Alan Lymbery

Freshwater mussels are important components of freshwater ecosystems as they can act as ecosystem engineers and improve water quality through filtration. Carter’s Freshwater Mussel, Westralunio carteri, is the only freshwater mussel species in the south ‐ west of Western Australia and is classified as Vulnerable on the IUCN Red List of threatened species, and as Threatened under the Western Australian Wildlife Conservation Act 1950. The species has lost approximately 50% of its former habitat through salinisation and habitat degradation. City of Busselton proposes to undertake an extension of the existing Causeway Road Bridge and additionally, construct a new bridge (the ‘Eastern Link’) east of the existing footbridge, on the Lower Vasse River (hereafter referred to as the ‘impact sites’). As W. carteri is known to inhabit the Lower Vasse River, construction at the proposed bridge development sites had the potential to affect mussels, should they be present at or near the impact sites; particularly given the proponents existing data that suggests bridges can attract the species. Therefore a baseline assessment of the distribution and population density and structure of the species at the impact and reference sites was undertaken with the aim of assessing the likelihood of impact on the species and provide recommendations to mitigate any impacts identified associated with the proposed bridge development activities. The study revealed that Carter’s Freshwater Mussel were present at both of the impact sites and also at each reference site upstream and downstream of the bridge development sites. Mussel density was greater at the impact sites compared with the reference sites, possibly reflecting its preference for occupying habitats under and adjacent to bridges. Given the disturbance of the river bed and potential increased turbidity and reductions in dissolved oxygen that may occur due to resuspension of anoxic sediments (including Monosulfidic Black Ooze) associated with the construction phase of the bridge developments, it is recommended that active management of the species occurs to mitigate the effects of the bridge construction at the impact sites.

This report contains the findings of the first survey of the exotic bacterium Edwardsiella ictaluri in wild freshwater fish populations in Australia. Edwardsiella ictaluri causes enteric septicaemia of catfish (ESC), which is a serious disease of farmed channel catfish in the USA. The bacterium has previously been detected in imported ornamental fish and in native catfish held in Australian aquarium facilities, but wild fish populations in Australia are considered free of the disease. The Australian Government Department of Agriculture, through the Fisheries Research and Development Corporation, funded an active surveillance program to provide further evidence for this claim of disease freedom.

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 current project aimed to provide a comprehensive quantitative understanding of the fishes and crayfishes of the VWWS across the full range of habitats in order to assess the current and future health of the ecosystem and guide effective management decisions and monitoring programs. Given the impact of introduced fishes, the study had a specific simultaneous aim to document, prioritise and control introduced fishes and crayfishes to directly address a major known impact on aquatic ecsystems. Finally, it aimed to provide management recommendations that could be implemented to mitigate the impacts of existing and future introduced species.

Introduced fishes are a major threat to the ecological health of the Vasse-Wonnerup Wetland. Having recently completed an extensive 18 month removal program, it is imperative that we minimise future introductions to the system. This is best done by implementing a community education program. As such, this project produced and distributed a leaflet to allow people to identify native and introduced fishes and provide advice on how to dispose of unwanted aquarium fish to avoid them being introduced to local waterways. We then held a community workshop to launch the leaflet and raise local awareness. Copies of this leaflet are now with local government/management and NRM groups, pet stores in the Busselton area and available for download from a number of websites.