Ichthyofaunas of nearshore, shallow waters of normally-closed estuaries are highly depauperate and influenced markedly by salinity and oxygen concentration

This study determined the ichthyofaunal characteristics of the nearshore, shallow waters of three normally-closed estuaries (Stokes, Culham and Hamersley inlets) on the south coast of Western Australia over three years, during which each system remained closed to the ocean through the recent formation of a sand bar at its mouth. The ichthyofauna in each estuary was depauperate (6 or 7 species) and overwhelmingly dominated by small euryhaline atherinid and gobiid species and a larger sparid, which each complete their life cycle within their natal estuary. The atherinid Atherinosoma elongatum contributed between 74.5 and 94.5% to the ∼168,000 fish collected from these three estuaries. In contrast, marine species were represented solely by six Aldrichetta forsteri (Mugilidae), likely derived from overwash of the bar. The degree to which salinity increased and oxygen declined during the three years of closure varied markedly among the three estuaries. Among estuary basins, the number of species and density of fishes declined most rapidly in Culham Inlet, where mean salinity rose sharply from 52 to 293 and oxygen concentration fell precipitously to 0.6 mg L−1, with only A. elongatum present in salinities up to 147 and oxygen concentrations down to 2.4 mg L−1. In contrast, number of species and density of fishes remained relatively constant in the basin of Stokes Inlet, where salinity rose to only 64 and oxygen concentration declined to only 5.5 mg L−1, with ichthyofaunal and environmental changes in the basin of Hamersley Inlet intermediate between those in Culham and Stokes. While species were variably partitioned between the basin and river, certain species, when subjected to extreme salinities in the basin, moved into the less saline riverine reaches of the estuary, with the isolated pools that form upstream in estuaries during very dry periods providing refugia. In estuaries on the extensive south-western Australian coastline, the number of fish species, and particularly the number and abundance of marine species, undergo an overall progressive decline down the lower west coast and eastwards along the southern coast where they eventually fall to minimal levels. This reflects the overall trend for estuaries to change from permanently-open to the ocean to intermittently-open, to seasonally-open and finally normally-closed.