AbstractHatcheries are used as a conservation strategy to supplement native fish populations. Mating strategies used by hatcheries can employ different numbers of males and females, varying sex ratios, and different strategies to mix gametes. Due to sex ratio skew and variation in reproductive success, the effective number of breeders (Nb) contributing gametes to subsequent generations is often significantly less than the total number spawned. Accordingly, mating strategies can affect levels of genetic diversity, coancestry, and, concomitantly, long‐term fitness of hatchery stocks or natural populations into which hatchery fish are placed. Using steelhead Oncorhynchus mykiss, we mimicked six commonly employed hatchery mating regimes. We genotyped both parents and offspring at microsatellite loci and used likelihood‐based estimators implemented in the program COLONY to determine parentage. We calculated the mean and variance in reproductive success for each sex in each mating regime. In addition, we calculated coancestry and the inbreeding effective number of breeding adults for each mating regime to provide guidelines more consistent with the conservation goal of maintaining genetic diversity. Finally, we used the Ryman and Laikre equation to evaluate how estimates of Nb for each mating regime can influence the total effective population size. For all treatments, when gametes from multiple females and/or males were pooled, we found that the mean number of offspring was less for males than females, and there was a significantly higher reproductive variance for males than for females. In addition, when male gametes were pooled, a comparatively lower Nb was estimated. Single‐pair mating regimes and those that limited repeated use of males were most efficient at reducing levels of coancestry and maintaining genetic diversity in a hatchery or other closed populations.