Pomacentridae) in One Tree Island Lagoon, Great Barrier Reef
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MARINE ECOLOGY PROGRESS SERIES Vol. 226: 157-164,2002 Mar EcolProg Ser Published January 31 Distribution changes after settlement in six species of damselfish (Pomacentridae) in One Tree Island lagoon, Great Barrier Reef David J. Booth" Department of Environmental Sciences, University of Technology, Sydney, Westbourne Street, Gore Hill NSW 2065, Australia ABSTRACT:While larval supply patterns playa major role in determining the distribution and abun- dance of reef fishes at a range of spatial and temporal scales, events occurring in the weeks after set- tlement may also significantly alter the demography of a juvenile population. I monitored over 2 sum- mers the arrival of new recruits of 6 species of benthic dams elfish onto continuous and patch reef habitat in One Tree Island lagoon. Each species had specific habitat preferences, and most showed some form of patchiness in spatial and temporal distribution at settlement; however, this was most likely a result of group settlement rather than habitat patchiness. Tagged recruits of Pomacentrus amboinensis remained within 1 m of their settlement site for at least the first 36 d on continuous reef, while known individuals of other species also moved little during this period, suggesting that disap- pearances were likely to be due to mortality. Mortality of recruits varied among species, years and habitats (patch vs continuous reef), and consequently spatial distributions of fish changed after set- tlement at the scale of lOs of metres (within-site) and several kilometres (among 8 sites). The spatial patchiness in distributions was not consistent among species, sites or years, suggesting that spatial variation in substrate did not have a major effect on settlement variation at this scale. These results show that the settlement signal may be obscured in the first few weeks post-settlement, and that less- frequent censuses, commonly used as proxies for high-frequency monitoring of settlement, may not accurately hind cast spatial or temporal patterns of settlement. KEY WORDS: Damselfishes· Group settlement . Migration . One Tree lagoon . Post-settlement mortality -----------Resale or republicationnotpermitted withoutwrittenconsentof the publisher----------- INTRODUCTION (Roughgarden et al. 1988). Despite this, in many mod- els of dispersal, larvae are treated as passive particles, While most marine species have a life cycle that leading to the prediction that dispersal follows ocean incorporates a pelagic larval stage and benthic or dem- current patterns (e.g. Roberts 1997). However, more ersal juveniles and adults, little is known of the pelagic recently it has been demonstrated that larvae have phase or processes that occur as individuals arrive at strong behavioural and physiological abilities to orient adult habitat. Some evidence that these stages can independent of currents (Leis & Carson-Ewart 1999). have strong influences on the demography of adults In other marine organisms, swimming behaviour can has been gathered, providing support for a recruit- determine vertical zonation on rocky shores (ascidians: ment-limitation hypothesis for reef fishes (e.g. Grosberg 1982). and the duration of the settling-search Williams 1980, Doherty 1983) and other organisms phase can influence spatial distribution of recruits (Davis & Butler 1989). In coral reef fishes, pre settlement fish are known to •E-mail: [email protected] have considerable swimming abilities (e.g. Stobutski & © Inter-Research 2002 . www.int-res.com 158 Mar Ecol Prog Ser 226: 157-164,2002 Bellwood 1997), and show preferences for certain damselfishes, either preferring or avoiding particular swimming directions around reefs (e.g. Leis & Carson- habitat types or conspecifics. Ewart 1999). Once they arrive in the vicinity of a reef, Habitat hotspots for settlement at larger spatial larvae may detect the reef's lagoonal waters and avoid scales have been identified for coral reef fishes, due entry (e.g. Doherty et al. 1996). Close to the reef, larvae to patchiness of high-quality habitat (Munday et al. may prefer certain habitats, and may be attracted to or 1997), oceanographic features such as currents that actively avoid conspecifics (e.g. Sweatman 1985, Booth direct larvae to certain locations (e.g. Sponaugle & 1992). Holbrook et al. (2000) demonstrated that habitat Cowen 1997) or combinations of habitat and oceano- distribution, not larval supply, accounted for spatial graphic features (e.g. Booth et al. 2000). It is important variation in distribution of newly settled dams elfish to identify such hotspots, as they may indicate patchi- Dascyllus aruanus among widely spaced reefs in French ness of post-settlement predation or settlement prefer- Polynesia, while Srinivasan et al. (1999) showed simi- ences for resources, and be useful identifying manage- lar control of distribution by habitat for species of ment options such as habitat enhancement. anemonefish Amphiprion spp. These taxa are known Understanding the demographic effects of early to have very specific habitat preferences, and it is not post-settlement processes of predation and migration surprising that habitat largely controls distribution. is also important in justifying the use of late season However, other reef fish species may be less depen- recruitment surveys as proxies for more frequent cen- dent on specific habitat type, and less is known of how suses in determination of larval supply to a reef (e.g. well settlement predicts subsequent post-settlement Williams et al. 1994). These assume that settlement distribution in these species (e.g. Lewis 1997). patterns are not modified between settlement and end- Settlement behaviour has the potential to alter the of-season censuses, but if mortality and/or migration distribution of new settlers, and subsequently older are variable in space and time (e.g. Levin 1998). they fish at a number of spatial and temporal scales (e.g. will obscure settlement patterns. Booth & Wellington 1998). However, little direct evi- In this study, the change in distribution of 6 species dence of such effects of settlement behaviour is avail- of damselfishes (Pomacentridae) over about 6 mo post- able. Breitburg (1991) showed that nearshore aggrega- settlement at a small spatial scale, and between 2 yr, tions of naked gobies on temperate oyster reefs is interpreted in the light of possible migration and influenced settlement distribution, while Sweatman mortality. Specifically, the study asks: (1) Are hotspots (1985) and Booth (1995) demonstrated that settling for settlement temporally persistent? (2) Do spatial Dascyllus spp. selectively chose coral heads already patterns of settlement suggest either habitat associa- supporting conspecifics. Ohman et al. (1998) recorded tions or group settlement? (3) Is persistence in the species-specific habitat choices for late-stage larval weeks post-settlement variable spatially and tempo- rally, and among species? And (4) as a consequence, are patterns of distribution established at settlement obscured over the first months post-settlement, and therefore are censuses made at the end of the settle- ment season (sensu Williams et al. 1994) indicative of actual settlement patterns? MATERIALS AND METHODS Study site and species. One Tree Island lagoon is lo- cated at the southern end of the Great Barrier Reef, in the Bunker Group (23030' S, 152006' E).The lagoon contains a convoluted series of continuous reef zones, or piecrust, extending from the surface to depths offrom 1to 4 m. Be- tween piecrust areas, deeper patch reefs (2to 7 m below One Tree Island the surface) are surrounded by sand. This study was car- ried out on piecrust and constructed patch reefs in Shark Alley, on the southeastern side of the lagoon, and at 7 Fig. 1. Aerial map of One Tree Island (dark) and One Tree other sites in the lagoon (Fig. 1).Shark Alley is the site of lagoon, showing locations of 8 sites within the first lagoon. SA = Shark Alley, GU = Gutter, EN = Entrance, SF = Sandflat, highest settlement of damselfishes (Pomacentridae) FB = First Bank, CB = Centre Bornmie, MA = Maze, BB = Big within the lagoon, which settle in both patch reef and Bommies piecrust habitats (Booth et al. 2000). Booth: Distribution changes after settlement in damselfishes 159 Spatial and temporal distribution of fish. Visual sur- of tattoo ink and observed during subsequent cen- veys were carried out for settler, recruit, juvenile and suses, including intensive searches of adjacent areas adult damselfishes on pie crust transects at 8 sites and up to 20 m away from transects. Therefore, migration 6 artificial patch reefs in Shark Alley. Transects com- over the scale of several metres could be estimated, prised two 75 m stretches of continuous reef at each and it was assumed that disappearance of these indi- site and were marked off into 1m portions using flag- viduals was tantamount to mortality. Recruit persis- ging tape and underwater buoys. The observer slowly tence was estimated as the ratio of juveniles remaining swam along the transect, which varied from 1 to 2.5 m of each species in April to recruits censused in the pre- in width, depending on piecrust depth and topogra- vious February, while settler persistence was the ratio phy, and noted the identity and location (to the nearest of juveniles to settlers. metre) of each pomacentrid. Fish were categorised as Microhabitat monitoring. Every 5 m along transects settler «15 mm TL with reduced caudal pigmentation in Shark Alley, microhabitat was monitored by placing and of similar length to pre-settlement fish caught a 1 m2 quadrat midway between the upper and lower adjacent to the reefs by light traps, unpubl. data). boundary of the transect. The percentage cover of sev- recruit (15 to 25 mm TL), juvenile (25 to 40 mm TL) or eral broad habitat categories was estimated within the adult (>40 mm TL, or with adult pigmentation). All quadrat. Broad categories used were: dead coral, bran- length estimates were verified by capturing fish ching corals (mainly Porites sp., Stylophora sp., Pocillo- throughout the study, with less than 5 % of estimates pore damicornis) and conspecific density. Multiple leading to incorrect classification (unpubl.