FISH AGGREGATION RESULTS IN INVERSELY DENSITY-DEPENDENT PREDATION ON CONTINUOUS CORAL REEFS

Abstract

Spatially density-dependent predation is a leading hypothesis describing mechanisms of population regulation in coral reef fish. However, studies supporting this hypothesis predominantly have been conducted on small, isolated patch reefs. Here, we searched for evidence of spatially density-dependent predation on the continuous reefs of the Netherlands Antilles in a study of a dominant planktivore, the blue chromis (Chromis cyanea). Across space, we quantified both the patterns of loss from site-attached aggregations of C. cyanea through time and the behavioral reaction of predators to these aggregations. Looking across C. cyanea densities, we found that loss from aggregations was not characteristic of direct density dependence, but instead was commonly inversely related to density. Individual C. cyanea in larger aggregations were less likely to be lost from the group than were individuals in smaller aggregations. Thus, the observed density dependence increased spatial heterogeneity of C. cyanea. Predators showed behaviors that were consistent with these demographic patterns. Using remote videography, we quantified predator visitation and strike rates across a range of C. cyanea aggregation sizes. Predators consistently visited and struck at individuals in C. cyanea aggregations in a pattern that was strongly inversely density dependent, suggesting that aggregation is an effective means of minimizing per capita risk of predation for prey reef fish. Differences in spatial distribution of resources for predators (i.e., prey fish) between continuous and patch reef habitats may explain the difference between these results and those of previous studies on patch reefs.