Fisheries-induced evolution of alternative male life history tactics in Coho salmon

Fisheries-induced evolution (FIE) can result when harvest imposes artificial selection on variation in heritable phenotypic traits. While there is evidence for FIE, it remains difficult to disentangle the contributions of within-generation demographic adjustment, phenotypic plasticity, and genetic adaption to observed changes in life history traits. We present evidence for FIE using dozens of Coho salmon (Oncorhynchus kisutch) populations in which males adopt one of two age-invariant, heritable life history tactics: most mature as large three-year-old "hooknose" and typically fight for spawning opportunities, while some mature as small two-year-old "jacks" and fertilize eggs through sneaking. The closure of a fishery targeting three-year-old fish provided an experimental test of the prediction that fishery-imposed selection against hooknose males drives an evolutionary increase in the proportion of males adopting the jack tactic. The data support the prediction: 43 of 46 populations had higher jack proportions during than after the fishery. The data further suggest that changes in jack proportion were not solely the result of demographic adjustments to harvest. We suggest that systems where fisheries differentially exploit phenotypically discrete, age-invariant life histories provide excellent opportunities for detecting FIE.