Marine Biology 106, 25-37 (1990) Marine ................ BiOlOgy o. ur~i. oo~,., © Springer-Verlag 1990 Factors influencing community structure and distribution of different life-cycle categories of fishes in shallow waters of a large Australian estuary * N.R. Loneragan ** and I. C. Potter School of Biological and Environmental Sciences, Murdoch University, Murdoch, Western Australia 6150, Australia Date of final manuscript acceptance: April 6, 1990. Communicated by G. E Humphrey, Sydney Abstract. Fish were collected at regular intervals over 5 yr (February 1977 to December 1981) from ten shallow-wa- ter sites located throughout the lower, middle and upper regions of the large Swan Estuary in temperate south- western Australia. Analysis of the catch data showed that the total number of species and total density of fishes were both influenced to a greater extent by site and sea- son within the estuary than by year. The number of spe- cies and density of fishes within the whole system were greatest during the summer and autumn, when salinities and temperatures were at a maximum, and declined with distance from the estuary mouth. This reflects the trends shown by marine species, which comprise many species that occur only occasionally in the estuary (marine strag- glers) and others which enter estuaries regularly and in considerable numbers (marine estuarine-opportunists). The density of marine estuarine-opportunists was also correlated with temperature, reflecting the tendency for the species of this category to congregate in the shallows during the summer and autumn. The similar seasonal aggregations of the single anadromous species and repre- sentatives of species that complete their whole life cycle in the estuary were frequently related to spawning. The den- sity of the estuarine category was correlated neither with salinity nor distance from the estuary mouth. The number of estuarine species was also not correlated with distance from the estuary mouth. The density of freshwa- ter species was inversely correlated with salinity and pos- itively correlated with distance from the estuary mouth. The composition of the fish fauna changed progressively through the estuary, with that of the lower estuary being the most discrete. The composition also changed season- ally, particularly in the upper estuary where, during the winter and spring, the volume of freshwater discharge increased greatly and as a consequence the salinity de- clined markedly. The species diagnostic of the lower * Please address all correspondence and requests for reprints to I.C. Potter at Murdoch University ** Present address: Department of Biology, University of Water- loo, Waterloo, Ontario N2L 3G1, Canada estuary were generally marine estuarine-opportunists, whereas those of the upper estuary typically belonged to either the estuarine or anadromous categories. The ma- rine estuarine-opportunist Mugil cephalus was, however, also one of the diagnostic species in the upper estuary during the winter and spring. Introduction Large numbers of the juveniles of some species of marine teleosts enter estuaries of the temperate regions of both the Northern and Southern Hemispheres (e.g. Day et al. 1981, Haedrich 1983, Blaber 1985, Potter et al. 1990). These species have been termed marine estuarine-oppor- tunists to distinguish them from marine stragglers, which are found irregularly and usually in small numbers in estuaries (Lenanton and Potter 1987, Potter et al. 1990). Estuaries are also used as a route by which the catadro- mous and anadromous species of fish migrate between their spawning and main feeding areas (Haedrich 1983, Dando 1984, Claridge et al. 1986). Some species ofteleost are able to complete the whole of their life cycle within estuaries, and a few freshwater species are occasionally found in the upper reaches of estuaries (Day et al. 1981, Haedrich 1983, Dando 1984, Loneragan et al. 1989). The distribution, abundance and community structure of fishes in estuaries are influenced by environmental vari- ables such as salinity and distance from the estuary mouth (Gunter 1961, Kinne 1964, Hoff and Ibara 1977, Blaber and Blaber 1980, Quinn 1980, Weinstein et al. 1980, Allen 1982, Loneragan et al. 1986, 1987, Bell et al. 1988), the effects of which differ among species (Kinne 1964, Whitfield et al. 1981). During the last 13 yr, extensive studies have been car- ried out on the fish faunas of the large Swan Estuary and nearby Peel-Harvey Estuary in temperate south-western Australia (Chubb et al. 1981, Prince et al. 1982, Potter et al. 1983, 1988, Prince and Potter 1983, Chubb and Potter 1984, 1986, Lenanton et al. 1984, Chrystal et al.