Composition of the fish fauna of a permanently open estuary on the southern coast of Australia, and comparisons with a nearby seasonally closed estuary

Abstract

The fish faunas of the outer basin (Nornalup Inlet), inner basin (Walpole Inlet) and saline region of the main tributary (Frankland River) of the permanently open Nornalup-Walpole Estuary on the southern coast of Western Australia, were sampled bimonthly for a year using seine and gill nets, and also during a further two months by the former method. Although the Nornalup-Walpole Estuary is permanently open, the catches of fish in its shallows were dominated (98.4%) by estuarine-spawning species, thereby paralleling the situation in the nearby and seasonally closed Wilson Inlet. In contrast, larger representatives of several marine species were present in appreciable numbers in the offshore, deeper waters of both of these estuaries. The delayed recruitment of marine species into these estuaries apparently reflects the distance that the juveniles of these species have to travel from the areas where they are believed predominantly to spawn. The larger representatives of marine species made a greater contribution to the fish faunas of the offshore, deeper waters in the Nornalup-Walpole Estuary than in Wilson Inlet (64.5 vs 36.9%) and, unlike the situation in the latter estuary, they included five species of elasmobranchs, two of which (Mustelus antarcticus and Myliobatis australis) were relatively abundant. Classification and ordination of the combined data for both estuaries demonstrated that the composition of the fish fauna in the offshore, deeper waters of the outer basin of the Nornalup-Walpole Estuary was particularly distinct, with some marine species being restricted to these waters. 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 estuary 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 Inlet were more similar to each other than they were to those in the more seawards end of either estuary. This similarity reflects the apparent preference of certain teleosts, such as the estuarine species Acanthopagrus butcheri and the marine species Mugil cephalus and Aldrichetta forsteri for reduced salinities and/or features associated with riverine environments.