Spatial patterns of the structure of reef fish assemblages at a pristine atoll in the central Pacific

Abstract

We conducted in situ diver surveys describing the spatial structure of reef fish assem- blages at Kingman Reef, an unexploited and remote atoll in the central North Pacific. Structural pat- terns reflect natural ecological processes that are not influenced by fishing or other anthropogenic factors. The most striking feature of this assemblage is an inverted biomass pyramid dominated by apex predators, primarily sharks and large snappers, across all depth and habitat strata examined. This pattern is most pronounced at greater depths (20 m) on the fore reef. Apex predators dominated to lesser extents in back-reef, patch-reef, and shallow fore-reef habitats. Prey assemblage size spec- tra showed fewer large prey and greater numbers of prey from small size classes at locations with greater piscivore biomass. Other patterns of prey abundance generally conformed to those previ- ously observed at more commonly encountered, human-altered reefs (e.g. highest herbivore abun- dance on back reefs and shallower depths on the fore reefs; greater planktivore prevalence deeper on the fore reef). The latter patterns, however, inadvertently miss the less obvious differences in assemblage dynamics that result from alterations in the size structures of prey fish populations where apex predators have been heavily exploited or extirpated. The present study of a fully intact coral reef suggests that (1) piscivores are common across all habitats and depths, (2) the presence of preda- tors does not lead to appreciable reductions in the biomass of other guilds, and (3) predators alter the size structure and therefore the potential productivity and energy flow of the ecosystem.