The populations and community structures of fishes in two large estuaries of south-western Australia

Abstract

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