COMPOSITION OF THE FISH FAUNA OF A PERMANENTLY OPEN ESTUARY ON THE SOUTHERN COAST OF AUSTRALIA, AND COMPARISONS WITH A NEARBY SEASONALLY CLOSED ESTUARY

The fish faunas of the outer basin (Nornalup Inlet), inner basin (Walp ole Inlet) and saline region of the main tributary (Frankland River) o f the permanently open Nornalup-Walpole Estuary on the southern coast of Western Australia, were sampled bimonthly for a year using seine an d gill nets, and also during a further two months by the former method . Although the Nornalup-Walpole Estuary is permanently open, the catch es of fish in its shallows were dominated (98.4%) by estuarine-spawnin g species, thereby paralleling the situation in the nearby and seasona lly closed Wilson Inlet. In contrast, larger representatives of severa l marine species were present in appreciable numbers in the offshore, deeper waters of both of these estuaries. The delayed recruitment of m arine species into these estuaries apparently reflects the distance th at the juveniles of these species have to travel from the areas where they are believed predominantly to spawn. The larger representatives o f marine species made a greater contribution to the fish faunas of the offshore, deeper waters in the Nornalup-Walpole Estuary than in Wilso n Inlet (64.5 vs 36.9%) and, unlike the situation in the latter estuar y, they included five species of elasmobranchs, two of which (Mustelus antarcticus and Myliobatis australis) were relatively abundant. Class ification and ordination of the combined data for both estuaries demon strated that the composition of the fish fauna in the offshore, deeper waters of the outer basin of the Nornalup-Walpole Estuary was particu larly distinct, with some marine species being restricted to these wat ers. This is presumably related both to the presence of a permanently open entrance channel and the relatively deep waters found in Nornalup Inlet, which allow the ready exchange of water between the sea and es tuary and the maintenance of high salinities in the deeper regions of the outer basin for much of the year. The fish faunas in Walpole Inlet and the tributaries of both the Nornalup-Walpole Estuary and Wilson I nlet were more similar to each other than they were to those in the mo re seawards end of either estuary. This similarity reflects the appare nt preference of certain teleosts, such as the estuarine species Acant hopagrus butcheri and the marine species Mugil cephalus and Aldrichett a forsteri for reduced salinities and/or features associated with rive rine environments.