The impact of environmental changes on the fish faunas of Western Australian south-coast estuaries

Data on the fish communities and environmental conditions in three normally-closed estuaries (Stokes, Culham and Hamersley inlets) on the central south coast of Western Australia have been obtained seasonally for three years. The sampling regime and analyses were designed so that the data and their implications would be of value to both fisheries and environmental managers. Salinities in all three estuaries rose as a result of a combination of salt loading through land clearing, dry winters and high evaporation rates during summer. These increases were most marked in the Culham and Hamersley inlets, eventually resulting in the salinities in these two estuaries exceeding by several times that of sea water. Massive mortalities of Black Bream occurred in these two estuaries when salinities were approximately twice that of sea water, a finding that has been published in an international journal. The development of extremely high salinities was accompanied by a reduction in the number of species and density of fish in Culham and Hamersley inlets, with only a small species of hardyhead surviving when salinities reached levels equivalent to four times sea water. Dietary data emphasise that Black Bream is a highly opportunistic omnivore and thus able to withstand major changes in potential food types. Survival by Black Bream over several years was greatest in Stokes Inlet, the most environmentally stable estuary. Growth of Black Bream varied greatly among estuaries, which appeared to reflect differences in density rather than diet. The results emphasise that (1) the stocks of Black Bream can only be sustained permanently in the basins of estuaries if the quality of environmental conditions in those systems is maintained at an appropriate level and (2) upstream pools can act as refugia for Black Bream when extreme conditions exist downstream