Growth Modulation during Juvenile Rearing Can Reduce Rates of Residualism in the Progeny of Wild Steelhead Broodstock

Abstract

Residualism is the failure of some hatchery-reared salmonid juveniles to out-migrate as smolts with the rest of their cohort. We released hatchery-reared juveniles from domesticated- and wild-origin broodstock steelhead and measured their relative rates of residualism. The residualism rate exhibited by the offspring of wild broodstock was more than one order of magnitude greater than that of domesticated stock. Further, we experimentally manipulated the growth trajectories of juvenile offspring of wild broodstock to decrease the size variance among released fish. Our expectation was that fewer fish would be too small or too large to smolt as yearlings. Small fish placed on an aggressive rearing regimen (reared separately from larger fish, fed more food more often) residualized at a lower rate than did comparable control fish (reared with large fish without supplemental feeding). We saw no effect of a lower feeding regimen on the residualism rate of large fish, but the lower feeding regimen did not significantly affect size during rearing. Residual steelhead were bimodally distributed by size. Most residual fish were male; among these fish, 11-ketotestosterone levels and visual scores of gonadal development indicated that 42% were mature or maturing. All fish showed the expected patterns of change in gill Na⁺,K⁺-ATPase and insulin-like growth factor-I (IGF-I) associated with smoltification, and small fish placed on the aggressive rearing regimen showed slightly, but significantly, higher IGF-I levels than did comparable control fish.