Neil Loneragan

The fish fauna of the large and adjacent Swan and Peel-Harvey estuaries in temperate south-western Australia, were sampled at regular intervals using beach seines, gill nets and otter trawls at a number of sites throughout these systems. A total of 673 037 individuals, representing 38 families and 76 species were caught in the Swan Estuary compared with 144 372 individuals, 29 families and 55 species in the PeelHarvey Estuary. Although the Clupeidae, Terapontidae, Mugilidae, Apogonidae and Atherinidae were the most abundant families in each system, the important species within the families differed between the estuaries. Of the 15 most abundant species in the shallows of the Swan Estuary, seven were marine teleosts which entered the estuary regularly and in large numbers (marine estuarine-opportunists), seven completed their life cycle within the estuary (estuarine) and one (Nematalosa vlaminghi) was anadromous. The contribution of individuals of the marine estuarine-opportunist category to catches in the shallows declined from nearly 95% in the lower estuary, to approximately 17% in the middle estuary and 6% in the upper estuary. The estuarine and anadromous groups together comprised 83 and 94% of the catches in the middle and upper estuaries, respectively. By contrast, marine estuarine-opportunists were the most abundant group in all regions of the Peel-Harvey, including the saline reaches of tributary rivers. The number of species and density of fish in the shallows of the Swan and PeelHarvey systems declined with distance from estuary mouth and rose with increasing salinity and temperature. Classification and ordination of the data from the shallows of both estuarine systems distinguished the ichthyofauna of the saline reaches of the rivers from that of the lower reaches of the estuary. However, the faunal composition of the middle estuary of the Swan was also relatively distinct from those of the lower and upper estuary. The number of species and catch rates in the deeper waters of the Peel-Harvey were influenced to a greater extent by salinity, than those in the shallows. This implies that the larger fish which characterise the deeper waters may thus be less tolerant to low salinities than the smaller fish, typically found in the shallows. Site within the Swan Estuary generally influenced the densities of individual species to a greater extent than either season or year, or the interactions between these factors. When seasonal effects were pronounced, they could be related to summer spawning migrations into the upper estuary (N. vlaminghi, Amniataba caudavittatus ), spring immigrations into the lower estuary (Mugil cephalus ) or winter movements into deeper and more saline waters (Apogon rueppellii ). Marked annual variations in the density of Torquigener pleurogramma were related to large differences in the recruitment of the 0+ age class between years.

A method of processing data has been developed to facilitate the storage, retrieval and analysis of data collected during ecological studies of fish populations. This method was also used to compare the composition of the fish community in the shallow water of the Peel-Harvey and Swan-Avon estuaries. A total of 42 species representing 22 families were recorded from the Peel-Harvey compared with 50 species and 28 families in the Swan-Avon. Fifteen species accounted for approximately 95% of the total numbers of fish caught in both estuaries. The most prevalent species in the Peel-Harvey was the marine species, Pelates sexlineatus, whose abundance might be related to the presence of large quantities of Cladophora, which constitutes a major proportion of its diet and which is found throughout much of the Peel Inlet. By contrast, the estuarine species, Nematalosa vlaminghi, was the most abundant species in the Swan-Avon in which it utilised both the riverine regions of the upper estuary and also the broad expanses of water in the middle estuary. Amongst the 15 most abundant species, a greater number were represented by populations capable of completing their entire life cycle within the estuary in the Swan-Avon (9) than in the Peel-Harvey (6). The relative abundance of estuarine species was also higher in the Swan-Avon than in the Peel-Harvey. Species richness decreased through the Peel-Harvey but reached a maximum in the middle estuary of the Swan-Avon.