Diel, seasonal, regional and annual variations in the characteristics of the ichthyofauna of the upper reaches of a large Australian microtidal estuary

Nearshore, shallow waters in three regions along ca. 14.5 km of the upper Swan River Estuary in south-western Australia, which undergo marked cyclical annual changes in freshwater discharge, and thus also in salinity, were sampled for 24 consecutive months. The atherinid Leptatherina wallacei and the gobiids Pseudogobius olorum and Afurcagobius suppositus dominated the ichthyofauna, ranking, in terms of abundance, first, second and third, respectively, in each of the upstream, middle and downstream regions. These three species and 11 others, which also complete their life cycles within estuaries, constituted 46.4% of the number of species recorded and contributed 93.5% to the total catch of fish. The characteristics of the ichthyofauna in the upper reaches of this microtidal estuary underwent annual cyclical changes and differed between day and night, regions and years. The number of species and density of fish peaked during summer and typically declined to their minima in winter and then rose markedly in spring. The annual cyclical change undergone by the species composition of each region during both the day and night was due to the densities of certain species peaking at different times of the year. The cyclical changes in composition were less conspicuous in the year when, in the summer, highly atypical heavy rainfall led to a sharp rise in freshwater discharge and thus a pronounced decline in salinity. The number of species and density of fish were far greater at night than during the day and the species composition underwent diel changes. These differences were due, in particular, to relatively greater abundances and frequencies of occurrence at night of the estuarine species A. suppositus, P. olorum, L. wallacei and Acanthopagrus butcheri and of the introduced freshwater species Gambusia holbrooki. Overall, marine species were relatively more abundant in the downstream region, whereas the reverse was true for the freshwater and semi-anadromous species. The freshwater species G. holbrooki, the estuarine A. suppositus, the semi-anadromous Nematalosa vlaminghi and the marine species Mugil cephalus were relatively more abundant in the upstream and middle regions, while the opposite was true for the marine species Aldrichetta forsteri and the estuarine species P. olorum, A. butcheri and Amoya bifrenatus, the last of which is also represented by discrete marine populations.