The impact of small and large impoundments on freshwater mussel distribution in the Hawkesbury‐Nepean River, southeastern Australia

Abstract

The influence of weirs on the distribution of freshwater mussels was investigated in the Hawkesbury-Nepean River, Australia. Distribution of species and densities of size classes were strongly correlated with catchment level factors (e.g. location around a major impoundment, stream order). At catchment scale, weir height, presence of a fish barrier, fish ladder type and position above or below small weirs did not influence the presence/absence of mussel populations. Lower mussel densities in the upper catchment may therefore reflect inhibition of host fish migration.Where present, weir height and geomorphic reach type were linked to differences in densities among species. Geomorphic reach-based differences were reflected by the Hyridella species, but not Velesunio ambiguus. When population structure was described by size class distribution, there were significant differences between densities of small and medium mussels from weirs above, compared to weirs below, a major impoundment, but not for large mussels. Upstream populations may therefore be functionally extinct. Distribution of mussel size classes differed among geomorphic reach types with highest densities for each class found in the least human-impacted reaches. Small mussels were almost invariably found below the major impoundment, most frequently below weirs.Distribution patterns were inconsistent across species, suggesting habitat preference. V. ambiguus and Hyridella australis were most abundant in shale reaches, where assemblages were influenced by fish ladder type. Hyridella depressa and H. australis dominated in sandstone gorges and straights with assemblage density related to weir height. In upper catchment sandstone reaches, mussel assemblages comprising predominantly V. ambiguus were influenced by fish ladder type and weir height.While multiple factors defined localized distribution, large impoundments were linked with reduced population densities. The probable mechanism is the restriction of host fish movement and resulting lack of recruitment. In the Hawkesbury-Nepean River, smaller weirs also seriously impacted recruitment.

The impact of catastrophic channel change on freshwater mussels in the Hunter River system, Australia: a conservation assessment

“…In a landscape-scale study of this system the present distribution of mussels in the Hunter River was assessed with respect to physical habitat characteristics at the reach scale and past distribution. This work complements similar research on mussel populations in the adjoining Hawkesbury-Nepean River system (Brainwood et al, 2006(Brainwood et al, , 2008a. Several strongholds of mussel diversity and abundance were located in the Hunter River system in relatively intact reaches.…”

“…The microhabitats of the species in the Hunter River system are similar to observations made in nearby river systems (Jones, 1983;Brainwood et al, 2008a). The affinities of the Hyridella species for protected pockets along stream margins have implications for the rehabilitation of instream habitats.…”

“…In Australia, mussel populations have undergone declines due to river regulation and intensification of catchment land use (Brainwood et al, 2006(Brainwood et al, , 2008aPlayford and Walker, 2008). This worldwide trend is associated with extinctions, as seen in the extensive loss of endemic mussel species in North America (Bogan, 1993).…”

Impacts of Small Scale Flow Regulation on Sediment Dynamics in an Ecologically Important Upland River

“…There is evidence from fishery survey data that juvenile salmonid densities in the Ehen are below the optimum required for mussel recruitment (Gibbins et al 2004). Moreover, barriers to fish migration can affect mussel distributions (Kelner and Sietman 2000;Brainwood et al 2008;Newton et al 2008). Although Ennerdale weir has a fish pass built into it, the design of the pass may still limit upstream migration and, in turn, constrain dispersal of glochidia (Cascade 2013).…”

“…The work was undertaken in the River Ehen (NW England) which supports an important but declining population of the endangered Freshwater Pearl Mussel (FPM), Margaritifera margaritifera. Small weirs have been shown to impact Australian mussel species (Brainwood et al 2008) so the context of the work was therefore to understand whether flow regulation in the Ehen might be contributing to the declines observed in its mussel population.…”