Deleterious genetic change in wild anadromous salmonids is expected from fisheries that differentially harvest fish that spawn at particular times within a season, mature at particular sizes or ages, or grow at particular rates. Other sources are overfishing, habitat degradation or destruction, and interactions with hatchery fish, particularly when these phenomena severely reduce populations size. Gene flow from hatchery populations of fish also is deleterious because hatchery populations genetically adapt to the unnatural conditions of the hatchery environment at the expense of adapted ness for living in natural streams. This domestication is significant even in the first generation of hatchery rearing. Spawner-recruit theory serves as a framework for discussing the consequences of deleterious genetic change, and can illustrate how the fitness or productivity of a population is reduced whether deleterious genetic change is largely offset by natural selection within one generation, or accumulates over many generations. Although our knowledge is far from complete, sufficient information exists to demand actions to reduce or avoid deleterious genetic change. As additional information becomes available, these actions may be changed, perhaps relaxed. One suggested action is to establish or maintain refuge populations of wild fish that are to be protected from habitat degradation, selective or intense fishing, and interactions with hatchery fish.
Interbreeding of hatchery and wild fish is the intended result of out planting or supplementation hatchery programs which are designed to increase the number of salmon or steelhead produced in streams by increasing the number of naturally spawning fish. If the hatchery fish are genetically different from wild fish, particularly if they are less well adapted for rearing in natural streams than are wild fish, interbreeding will reduce the fitness of the wild population.