Abstract
Freshwater mussels and fishes are heavily influenced by thermal stress and can only tolerate increasing temperatures until they reach their individual thermal maximum. Freshwater mussels are ecologically important as ecosystem engineers in aquatic systems and are a First Food (related to the tribal creation belief) for Columbia Plateau tribes like the Confederated Tribes of the Umatilla Indian Reservation (CTUIR). However, globally freshwater mussels have the highest extinction and imperilment rate compared to any other animal group on the planet. The Western Ridged Mussel (Gonidea angulata) and the Western Pearlshell (Margaritifera falcata) in particular have disappeared from areas in their historic range and are experiencing mass die-offs. To undergo metamorphosis to the juvenile life stage, G. angulata utilizes the gills of sculpins (Cottus spp.) and M. falcata uses the gills of salmonid species to host their larval mussels. The purpose of this study was to determine upper thermal tolerances of adult G. angulata and M. falcata, and two sculpin species, Margined Sculpin (C. marginatus) and Paiute Sculpin (C. beldingi). Mussels were collected from the Salmon River, ID and Grande Ronde River, OR, and sculpins were collected from rivers near the Blue Mountains of southeastern WA and northeastern OR. Mussels and fish were transported to the CTUIR Aquatic Laboratory in Walla Walla, WA, where they were acclimated in 13°C water. Upper thermal tolerance was determined using acute and chronic tests. The acute test consisted of increasing the water temperature until critical thermal maximum (CTmax) behaviors were observed. The mean CTmax temperature calculated for the mussels was 34.1°C for G. angulata and 33.2°C for M. falcata, and the mean for the two Cottus spp. was 28.9°C. The chronic test gradually increased the water temperature surrounding the test organism to an arbitrary 2°C below the CTmax determined for each species and maintained that temperature for seven days. Survival was recorded over the 7-day period. In the chronic test, no mussels of either species survived until the end of the 7-day test period, while 58.3% of the sculpins survived. Based on the acute test results, G. angulata and M. falcata have a higher CTmax than their host fish, but based on the chronic test, C. marginatus and C. beldingi have a higher upper thermal tolerance than G. angulata. These differences in thermal tolerance could create a thermal separation between mussel and host fish, thus limiting mussel reproduction. These results will help focus attention on stream reaches in the Columbia River Basin at risk of high summer temperatures where sculpin and mussels are present.