Upstream recolonization by freshwater mussels (Unionoida: Hyriidae) following installation of a fishway

Freshwater mussels provide important benefits to aquatic ecosystems by filtering water, bioturbating sediments, and cycling and transforming nutrients. The global decline in mussel diversity, distribution and abundance has led to concerns that ecological functioning in freshwater systems will be diminished. Mussels from the order Unionoida have an obligate larval stage that parasitizes a fish host, developing into a juvenile while being dispersed throughout the ecosystem. Barriers that obstruct fish movement can lead to localized extinctions of fish and mussels. In many cases, fishways have successfully restored habitat connectivity for fish; however, mussel recolonization is rarely assessed. This paper provides evidence for recolonization by Carter's freshwater mussel (Westralunio carteri, Iredale 1934) in habitats upstream of a weir following fishway installation. Mussels were present at all sites both above and below the weir, although they were far more abundant downstream. A lack of larger size classes upstream highlights the historical lack of recruitment in that area. Recent recruitment post-fishway installation suggests that the population will eventually recover above the weir. The return of mussels above the weir is likely to benefit the ecosystem owing to the key role mussels play in aquatic habitats. Fishways may therefore be an important tool for the restoration of mussels, and broader ecological functioning.