Evolutionary Pressures on Pacific Salmon: Selective Fishing on Body Size and Migration Timing

Life history traits of fishes are affected by a variety of forms of natural selection, notably the rates of mortality experienced by larval, juvenile, and adult individuals, and reproductive success, including aspects of egg production, parental care, and sexual selection.  Fishing not only increases the mortality rate on adults but it is often selective with respect to size, thus altering the natural selection regime, and there is concern that long-term exposure to such selection may be altering the evolutionary trajectory of exploited fish species.  In this presentation, we first explain why semelparous, anadromous Pacific salmon are especially suitable for studies of fishery-induced selection.  We then provide examples from fisheries in Alaska that reveal a more nuanced and complex picture than the conventional wisdom that “bigger is more vulnerable”.  However, body size and timing of migration by salmon are linked, and fisheries are often highly selective with respect to return timing.  Evidence indicates that this selection may be altering the timing of migration of salmon runs.  We conclude that fishing can be a powerful force in the contemporary evolution of life history in fishes, and that selection on timing should receive more attention because it can be an evolutionary pressure in and of itself, and can co-vary with body size in migratory fishes.