The Peel-Harvey Estuary in south-western Australia covers an area of ca 136km2. The natural entrance channel at Mandurah is ca 5km long and opens into the north-western corner of the circular Peel Inlet, which occupies an area of ca 75km2. The south-western corner of the Peel Inlet in turn opens into the elongated Harvey Estuary, which has an area of ca 56km2. The Serpentine and Murray rivers discharge into the north-eastern corner of the Peel Inlet, which the Harvey River discharges into the southern end of the Harvey Estuary.

The discharge of nutrients into the Peel-Harvey Estuary from agricultural land and piggeries during the 1970s and 1980s resulted in the development of massive growths of macroalgae in Peel Inlet and prolific seasonal grows of the toxic blue-green algae Nodularia spumigena in the Harvey Estuary. In 1994, an artificial channel was opened between the northern end of the Harvey Estuary and the ocean at Dawesville in order to increase the amount of water exchanged between the estuary and the ocean, and thereby facilitate the flushing of nutrients out to sea, and to raise salinities in the Harvey Estuary to levels that would restrict the germination and growth of the blue-green algae.

The aim of this study on the Peel-Harvey Estuary was to determine the influence of the Dawesville Channel on such features as the migratory patterns, abundances, size compositions and distributions of the blue swimmer crabs and western king prawns, the species composition of the fish fauna, and the abundances, distributions and commercial catch of the main commercially-fished species. Relevant biological data were thus collected for crustaceans and fish in the Peel-Harvey Estuary between March 1995 and July 1998, i.e. post-Dawesville Channel, and compared with data collected for the same sampling sites in periods between July 1979 and April 1988, i.e. pre-Dawesville Channel.

Our results demonstrate that the blue swimmer crab and western king prawn are now present in far greater numbers and for far longer periods in the Harvey Estuary than was the case prior to the construction of the Dawesville Channel.