To what extents are species richness and abundance of reef fishes along a tropical coast related to latitude and other factors?

Abstract

This study has employed an integrated statistical approach to determine the extents to which species richness and abundance (catch rate) of fishes over reefs along an extensive coastline were related to various factors. Fish were thus sampled by trapping over deep (~ 22 m) and shallow (~ 12 m) reefs along the ~ 1500 km tropical coast of north-western Australia (NWA). Fish were caught during day and night in both dry and wet seasons at two well-spaced locations in each of the Kimberley (13–16°S), Canning (16–19°S) and Pilbara (20–22°S) bioregions. Species richness and abundance were both typically less at the two locations at the lowest latitude than at the two at the highest latitudes. This trend, which contrasts with the paradigm regarding latitudinal trends for these biotic variables, is attributable to the far more extreme hydrological conditions in the Kimberley than Pilbara. Indeed, both biotic variables peaked in the Canning, presumably reflecting, inter alia, the better development of reef and suspension feeder habitats in that bioregion. The peak in abundance was attributable to particularly high numbers of the NWA endemic Lethrinus punctulatus. A greater species richness and abundance of fishes in deeper than shallow water during the wet season, and particularly in areas of greater cyclonic activity, reflect the movement of species such as Lethrinus laticaudis and Lutjanus carponotatus from shallow to calmer, deeper waters. These two biotic variables were greater during the day than night, with a greater number and abundance of diurnally-active species, e.g. lethrinids, feeding overall in the traps than is the case with nocturnal species, e.g. lutjanids. The results imply that the typical poleward latitudinal gradients in species richness and abundance are over-ridden along the NWA coast by environmental differences among bioregions.