Associations between the abundance of piscivorous fishes and their prey oncoral reefs: implications for prey-fish mortality

Few studies have examined predator-prey relationships in diverse communitie s such as those found on coral reefs. Here we examined patterns of spatial and temporal association between the local abundance of predator and prey f ishes at Lizard Island on the Great Barrier Reef, Australia. We predicted t hat the nature of this association would have implications for patterns of prey-fish mortality. Strong positive relationships between prey and piscivo re abundance were found throughout the study. Greater densities of predator s and of prey were found on patch-reef habitats, compared with contiguous r eef-slope habitats. Declines in prey-fish abundance on patch reefs were den sity-dependent and correlated with the densities of predators. The relative roles of recruitment and piscivore movement in determining patterns of pre dator and prey abundance were assessed from surveys of recruit densities an d an intensive programme of tagging two species of rock-cod, Cephalopholis cyanostigma and C. boenak (Serranidae), over 2 years. Patterns of recruitme nt explained little of the variation in the abundance and distribution of p iscivorous fish. If movement explains large-scale patterns of distribution, this was not evident from the tagging study. The two rock-cod species were highly sedentary, with individuals on patch reefs seldom moving among reef s. Individuals on reef slopes were also highly site-attached, although they moved greater distances than those on patch reefs. Although the mechanisms responsible remain to be determined, this study demonstrated strong associ ations between the abundance of piscivorous fish and their prey on coral re efs. This relationship appeared to be an important factor in producing dens ity-dependent declines in the abundance of prey.