Fish feeding and dynamics of soft-sediment mollusc populations in a coral reef lagoon

Abstract

Large coral reef fish were experimentally excluded from enclosed plots for 2 yr to examine their effect on the dynamics of soft sediment mollusc populations from areas in One Tree lagoon (Great Barrier Reef). Three teleost fish which feed on benthic molluscs. Lethrinus nebulosus, Diagramrna pictum and Pseudocaranx dentex, were common in the vicinity of the cages. Surveys of feeding scars in the sand indicated similar use of cage control and open control plots and effective exclusion by cages. The densities of 10 common species of prey were variable between locations and among times. Only 2 species exhibited an effect attributable to feeding by fish, and this was at one location only. The effect size was small relative to the spatial and temporal variation in numbers. The power of the test was sufficient to detect effects of fish on most species, had they occurred. A number of the molluscs exhibited annual cycles in abundance, with summer peaks due to an influx of juveniles but almost total loss of this cohort in winter. There was no evidence that predation altered the size-structure of these populations. While predation by fish is clearly intense, it does not have significant effects on the demography of these molluscs. The results cast doubt on the generality of the claim that predation is an important structuring agent in tropical communities.