Settlement strategies and distribution patterns of coral-reef fishes

Abstract

Patterns of habitat use established at settlement may be a primary determinant of the distribution of coral-reef fishes within and among habitats. However, due to the difficulty of observing fish larvae in the wild, the behaviour of individuals at settlement has rarely been observed. Here, we examined the behaviour at settlement of five species of damselfishes (family Pomacentridae) by conducting multi-choice experiments in large outdoor aquaria at Lizard Island on the Great Barrier Reef, Australia. Late-stage larvae that were competent to settle were collected using light traps and placed in large circular aquaria at night where they could choose to settle among live coral, dead branching coral, coral rubble and bare sand habitats. We examined patterns of habitat choice at settlement by larvae with no prior experience of the reef (naive larvae). The effect of experience in the reef environment on habitat preferences was then determined by studying the behaviour of juveniles collected soon after settlement and exposed to the same experimental protocol. We also examined the importance of interspecific interactions among late-stage larvae and interactions between newly settled juveniles and adults in determining patterns of habitat choice exhibited by larvae and newly settled juveniles. Late-stage larvae and newly settled juveniles made distinct habitat choices but the choices made varied widely among species. Chromis viridis (Cuvier) late-stage larvae selected live coral and always settled as a group. Post-settlement C. viridis also selected live coral but exhibited stronger schooling behaviour and moved around the aquaria more frequently than late-stage larvae. Pomacentrus moluccensis (Bleeker) consistently selected live coral and also preferentially selected habitats where conspecifics, either settlers or adults, were present. P. amboinensis (Bleeker) late-stage larvae preferentially selected both live and dead standing coral habitats but avoided coral rubble. Post-settlement P. amboinensis selected live coral more frequently than late-stage larvae. P. coelestis (Jordan and Starks) late-stage larvae initially settled with equal frequency among the habitats presented but exhibited an increasing use of coral rubble and dead coral throughout the day. In contrast, early post-settlers always selected the coral rubble habitat with significantly greater frequency than other habitats. Therefore, experience on the reef may be associated with habitat preferences in P. coelestis. P. chrysurus settlers were distributed evenly among habitats due to agonistic interactions among individuals observed within hours of settling. Also, early post-settlement P. chrysurus avoided habitats containing adults. For the pomacentrids we tested, the patterns of habitat selection observed in our experiments were similar to the distribution of adults among habitats on the reef at Lizard Island. Also, interactions among individuals at settlement could explain the spacing of individuals within habitats. Therefore, precise selection of habitats at settlement and behaviour among conspecifics within hours of settling may have a major influence on the distribution of coral-reef fishes within and among habitats.