Thermal tolerance of Carter’s freshwater mussel - Westralunio carteri Iredale 1934

Dulmini Perera

The Carter’s freshwater mussel Westralunio carteri is a species of freshwater mussel endemic to the southwest region of Australia. During recent decades, the range of W. carteri has declined by almost 50 %, with past research suggesting these declines are primarily a result of secondary salinisation and reduced water permanency. It was predicted that elevated water temperatures in systems with reduced flow and minimal riparian shading were further driving declines, however, no information existed on the temperature tolerance of the species. This thesis aimed to determine the thermal tolerance for W. carteri in the laboratory and relate the resultant parameters to the distribution of mussel populations in the Harvey River, a heavily channelised river system with limited riparian shading. Mussels were exposed to four temperature treatments (20, 26, 32, and 38 C; 40 mussels per treatment) for a period of ten days following an initial acclimation period. Mussel mortality was measured daily, and no mortalities were recorded in the 20 or 26 C treatments during the trial period. Total mortality (100%) was observed in the highest temperature treatment (38 C). Estimated lethal temperature that led to 50% mortality (LT50) was 32.8 C and for 95% mortality (LT95), the temperature was 33.4 C. Temperature loggers were deployed across 29 sites in the Harvey River over the summer period of 2022/2023 and population surveys for W. carteri were conducted at each site at the end of the study. Eleven of the sites recorded populations of W. carteri, with maximum water temperatures at ten of those sites <LT50. The maximum temperature of the sites appeared to have a negative correlation with mussel abundance. The results of this study demonstrate that high temperatures are likely to contribute to the mortality of W. carteri, with peak summer temperatures exceeding laboratory-derived thermal tolerances at numerous sites in the Harvey River. This would likely similarly be the case in many other rivers in this region that lack riparian shading. It is also expected that temperature will become a more prominent contributor to population declines across southwest Australia due to warming associated with climate change. Therefore, active management to restore riparian shading is recommended.

Thermal tolerance of Carter’s freshwater mussel - Westralunio carteri Iredale 1934

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