Demographic and genetic consequences of a Steelhead supplementation program

Abstract 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. We present an analysis of steelhead, the anadromous form of Rainbow Trout ( Oncorhynchus mykiss ), returning to an integrated supplemented population in Southwest Washington over the course of 15 years. The goal of the supplementation program was to evaluate whether use of a juvenile captive broodstock and an integrated paradigm could be used to increase adult returns, while avoiding negative genetic impacts to the population. Estimates of relative reproductive success (RRS) for fish spawned in the hatchery ranged from 2.4 for hatchery‐origin females to 6.4 for natural‐origin males, indicating that fish spawned in the hatchery produced more returning adult progeny than did fish allowed to spawn in the natural environment. We observed a slight reduction in reproductive success (RS) for hatchery‐origin (relative to natural‐origin) fish when spawning in the natural environment, but the difference was non‐significant for males and marginally significant for females. In contrast to the relatively weak relationship between RS and origin (male P = 0.347, η 2 = 0.008; female P = 0.066, η 2 = 0.037), we observed a strong relationship between RS and return year (male P < 0.001, η 2 = 0.896; female P < 0.001, η 2 = 0.867) (i.e., hatchery‐ and natural‐origin fish did well or poorly together each year). Hatchery origin fish exhibited reduced genetic diversity, as well as evidence of increased temporal population structure among hatchery fish. We suspect the latter is an artifact of culture practices that reduce diversity in age at smoltification. We conclude that the program was successful in achieving an increase in adult return, but not in avoiding negative genetic effects on the population, and that any lasting impacts of supplementation remain to be determined.