Abundance and distribution of nocturnal fishes over a coral reef during the night

Abstract

Coral reefs are characterized by high abundance of fishes, which often have marked effects on the recruitment, nutrition, and reproduction of coral reef inhabitants. Despite the fact that fish are active over the coral reef at night, very little is known of their whereabouts or about their contribution to those processes. In the present study, the abundance and distribution of nocturnal fishes were studied over a coral reef near Eilat, Red Sea. Sampling was conducted between the surface and 15 m depth, using a stereoscopic camera system that surveyed 3.5 m(3) of water every 2 min. The density of zooplanktivorous species ranged from 0.7 to 7.4 fish 100 M-3, accounting for > 60 % of the total nocturnal fishes recorded. Fish abundance decreased with increasing distance from shore and with increasing height above bottom. Larger fish foraged at greater heights above bottom. While the appearance of zooplanktivorous fishes after sunset and their disappearance before sunrise corresponded to that of demersal zooplankton, inter-night variation in fish abundance was unrelated to variation in zooplankton abundance. Furthermore, neither current speed nor moon illumination accounted for inter-night variations in fish abundance, perhaps indicating some degree of site fidelity. The abundance of zooplanktivorous fishes was similar to 30-fold lower during the night than day. However, since nocturnal fishes feed on larger zooplankters, the biomass of zooplankton preyed upon by nocturnal fishes was similar to that of diurnal species. This study suggests that the paradigm of negligible predation by nocturnal fishes should be reconsidered.