Does the composition of the demersal fish assemblages in temperate coastalwaters change with depth and undergo consistent seasonal changes?

The aim of this study was to determine whether the composition of the demer sal fish fauna in coastal marine waters in temperate Australia changes mark edly with increasing water depth and distance from the shore and whether th e composition of the fish fauna in water depths of 5 to 35 m undergoes cycl ic, seasonal changes. Samples of demersal fishes were therefore collected b y trawling over the predominantly sandy substrate at nine sites located in water depths of 5 to 15 m or 20 to 35 m and within 20 km of the shore in fo ur regions along similar to 200 km on the lower west coast of Australia. Th e sampling regime involved trawling for fishes at each site at night in sev en consecutive seasons between the summer of 1990/1991 and winter of 1992. A total of 72 435 fishes, representing 77 families, 143 genera and 172 spec ies was caught. The compositions of the fish faunas in offshore waters with depths of 5 to 35 m were shown to differ markedly from those previously re corded for nearshore marine waters in the same regions. However, as some sp ecies, such as Sillago burrus, S. vittata, S. bassensis and Rhabdosargus sa rba, increase in size, they move out from their nursery areas in nearshore waters into deeper and more offshore waters, where spawning occurs. Ordinat ion showed that, in each of the four regions, the composition of the fish f auna in depths of 5 to 15 m differs from that in depths of 20 to 35 m. This difference is attributable to the fact that some species, such as S. burru s, S. vittata and Upeneichthys lineatus, are far more abundant in depths of 5 to 15 m, whereas other species, such as S. robusta, U. stotti and Lepido trigla modesta, occur predominantly in depths of 20 to 35 m. However, the s amples collected from the single site that was inshore but in deeper water demonstrate that the composition of the fish fauna is influenced by distanc e from shore as well as by water depth. The compositions of the fish faunas differed with latitude, largely due to the fact that some subtropical spec ies, such as Polyspina piosae, S. burrus and S. robusta, did not extend dow n into the more southern regions. Ordination also showed that the compositi on of the fish faunas at all but one of the nine sites underwent pronounced and consistent cyclic, seasonal changes. This seasonal cyclicity at the di fferent sites was attributable to sequential patterns of immigrations and e migrations by a number of fish species during the course of the year. These seasonal migrations involved, inter alia (1) movements of certain species from their nursery areas into these deeper waters, e.g. S. bassensis and Sc obinichthys granulatus; (2) migrations into and off the sandy areas of the inner continental shelf, e.g. Arnoglossus muelleri; (3) migrations to spawn ing areas, e.g. Sillago robusta; and (4) movements into areas where detache d macrophytes accumulate in winter, e.g. Cnidoglanis macrocephalus and Apog on rueppelii.