Changes to demersal fish communities in Port Phillip Bay, Australia, over two decades, 1970-91

Changes to the structure of fish communities in Port Phillip Bay between 19 70 and 1991 were analysed by trawling at 14 stations sampled regularly duri ng 1970-75 and in 1990/91. Differences between stations were compared by us ing Bray-Curtis dissimilarity indices, multidimensional scaling and analysi s of variance for the most abundant species. Differences between two sampli ng periods in the 1970s appear to be the result of the use of different ves sels for trawling. A third vessel was used during 1990/91, but differences in catches between 1972-75 and 1990/91 appear to represent real temporal ch anges rather than differences between trawl efficiencies. Between 1972-75 a nd 1990/91, increased fishing pressure is the most likely explanation for d eclines in several important commercial and recreational species. A consequ ent decrease in competition may have caused an increase in the abundance of stingarees. A decline in seagrass abundance in the western bay probably re duced the abundance of several seagrass-dependent species. The Japanese gob y was introduced into the bay after the mid 1970s, and populations of littl e rock whiting in the western bay and globefish in deep regions of Port Phi llip Bay appear to have increased because of the recent establishment of ad ditional exotic invertebrates.