Thermal Metabolic Performance of Wild Juvenile Oncorhynchus mykiss in the Lower Tuolumne River: A Case for Local Adjustment to High River Temperature

Oncorhynchus mykiss and other fishes can adjust to local thermal conditions. This study tested the hypothesis that the Tuolumne River O. mykiss population below La Grange Diversion Dam is locally adjusted to the relatively warm, summer river conditions. Wild juvenile O. mykiss (100-160 mm fork length) were locally caught, tested, and returned safely within one day of capture to the Tuolumne River. We used swim tunnel respirometers to measure the fish’s basic oxygen requirement (routine metabolic rate; RMR) and maximal oxygen demand when it swam maximally (maximum metabolic rate; MMR). In total, 44 individual fish were tested at temperatures from 13°C to 25°C and paired measurements of RMR and MMR were obtained for 37 fish. We calculated absolute aerobic scope (AAS=MMR-RMR), which is the capacity of each fish to supply oxygen to tissues above and beyond a basic routine need.  The AAS and factorial aerobic scope (FAS = MMR/RMR) were used to define the optimum thermal range for the Tuolumne River O. mykiss.  With this approach we apply the basic tenet that every activity of a fish requires oxygen above a basic need and that salmonids have evolved to maximize their oxygen supply to fuel muscles during exhaustive swimming.