Fish species compositions over reefs and soft substrates in north-western Australia

Trawling and trapping was conducted in water depths of 5 to 30 m at seven locations along the Pilbara, Canning and Kimberley coasts of north-western Australia. 361 species were recorded, of which only 56 were recorded over both soft substrates and reefs. 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 habitat types changed progressively with increasing latitude and grouped according to bioregion. Different species were responsible for these latitudinal and bioregional differences in the two habitat types. Indo-west Pacific species comprised 31.2 and 24.2% of the number of species over soft substrates and reefs, respectively, and contributed 72.5% to the total catch over soft substrates, but only 9.7% over reefs. Australian endemics made an important contribution in both habitat types.The ichthyofaunal compositions of both habitat types undergo similar patterns of progressive change with latitude, due to site-staggered changes in the relative abundances of key fish species. The latitudinal differences over reefs are correlated with water temperature. Bioregional differences in composition can be related to such factors as turbidity and whether mangroves are nearby. Although the Indo-west Pacific was the most speciose of the ten main zoogeographical categories over both habitat types, its relative contribution to total catches was 7.5 times greater over soft substrates than over reefs.