Abstract
Among the many environmental conditions that structure estuarine fish assemblages, salinity is perhaps the most influential. While most estuaries have a positive salinity gradient, where salinity declines in an upstream direction, and several exhibit the reverse pattern, very few experience both salinity regimes within a typical year. One estuary that does, however, is the shallow Vasse- Wonnerup Estuary, which receives no freshwater discharge during summer. In this system, salinity ranges from 0 (upstream) to 15 (mouth) during winter and 130 (upstream) to 35 (mouth) in summer. The fish faunas of the shallow and deeper waters were sampled on a seasonal basis for two years. The nearshore fish fauna was dominated by atherinids and gobies, which complete their life cycle within the estuary and are highly euryhaline, but also included 18 species of marinespawning fish that utilise the system as a nursery area. Species richness and density decreased with increasing distance from the ocean and faunal composition changed markedly seasonally, in response to massive changes in salinity. The fish of the deeper waters comprised predominantly marine-spawning species, with the notable exception of Black Bream, which is solely estuarine. This species, together with two mugillids, largely dominated these deeper waters and their abundances remained relatively consistent. However, the prevalence of other species varied with changes in salinity and/or the frequency and duration of bar openings. This estuary is used as model for predicting the impact of climate change on seasonally-open estuaries in southern Australia, which will become more saline in the future.