Demographic and genetic consequences of a steelhead supplementation program.

Objective: Supplementation of naturally spawning populations by the addition of hatchery‐spawned individuals is commonly conducted for recovery of threatened and endangered populations and to support harvest opportunities. Our objective was to evaluate whether the use of a juvenile captive broodstock and an integrated paradigm could increase returns of steelhead, the anadromous form of Rainbow Trout Oncorhynchus mykiss, while avoiding negative genetic impacts commonly associated with hatchery propagation. Methods: We analyzed 291 genetic markers in adult steelhead returning to an integrated population in southwest Washington over the course of 15 years. Reproductive success (RS) of fish spawning in the natural environment was evaluated by origin (hatchery versus natural) and return year. Data were then pooled over years to maximize sample sizes for comparing RS estimates among groups (i.e., estimation of relative reproductive success [RRS]). Result: We observed a weak relationship between RS and origin (male p = 0.347; female p = 0.066), but a significant relationship between RS and return year (male p < 0.001, η2 = 0.896; female p ⟨ 0.001, η2 = 0.867) (i.e., hatchery‐ and natural‐origin fish did well or poorly together each year). Estimates of RRS for fish spawned in the hatchery ranged from 2.4 to 6.4, indicating that fish spawned in the hatchery produced more returning adult progeny than did fish allowed to spawn in the natural environment. Estimates of RRS were ⟨1.0 for hatchery‐origin fish spawning in the natural environment, but the difference was nonsignificant for males and marginally significant for females. Hatchery‐origin fish exhibited reduced genetic diversity as well as evidence of increased temporal population structure. Conclusion: We conclude that the program was successful in achieving an increase in adult returns but not in avoiding negative genetic effects on the population and that any lasting impacts of supplementation remain to be determined. Impact statementSteelhead spawned in the hatchery produced more returning adult offspring than did steelhead left to spawn in the natural environment, but hatchery‐origin fish exhibited reduced genetic diversity and increased temporal population structure. Return year was a much stronger predictor than origin (natural or hatchery) for reproductive success, presumably illustrating the relatively great influence of temporal habitat variation on productivity. [ABSTRACT FROM AUTHOR]