EFFECTS OF RESIDUAL HATCHERY-REARED STEELHEAD ON GROWTH OF WILD RAINBOW-TROUT AND SPRING CHINOOK SALMON

We investigated the effects of nonmigrant (residual) juvenile hatchery steelhead (anadromous rainbow trout Oncorhynchus mykiss) on growth of wild rainbow trout and juvenile spring chinook salmon O. tshawytscha to examine how increased densities of residual hatchery steelhead migh t affect the growth of preexisting wild rainbow trout and chinook salm on. We used screened enclosures in a natural stream to examine food ut ilization and physiological stress, factors that might affect fish gro wth. The presence of residual hatchery steelhead led to reduced growth of wild rainbow trout (1993: P = 0.019; 1994: P = 0.020) but not of s pring chinook salmon (P = 0.360). Enclosures did not reduce the total number of food items available but did influence the species compositi on of aquatic and terrestrial invertebrates. The food habits of paired and unpaired fish differed; however, the power of those tests was low . Cortisol level, a measure of physiological stress, did not differ be tween paired and unpaired fish held in enclosures. Cortisol levels wer e significantly lower in fish confined for 42 d than in wild fish outs ide the enclosures at the end of the experiment. Our results suggest t hat adverse effects on wild rainbow trout growth resulting from high d ensities (a doubling) of residual juvenile steelhead from hatchery rel eases may be significant.