areas. Depth-related patterns are morelikely to represent selection of habitats that maximise growth and increase lifeexpectancy. Both growth and mortality appear to interact, either to reinforce or disruptpatterns in distribution established at the time of recruitment. The nature of thisinteraction and the processes responsible for it need to be explored further.

Acknowledgments

My gratitude to all those who assisted in the field, whoever you were, wherever you


are now. I hope you weren’t waiting on these results. The manuscript benefited from 10years gestation, and recent comments by J. Caley, M. Hixon, M. McCormick, U. Kalyand anonymous reviewers.

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