The fish fauna of the Vasse-Wonnerup and the impact of a substantial fish kill event

J.R. Tweedley; J. Keleher; A. Cottingham; S.J. Beatty; A.J. Lymbery

Despite the high community value placed on the Vasse-Wonnerup and the many issues it faces, very little was known on its fish communities. This report details the results of the first comprehensive study of the fish communities in the nearshore (shallow) waters of the Vasse-Wonnerup. Over the course of the two year project, over 174,000 fish belonging to 31 species were recorded. Most individuals (94%) were either silversides or gobies, both of which are small fish that spend their whole life in the estuary. However, 18 species of marine fish (and invertebrates such as blue-swimmer crabs) were also found, emphasizing that the Vasse-Wonnerup provides a valuable nursery area for species found in Geographe Bay, many of which are targeted by recreational fishers. The distribution and abundance of fishes in the estuary changes throughout the year, due to the massive increases in salinity that occur during summer and autumn. Generally, the number of species and individuals decreased with increasing distance from the ocean (i.e. in an upstream direction). Higher densities of fish were also recorded during spring and summer than in autumn and winter, as most fish breed during the former period. While the large fish kill in April 2013 was undoubtedly a disturbing event, our analysis of the nearshore fish community failed to detect a change in the fish fauna between the May of 2013 (several weeks after the fish kill) and that same season in 2012. This is because there is a ‘natural’ decrease in the fish fauna at this time of year. Although no data were available on the larger fishes living in the deeper waters, like Mullets and Black Bream before the fish kill, we now have baseline data of those fishes which will be valuable in the long-term monitoring of those stocks. Worryingly, there has been negligible successful breeding of that population since the fish kill. Those deeper waters were found to contain approximately equal numbers of Sea Mullet, Yelloweye Mullet and Black Bream. The oldest Black Bream (which was 38 cm in length) was 13 years old, having been born in 2000. Our preliminary analysis of the Black Bream from the Vasse-Wonnerup also indicated that they were slow growing, due to them consuming a low calorie diet of mainly plant material, rather than molluscs, worms and fish like they eat in other estuaries. The robust baseline data on the entire fish community gathered in the current report will prove extremely valuable in the long-term monitoring of the ecological health of the Vasse-Wonnerup. Moreover, it will allow more informed management decisions to be made, and also better assessment of the efficacy of those decisions. It is recommended that annual monitoring of the fish community occurs in February to detect any changes in the ecological condition of the system. It is also recommended that the biology of Black Bream be investigated further to help guide specific management of that recreationally valuable species to ensure that its stocks remain healthy.

The fish fauna of the Vasse-Wonnerup and the impact of a substantial fish kill event

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