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

Abstract

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.

Interactions between Unionida and non‐native species: A global meta‐analysis

“…Directions for future research focused on conservation of the mussel–fish host relationship have been thoroughly reviewed by Modesto et al (). Pertinent to this review, and of particular relevance to New Zealand, where a large proportion of the native fish fauna is diadromous, is the need to consider fish‐passage connectivity when developing strategies for mussel conservation and invasive species management, particularly in relation to unionid source populations (Benson, Close, Stewart, & Lymbery, ; Bódis, Tóth, & Sousa, ). Although fish barriers may restrict native host‐fish movement and thus mussel dispersal, as well as affecting the recruitment of obligate diadromous fish hosts that require access to the sea (Clavero, Hermoso, & Cao, ; Vaughn, ), they may also prevent the spread of potentially unsuitable non‐native fish hosts that would reduce unionid recruitment through the mechanisms highlighted above.…”

Research priorities for freshwater mussel conservation assessment

“…For example, removing dams can harm freshwater mussels by releasing pollutants or destabilizing sediments. Fish passageways to allow migration past dams may be helpful in reconnecting upstream and downstream populations of both mussels and host fish (e.g., Benson et al, 2017). However, past designs often have been ineffective, and fish passageways sometimes are used to justify harmful dams (Brown et al, 2013).…”

Decline of unionid mussels enhances hybridisation of native and introduced bitterling fish species through competition for breeding substrate

“…Therefore, morphologically distinct hybrids should be removed, and the genetic composition of the remaining population monitored using microsatellite markers. Secondly, the construction of fishways would provide accessibility to upstream regions for the host fish and lead to a recovery of habitat for unionids (Benson et al., 2018). Subsequently, larger‐scale habitat restorations need to be implemented to improve stream substrates for the survival of unionid juveniles.…”