Development of natural growth regimes for hatchery-reared steelhead to reduce residualism, fitness loss, and negative ecological interactions
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- Berejikian, B.A., Larsen, D.A., Swanson, P. et al. Environ Biol Fish (2012) 94: 29. doi:10.1007/s10641-011-9788-0
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Wild steelhead (Oncorhynchus mykiss) typically spend two or more years in freshwater before migrating to sea, but hatchery steelhead are almost ubiquitously released as yearlings. Their large size at release coupled with life history pathways that include both male and female maturation in freshwater present ecological risks different from those posed by hatchery populations of Pacific salmon. Yearling hatchery reared steelhead that fail to attain minimum thresholds for smoltification or exceed thresholds for male maturation tend to ‘residualize’ (i.e., remain in freshwater). Residuals pose ecological risks including size-biased interference competition and predation on juvenile salmon and trout. Three hatchery populations of steelhead in Hood Canal, WA were reared under growth regimes designed to produce a more natural age at smoltification (age-2) to aid in rebuilding their respective natural populations. Mean smolt sizes and size variability at age-2 were within the range of wild smolts for two of the three populations. The third population reared at a different facility under similar temperatures exhibited high growth rate variability and high male maturation rates (20% of all released fish). Experimentally comparing age-1 and age-2 smolt programs will help identify optimal rearing strategies to reduce the genetic risk of domestication selection and reduce residualism rates and associated negative ecological effects on natural populations. Investigations of Winthrop National Fish Hatchery summer-run steelhead will measure a) selection on correlated behavioral traits (‘behavioral syndromes’), b) degree of smoltification, c) changes in hormones that regulate gonad growth at key developmental stages, and d) conduct extensive post-release monitoring of fish reared under each growth regime.