Numbers of wild Pacific salmon or steelhead trout spawning in streams are often smaller than the estimated numbers for maximum recruitment or maximum sustainable yield, or for meeting other goals designated as desirable for wild populations. An alternative to reducing fishing or other mortality factors is to release hatchery-reared fish at natural spawning or rearing areas remote from the hatchery (outplanting), thereby increasing the number of salmon or steelhead produced in streams. A simple genetic model (one gene locus with two alleles) was developed to incorporate gene flow and density-dependent mortality, and was adapted for computer-based simulation. Simulations show that density-dependent mortality and gene flow constitute a potent force for eliminating advantageous alleles and, by inference, for effecting other potentially damaging genetic change in wild fish populations.