Hybridization with domesticated rainbow trout reduces seasonal variation in seawater adaptability of steelhead trout ( Oncorhynchus mykiss)

Abstract

The hypothesis that interbreeding with domesticated rainbow trout reduces seasonal variation in seawater adaptability in wild-type anadromous steelhead trout was tested. For this purpose, the growth of 18 maternal half-sib families of wild-type steelhead trout and steelhead/domesticated rainbow trout hybrids along with a control family of domesticated rainbow trout was monitored in a laboratory experiment. The trout were reared under ambient temperature and simulated natural photoperiod conditions from June 1989 to the end of May 1990. The seasonality of seawater growth was examined by comparing the growth rates of early (December) and late (April) transfers of the experimental strains to seawater. Further, on eleven occasions from September to late May, steelhead and hybrid trout were subjected to 24-h seawater (SW) challenge tests in 30‰ seawater. Significant male parent type by environment interaction for the growth rates showed that seawater transfer during winter was relatively more favourable for the rainbow and hybrid trout than for the wild-type steelhead trout. These results were supported by the results of the SW challenge tests. Winter transfers to seawater revealed significantly lower plasma sodium levels in hybrid trout compared to steelhead trout, while when SW-transferred in May, the wild-type steelhead trout had a higher hypo-osmoregulatory capacity than the hybrid trout. Our results support a genetic basis for the seasonality of seawater adaptability, and the hypothesis that hybridization with domesticated rainbow trout reduces seasonal variation in the seawater adaptability of steelhead trout.