Relationships between latitude and environmental conditions and the species richness, abundance and composition of tropical fish assemblages over soft substrata

Large-scale studies are required to elucidate the environmental factors that structure faunal communities. The relationships between the characteristics of the coastal ichthyofaunas over the soft substrata of tropical north-western Australia and both latitude and environmental factors were thus explored by analysing trawl data obtained for deep and shallow inshore waters at 7 regularly spaced locations along this 1500 km coast during both the dry and wet seasons. In the dry season, species richness and density were greater in the Kimberley and Pilbara bioregions than in the intervening Canning bioregion, where, in contrast to particularly the Kimberley, rivers and mangroves are largely absent. Species richness and density were greatest in the most northern bioregion (Kimberley) during the wet season, when nutrient input from rivers and water temperatures were highest. The high species richness and density at 1 Canning location during the wet season was presumably related to increased productivity brought about by local cyclonic events. Ichthyofaunal compositions in the Kimberley differed markedly from those in the Canning and Pilbara, where tidal range was less and water clarity greater due, in particular, to far greater densities of leiognathids and terapontids. Compositions at all locations in the dry season differed from those in the wet, when chlorophyll a concentrations and/or water temperatures were greatest and large numbers of certain species were recruited. Ichthyofaunal composition at each location almost invariably differed markedly between water depths, reflecting, inter alia, the tendency for some species to use nearshore waters as nursery areas and for others to occupy particular habitats.