Baseline assessment of Carter’s Freshwater Mussel, Westralunio carteri, at proposed bridge construction sites on the lower Vasse River

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