Thermal tolerance and metabolic physiology among redband trout populations in south-eastern Oregon

Streamside measurements of critical thermal maxima (Tcrit), swimming performance (Ucrit), and routine (Rr) and maximum (Rmax) metabolic rates were performed on three populations of genetically distinct redband trout Oncorhynchus mykiss in the high-desert region of south-eastern Oregon. The Tcrit values (29
40
1 C) for small (40–140 g) redband trout fromthe three streams, and large (400–1400 g) redband trout at Bridge Creek were not different, and were comparable to published values for other salmonids. At high water temperatures (24–28 C), large fish incurred higher metabolic costs and were more thermally sensitive than small fish. Ucrit (3
60
1 LF s1), Rr (20013 mg O2 kg0
830 h1) and metabolic power (53322 mg O2 kg0
882 h1) were not significantly different between populations of small redband trout at 24 C. Rmax and metabolic power, however, were higher than previous measurements for rainbow trout at these temperatures. Fish from Bridge Creek had a 30% lower minimum total cost of transport (Cmin), exhibited a lower refusal rate, and had smaller hearts than fish at 12-mile or Rock Creeks. In contrast, no differences in Ucrit or metabolism were observed between the two size classes of redband trout, although Cmin was significantly lower for large fish at all swimming speeds. Biochemical analyses revealed that fish from 12-mile Creek, which had the highest refusal rate (36%), were moderately hyperkalemic and had substantially lower circulating levels of free fatty acids, triglycerides and albumin. Aerobic and anaerobic enzyme activities in axial white muscle, however, were not different between populations, and morphological features were similar.