Migrating wild (W) and hatchery‐reared (H) chinook salmon Oncorhynchus tshawytscha and steelhead Oncorhynchus mykiss juveniles were sampled after loading into fish‐transport barges at Lower Granite Dam on the Snake River, Washington, and after barge transportation downstream to Bonneville Dam on the Columbia River. Stress indices (increased plasma cortisol and glucose concentrations and decreased plasma chloride concentrations) were higher (P < 0.001) for chinook salmon sampled during midseason (early to mid‐May), when fish loading densities in barges were at seasonal maximums, than were stress indices for those sampled earlier or later. Cortisol concentrations in chinook salmon were correlated with steelhead densities after loading of barges (P < 0.0001, R2 = 0.41) and after arrival of barges at Bonneville Dam (P < 0.0001, R2 = 0.65). Cortisol concentrations were not correlated with gill Na+, K+–adenosine triphosphatase activities, which were higher in W than in H fish of both species. Cortisol concentrations were higher (P < 0.0001 in 1994, P = 0.02 in 1995) in W than in H chinook salmon, and concentrations declined in both groups during barge transportation early and late in the migration season but not during midseason. In contrast, cortisol concentrations were lower (P < 0.001) in W than in H steelhead, were not correlated with steelhead loading densities, and declined in both W and H fish during barge transportation on all sampling dates. Electrolyte disturbances were greater in chinook salmon than in steelhead, but disturbances were similar for W and H fish of both species. Stress‐related water gain was, however, greater (or was compensated more slowly) in W than in H fish. These results indicate that chinook salmon are more stressed by barge transportation than are steelhead. If the viability of juvenile chinook salmon is reduced by adverse physiological, immunological, or behavioral responses to transportation stress, reductions in survival rates should be largest for fish transported during midseason, when densities of juvenile steelhead in the fish‐transport barges are highest.
The use of stream-margin habitat by age-0 salmonids has been studied, but differences in use among various types of habitat along stream margins has not been addressed . We described the nighttime use of habitat features by age-0 brown trout (Salmo trutta) among three types of stream-margin habitat late in the growing season (AugustSeptember) and assessed the extent to which use of habitat features within each type differed over the sampling period . Differences in water depths, water velocities, distances from shore, and substrate at the locations of fish along the margins of pools, the margins of riffles, and in backwaters were studied . Variation in habitat use also was observed during the study period as fish increased in length . Our observations are important considerations when developing habitat suitability criteria for assessment of instream-flow needs of age-0 brown trout .
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