Rapid evolution of egg size in captive salmon

Captive breeding and release programs, widely used to supplement popu-lations of declining species, minimize juvenile mortality to achieve rapid population growth. However, raising animals in benign environments may promote traits that are adaptive in captivity but maladaptive in nature. In chinook salmon, hatchery rearing relaxes natural selection favoring large eggs, allowing fecundity selection to drive exceptionally rapid evolution of small eggs. Trends toward small eggs are also evident in natural populations heavily supplemented by hatcheries, but not in minimally supplemented populations. Unintentional selection in captivity can lead to rapid changes in critical life-history traits that may reduce the success of supplementation or reintroduction programs. The use of captive breeding programs is widespread, and increased use of such pro-grams for species preservation has been rec-ommended (1, 2). However, there is consid