The fish faunas of south coast estuaries: final report to the community

The estuaries on the south coast of Western Australia are important in terms of their ecological, economic and social values. They support diverse and sometimes very abundant fish faunas that are utilised by both recreational and commercial fishers. These fish faunas consist of a combination of species that only spawn within estuarine systems, e.g. Black Bream, those that can spawn within estuaries or in the marine environment, e.g. Estuary Cobbler and Bluespotted Flathead, and those that spawn only in the marine environment and either enter estuaries in large numbers, e.g. King George Whiting, Australian Herring and mullet species, or in small numbers, e.g. sharks and leather jackets.

These estuaries, which are situated in a microtidal region, i.e. with a tidal range that is usually less than 1 m, typically comprise a narrow and short entrance channel, a wide central basin area and the saline lower reaches of their rivers. Sand bars form at the mouths of most of these estuaries and prevent exchange between the estuary and the ocean. Although a few estuaries on the south coast remain permanently open, most are either seasonally open or normally closed to the sea, and many of those that are seasonally open are mechanically breached each year to avoid flooding of agricultural land and infrastructure. The estuaries on this coast have become variably eutrophic through the input of nutrients from surrounding agricultural land and/or markedly hypersaline through increases in salt runoff from cleared areas. The salinities in some estuaries have become so extreme that they have sometimes led to substantial fish mortalities, such as the massive fish kill that occurred in Culham Inlet in 2001 in which 1.4 million Black Bream were estimated to have died.

In light of the continuing degradation and increasing use of many south coast estuaries as a result of population increases on south coast, South Coast Natural Resource Management Inc. and Murdoch University’s Centre for Fish and Fisheries Research have undertaken an intensive two year study of the fish faunas of the Broke, Irwin, Wilson and Stokes inlets, Oyster Harbour and Wellstead Estuary. This study aimed to determine how the fish communities and the population biology of six key species, i.e. Black Bream, King George Whiting, Estuary Cobbler, Australian Herring, Sea Mullet and Yelloweye Mullet, varied among those estuaries and how any differences were related to estuary type, i.e. permanently open vs seasonally open vs normally closed.