Genetic Factors Contributing to Declines of Anadromous Salmonids in the Pacific Northwest

Abstract

Deleterious genetic change in wild anadromous salmonids (Oncorhynchus spp.) is expected from fisheries differentially harvesting fish that spawn at particular times within a season, mature at particular sizes or ages, or grow at particular rates. Other sources include overfishing, habitat degradation or destruction, and interactions with hatchery fish, particularly when these phenomena severely reduce population size. Gene flow from hatchery to wild fish populations also is deleterious because hatchery populations genetically adapt to the unnatural conditions of the hatchery environment at the expense of adaptation 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. This theory can illustrate how the fitness or productivity of a population is reduced and whether 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.