Leschenault Inlet is a long, coastal "lagoon-like" estuary covering an area of 27 km² and rarely exceeds 2 m in depth. ³⁷ This estuary, which is located in an interdunal depression, comprises the drowned lower reaches of the Preston and Collie River systems. The overall catchment area of the estuary is 2900 km². The mouth of this estuary, which is represented by an artificial cut constructed in 1951, is located just north of the town of Bunbury, approximately 150 km south of Perth and 85 km south of Mandurah (Figure 1 ). Leschenault Inlet, which is supplied by the Collie and Preston Rivers, remains essentially marine for much of the year, with the northern (upper) end becoming hypersaline in summer. ³⁷

Average annual rainfall in the region is 875 mm, most of which falls between May and August. Evaporation is lower than in the Peel-Harvey Estuary, averaging 1200 mm annually.

Glover ²⁹ describes Leschenault Inlet as mesotrophic to mildly eutrophic. Although the areal biomass of macrophytes in Leschenault Inlet is similar to that in the Peel-Harvey (123 and 160 g dry wt m^-2 , respectively), the species compositions of the macrophytes in the two systems differ markedly. ³⁷ Thus, in contrast to the Peel-Harvey, the macrophytes of Leschenault Inlet contain a larger contribution by seagrasses (>30% of biomass) and brown algae (>20% of biomass) and a far lower contribution by green algae. ³⁷

The commercial fishery of Leschenault Inlet in the financial year 1990/91 landed 123 t, i.e., almost the same as that of the Swan Estuary and nearly half that of the Peel-Harvey Estuary. ³⁹ The major commercially exploited species are yelloweye mullet, sea mullet, Perth herring, King George whiting, western sand whiting (Sillago schomburgkii), cobbler and the blue manna crab. The two whiting species, together with cobbler, western school prawn, western king prawn and the blue manna crab, are major components of the recreational fishery of Leschenault Inlet.