HYBRIDIZATION WITH DOMESTICATED RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) REDUCES SEASONAL-VARIATION IN GROWTH OF STEELHEAD TROUT (O-MYKISS)

We tested the hypothesis that hybridization with domesticated rainbow trout (Oncorhynchus mykiss) reduces the seasonal variation in growth i n both fresh and salt water in anadromous steelhead trout (O. mykiss). The seasonal variation in freshwater growth was more pronounced in th e steelhead trout than in the hybrid and domesticated trout. Consequen tly, differences in growth rate among groups were considerably higher during winter than in fall or spring. We suggest that high feeding act ivity during unfavourable winter conditions in natural waters is malad aptive, but is not selected against in protected hatchery environments with a continuous food supply. This may lead to genetic divergence in feeding physiology and/or behaviour between steelhead trout and domes ticated rainbow trout. Growth rates in experimental groups transferred to seawater in December relative to groups transferred in April were faster in rainbow and hybrid trout than in steelhead trout. The result s support the hypothesis that seasonal variation in seawater growth in steelhead trout is reduced by crossing with domesticated rainbow trou t.