The inshore fish faunas over soft substrates and reefs on the tropical west coast of Australia differ and change with latitude and bioregion

Aim To test the following hypotheses regarding ichthyofaunal compositions along an extensive tropical coastline. The compositions over soft substrates and reefs: (1) consistently differ markedly; (2) change progressively with latitude and temperature through sequential changes in the abundances of certain species; and (3) vary among bioregions, as those regions differ markedly in their environmental characteristics. Location Tropical north‐western Australia. Methods Similarity matrices, derived from percentage contributions of each fish species to catches obtained over soft substrates by trawling and over reefs by trapping at seven regularly spaced sites along 1500 km of coast, were subjected to cluster analysis, non‐metric multidimensional scaling ordination and associated tests. Results In total, 361 species were obtained by trawling and trapping along the tropical coast of north‐western Australia (NWA). Only 56 species were recorded over both soft substrates and reefs, whereas 229 and 76 species were caught exclusively over soft substrates and reefs, respectively. The Leiognathidae, Carangidae and Terapontidae contributed most individuals (62.2%) to catches over soft substrates, whereas the Lethrinidae and Lutjanidae dominated those over reefs (81.9%). The species compositions in both habitats were related to latitude and water temperature. Ichthyofaunal compositions in the Kimberley region differed markedly from those in the Canning/Pilbara regions further south, which, in turn, each had distinct characters. The vast majority of species found over both habitats also occur in the Pacific Ocean to the north. Main conclusions The most important fish families over soft substrates and reefs in inshore marine waters of tropical NWA differ markedly. The ichthyofaunal compositions of both habitats undergo similar patterns of progressive change with latitude, due to site‐staggered changes in the relative abundances of key fish species in their respective habitats. Ichthyofaunal composition in both habitats was found to be influenced by water temperature. The latitudinal trends exhibited by species composition are overlaid on a strong bioregional effect, reflecting the influence of the very different environmental conditions in those bioregions, which include marked differences in such factors as tidal regime, turbidity and whether mangroves are nearby. The important contribution of species with a Pacific affinity was presumably facilitated by the polewards‐flowing Indonesian Throughflow, which links the Pacific Ocean with waters bordering the NWA coast.