Factors influencing the characteristics of fish assemblages in a large subtropical marine embayment

Abstract

Mean number of species and density of fishes in nearshore shallow waters of Shark Bay, a large subtropical embayment, were c. seven and 19.5 times greater in seagrass than over bare sand, where protection from predators and the abundance of potential invertebrate prey were less. The number of fish species and density of fishes over bare sand were lower in nearshore than offshore waters, where there was a greater amount of organic material and thus presumably a greater density of benthic macroinvertebrate prey. Species composition in vegetated and unvegetated habitats differed markedly, with species such as Monacanthus chinensis, Apogon rueppellii and Pelates quadrilineatus being largely confined to seagrass, whereas others such as Pseudorhombus jenynsii, Torquigener whitleyi and Engyprosopon grandisquama were found predominantly or exclusively over bare sand. The ichthyofauna in beds of Posidonia australis, in which the canopy is uniformly dense, differed in composition and comprised a greater number of species and density of fishes than that in Amphibolis antarctica, in which an open space is present beneath the terminal clusters of relatively short leaves. Species composition in the beds of both of these seagrass species underwent well defined cyclical changes, caused by out‐of‐phase sequential changes in the densities of certain species. Such changes were less common over bare sand, where the ichthyofaunal composition was more variable. The number of species and density of fishes over bare sand were greater at night than during the day, reflecting, in part, a tendency for species such as A. rueppellii to move into unvegetated areas to feed at night, when the likelihood of predation by visual predators would be reduced. Within Shark Bay, ichthyofaunal composition is influenced most by habitat type (vegetated v. unvegetated), followed in general by water depth and then region in the bay and time of year.