Abrupt shifts in the fish community of the hydrologically variable upper estuary of the Swan River

Abstract

AbstractFish were sampled in nearshore, shallow waters (<1·5 m deep) and offshore, deeper waters (2·5–5 m) of the saline lower reaches of the Swan and Canning Rivers, which collectively represent the upper Swan–Canning Estuary, by using seine and gill nets, respectively, in each season between winter 1995 and autumn 1997. Seventeen of the 34 fish species caught during the study spawn in the upper estuaries. These species, which include the semi‐anadromous Nematalosa vlaminghi, comprised 50·0% of the number of species and 88·8% of the number of fish in shallow waters and 43·8 and 89·9%, respectively, of those in deeper waters. The two most abundant species in shallow waters, Engraulis australis and N. vlaminghi, contributed 36·0 and 19·6%, respectively, to the total numbers of fish in those waters, and the latter species comprised nearly 50% of the catch in deeper waters. During winter, when freshwater discharge increased sharply and salinities declined precipitously, the number of species and abundance of fish in shallow and deeper waters were generally at their lowest and the species composition was the most discrete. This reflected the downstream movement out of the upper estuary of substantial numbers of individuals of species such as N. vlaminghi, Acanthopagrus butcheri, Amniataba caudavittata, Atherinomorus ogilbyi, Atherinosoma elongata and Craterocephalus mugiloides and the upstream movement into the upper estuary of juvenile Mugil cephalus. The overall fish fauna then changed quite abruptly in spring, when large N. vlaminghi, A. butcheri and A. caudavittata became abundant as they migrated into the upper estuary where they then spawned. During summer and autumn, the fauna in the shallows then changed more gradually through, inter alia, the recruitment of juvenile A. caudavittata and Sillago burrus and the influx of the small atherinids A. elongata and C. mugiloides. The abrupt change in the fish community in winter contrasts with the gradual, cyclical changes undergone during the year by the ichthyofaunas of holarctic, macrotidal estuaries, such as the Severn Estuary in the UK, in which the hydrology does not undergo such sudden, extreme seasonal changes and the ichthyofauna is dominated by marine and diadromous species that each enter the estuary at specific, but varying times of the year. Differences between the fish faunas of the shallow waters of the Swan and Canning Rivers are probably related to differences in the hydrological characteristics and locations of the mouths of these two tributaries. Copyright © 2001 John Wiley & Sons, Ltd.