Density- and size-dependent mortality of a settling coral-reef damselfish ( Pomacentrus moluccensis Bleeker)

Abstract

Density-dependent mortality may regulate many populations, but due to an offshore larval phase in benthic marine organisms, it is often difficult to quantify the effects of mortality of arriving individuals. We added approximately 600 recruit-sized individuals onto coral reef that parallels patterns in naturally settling fish. Strong, positive density-dependent mortality occurred 3 days, 1–2 weeks, and 4 months after release. Since our study species was patchily distributed, we estimated both mean group size and overall density in a transect. Mortality was more strongly related to mean group size than overall density in a transect, indicating that recruit patchiness was important. Cohesive groups may suffer higher mortality than those spread over larger areas, even if overall density of the latter is greater. Aggregative responses of predators may occur in response to larger groups, so may have contributed to positive density-dependent mortality. Increased conspicuousness to predators and congeneric aggression are additional factors that may vary positively with group size. Tagging of recruits showed migration within but not between transects, so persistence was tantamount to survival. Standard otolith back-calculation techniques employed to reconstruct original size of tagged recruits that persisted 4 months after additions indicated that mortality was also size-dependent. Size-dependent mortality was apparent at the site with the highest mortality but not at the site with the lowest mortality, resulting in different mean body sizes of recruits between sites. Size-dependent mortality may influence estimation of growth and lead to onset of size-based maturity in these fish. Strong, positive density-dependent mortality may regulate recruitment, and if coupled with size-dependent mortality, may increase maturity rate, adding to reproductive schedules of those that survive.