Metabolism, swimming performance, and tissue biochemistry of high desert redband trout (Oncorhynchus mykiss ssp.):: Evidence for phenotypic differences in physiological function

Redband trout (Oncorhynchus mykiss ssp.) in southeastern Oregon inhabit high-elevation streams that exhibit extreme variability in seasonal flow and diel water temperature. Given the strong influence and potential limitations exerted by temperature on fish physiology, we were interested in how acute temperature change and thermal history influenced the physiological capabilities and biochemical characteristics of these trout. To this end, we studied wild redband trout inhabiting two streams with different thermal profiles by measuring (1) critical swimming speed (U-crit) and oxygen consumption in the field at 12degrees and 24degreesC; (2) biochemical indices of energy metabolism in the heart, axial white skeletal muscle, and blood; and (3) temperature preference in a laboratory thermal gradient. Further, we also examined genetic and morphological characteristics of fish from these two streams. At 12degreesC, maximum metabolic rate (Mo-2max) and metabolic power were greater in Little Blitzen redband trout as compared with those from Bridge Creek (by 37% and 32%, respectively). Conversely, Bridge Creek and Little Blitzen trout had similar values for Mo-2max and metabolic power at 24degreesC. The U-crit of Little Blitzen trout was similar at the two temperatures (61 +/- 3 vs. 57 +/- 4 cm s(-1)). However, the Ucrit for Bridge Creek trout increased from 63 +/- 3 cm s(-1) to 75 +/- 3 cm s(-1) when water temperature was raised from 12degrees to 24degreesC, and the U-crit value at 24degreesC was significantly greater than for Little Blitzen fish. Cost of transport was lower for Bridge Creek trout at both 12degrees and 24degreesC, indicating that these trout swim more efficiently than those from the Little Blitzen. Possible explanations for the greater metabolic power of Little Blitzen redband trout at 12degreesC include increased relative ventricular mass (27%) and an elevation in epaxial white muscle citrate synthase activity (by 72%). Bridge Creek trout had 50% higher lactate dehydrogenase activity in white muscle and presumably a greater potential for anaerobic metabolism. Both populations exhibited a preferred temperature of approximately 13degreesC and identical mitochondrial haplotypes and p53 gene allele frequencies. However, Bridge Creek trout had a more robust body form, with a relatively larger head and a deeper body and caudal peduncle. In summary, despite the short distance (similar to10 km) and genotypic similarity between study streams, our results indicate that phenotypic reorganization of anatomical characteristics, swimming ability at environmentally pertinent temperatures and white axial muscle ATP-producing pathways occurs in redband trout.