Mitigating the impact of Serpentine Dam works on Carter’s Freshwater Mussel

In order to mitigate the potential negative impacts on aquatic fauna caused by the draining of impoundment reservoirs, surveys and management programs within numerous reservoirs in south-western Australia have been successfully undertaken over the past decade (e.g. Beatty et al. 2003; Molony et al. 2003; Molony et al. 2005; Morgan & Beatty 2004; Beatty et al. 2006; Klunzinger et al. 2011a). Freshwater mussels are important in maintaining functional freshwater ecosystems (Vaughn & Hakenkamp 2001) with a great capacity to filter plankton, algae and other particulate material from the water column, thus maintaining water clarity and quality. Their empty shells also provide refuge for other freshwater fauna, including juvenile Marron (Cherax cainii and Cherax tenuimanus), gobies (Pseudogobius olorum and Afurcagobius suppositus) and other freshwater crayfishes. They oxygenate the sediments through their burrowing habit, as evidenced by tracks that can be seen in the sediments. They are also a favoured food item by water birds (e.g. Porphyrio porphyrio, Threskiornis molucca, Threskiornis spinicollis), water rats (Hydromys chrysogaster), aquatic reptiles and Marron (see Vestjens 1972; Woollard et al. 1978; Shannon & Mendyk 2009; Klunzinger et al., unpublished data). Their sensitivity to environmental changes makes freshwater mussels important bio-indicators of freshwater ecosystem health (Ponder & Walker 2003). Most freshwater mussels have a life cycle intricately associated with freshwater fishes. The larvae of most species have an obligate parasitic phase which generally requires a fish as a host for development and dispersal. A recent study by Klunzinger et al. (2012a) has revealed that glochidia (parasitic larvae) of Carter’s Freshwater Mussel (Westralunio carteri), the only freshwater mussel species found in south-western Australia, undergo metamorphosis to the juvenile stage on most native freshwater fishes and Eastern Gambusia (Gambusia holbrooki), but not Goldfish (Carassius auratus) or Pearl Cichlid (Geophagus brasiliensis). Failure of these feral species to support the life cycle of W. carteri has serious implications for the species in areas dominated by these feral fishes. Glochidia were generally restricted to the fins of host fishes and virtually non-existent on the gills or body. The fishes which were exposed to glochidia in captivity showed no ill effect. Rather than being a threat to fish stocks, freshwater mussels improve and maintain water quality in freshwater rivers, lakes and other water bodies, which benefit freshwater fishes (Treasurer et al. 2006). Glochidia attachment to host fishes is a natural phenomenon and a necessary part of the mussel’s life history, which also functions as a dispersal method. In the host fishes which were experimentally exposed to glochidia of W. carteri, juvenile mussels (i.e. metamorphosed glochidia) detached from the fishes 21-27 days post-exposure (Klunzinger et al. 2012a). Westralunio carteri is a seasonal spawner, spawning in late winter (July); embryos are brooded until mid-spring/early-summer developing into mature glochidia, which are released on mucus strands in response to warming water temperatures and external stimuli during late August, peaking in October to November. Females are virtually spent by December (Klunzinger 2011; Klunzinger et al. 2011b, 2012a and unpublished data). Little is known about the juvenile stage of freshwater mussels due to their difficulty to locate in wild populations, but mortality is generally thought to be high from predation by other macroinvertebrates and they are quite sensitive to interstitial ammonia (Walker et al. 2001). Results of a PhD study have shown that W. carteri has undergone major population loss from salinity, climate change and human activity (Klunzinger et al., unpublished data), suggesting the species is probably more threatened than previously believed to be and its current IUCN listing as ‘Least Concern’ is probably inappropriate. Significant mortalities of the species resulting from water removal and dehydration within the Helena Pipehead Dam, necessitated the translocation of remaining live mussels (n = 1205) into a refuge pool downstream from the dam in April 2011 (Klunzinger et al. 2011a). The current study is the second to focus on freshwater mussels within reservoirs of the south-west