Habitat preference, environmental tolerance and population viability of Westralunio carteri Iredale 1934, a threatened freshwater mussel of south-western Australia

Le Ma

Freshwater mussels can play major roles in aquatic ecosystems by improving water quality through filter feeding, providing shelter to other animals, and enhancing the connection between benthic and pelagic systems through burrowing, depositing nutrients and releasing biological waste. Carter’s freshwater mussel, Westralunio carteri, is the only freshwater mussel species in south-western Australia and is endemic to the region. The range of W. carteri has declined by 49% over the past 50 years, resulting in its classification as Vulnerable by both international and Australian conservation regulatory bodies. While recent studies have clarified many aspects of the biology of W. carteri, there remains a number of knowledge gaps that are hindering the development of an effective conservation management plan. This study addresses a number of these knowledge gaps, including the population structure and microhabitat preference of extant populations; and the tolerance of mussels to increasing salinity and water emersion, which are believed to be the key ongoing environmental stressors to the species. Living individuals of W. carteri were found in 31 of 47 sampled sites (19 of 30 sampled rivers) and most of these populations had evidence of recent (within the last 10 years) recruitment. Density ranged from 2-169 individuals m-2, with a mean over all sampled sites of 29.5 individuals m-2. Mussels were strongly aggregated in the habitat (variance:mean ratio = 5.9, Lloyd’s Patchiness Index = 3.6). The abundance of W. carteri was positively associated with fine substrate grain size, closeness to the river bank, and the presence and height of debris above the surface of the stream bed, suggesting that burrowing difficulty and protection from water flow are important habitat requirements for the species. Secondary salinisation has been primarily responsible for the dramatic recent decline in the range of W. carteri and remains a major threat to the survival of the species. Laboratory assessment of tolerance to acute increases in salinity in adult mussels from two populations found LC50 values of 5.87-5.96 gL-1, substantially greater than values found in a previous study. Mussels were able to acclimatise to gradual increases in salinity, with gradual LC50 values ranging from 6.43-6.45 gL-1. Furthermore, survival times increased with a slower rate of increasing salinity. Future research should determine both the salinity tolerance of glochidia and juvenile mussels and the non-lethal impacts of increased salinity on mussel fitness, so that the impacts of rising salinity levels in the rivers of south-western Australia can be more accurately predicted. The salinity tolerance of teleost hosts for the glochidia also warrants attention, as the decline in host abundance or presence may also impact mussel viability. South-western Australia has undergone dramatic declines in rainfall and river flow since the mid-1970s and all climate models project this drying trend to continue. In two recent cases of mass mortality in populations of W. carteri associated with drying rivers, larger shells were significantly over-represented in the cohort of dead mussels. Field and laboratory experiments suggested that mussels respond to water emersion by tracking the receding water levels and burrowing into the substrate. There was no relationship between mussel size (shell length) and horizontal movement, but smaller mussels burrowed earlier and were more likely to be completely buried in the substrate. Smaller mussels were also more likely to be removed through predation or scavenging after death. In laboratory experiments, mussels were rapidly killed by water emersion in the open, but shading and burrowing into the substrate increased the tolerance of mussels to emersion, allowing them to survive for at least 60 days out of water. Finer substrate allowed for greater depth of burrowing and increased survival rate, independent of size. These findings have a number of implications: (1) projected future reductions in water flow are likely to increase the mortality rate in W. carteri; (2) mortality risk may be greater for larger, older, more fecund mussels, producing a disproportionate decrease in recruitment; (3) factors that affect burrowing ability, such as presence of appropriate substrate, will have a synergistic effect on mortality rate; (4) the adverse effects of drying rivers may be at least partially averted by increasing riparian shading. The ongoing global decline of ecologically important but less charismatic aquatic fauna is of great concern and can only be halted by increased research, monitoring and management attention. It is hoped the findings of this thesis, along with related research on the ecology of W. carteri, will help the plight of the species and the unique ecosystems upon which it relies.

Habitat preference, environmental tolerance and population viability of Westralunio carteri Iredale 1934, a threatened freshwater mussel of south-western Australia

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