The Swan Estuary was formed as a drowned river valley about 6000 years ago 43 and is consequently deeper than the Peel-Harvey Estuary. 44 This estuary, which flows through the city of Perth, the capital of Western Australia (Figure 1), receives water from the Avon River and Swan coastal catchments, which together cover an area of approximately 129,000 km2. The zone of estuarine influence extends approximately 60 km upstream in the Swan River. 28 The upper estuary, which comprises the tidal reaches of the Swan and Canning Rivers, expands into the large basins of the middle estuary, which in turn enter a long narrow inlet channel before discharging into the sea at Fremantle (Figure 1). The basin of the Swan Estuary comprises marginal shallows of 2 to 3 m depth and channels that are up to 21 m in depth. 45 Average annual rainfall for Perth is 872 mm, with the majority of the rain falling between the months of May and August. 28,46 This estuary is exposed to a highly seasonal pattern of river flow, resulting in the upper parts of the estuary becoming fresh in winter and markedly saline in summer. 47 The salinity in the waters of this estuary rarely exceed full strength sea water (35‰) and, in those areas where this does occur, it is never by more than 3-4‰. 47
Since nutrients tend to settle in the sediments of the deep channels of the broad expanses of the basins of the Swan, they are below the photic zone and are therefore not available to either macrophytes or phytoplankton. Furthermore, the Swan Estuary is far better flushed than the Peel-Harvey. Thus, although the Swan Estuary is nutrient-enriched, and its upper reaches can be regarded as eutrophic, the basins and entrance channel are essentially mesotrophic. 48 The basins of the estuary, in which the commercial fishery is largely based, do not experience the same large phytoplankton blooms and massive growths of macrophytes that occur in the basins of the Peel-Harvey Estuary.
The Swan Estuary contains an important commercial and recreational fishery.
The commercial fishery is similar to that of the Peel-Harvey and is likewise
managed as a limited entry fishery. The total commercial catch in the financial
year 1990/91 was 112 t. 39 The
major commercial species have typically been the sea mullet, cobbler, yelloweye
mullet, Perth herring (Nematalosa vlaminghi) and to a lesser extent the
blue manna crab. 49-52
The most important recreational species are the cobbler, black bream (Acanthopagrus
butcheri), tailor, mulloway, western school prawn (Metapenaeus dalli),
blue manna crab and western king prawn. 52,53