One of the potential problems in stock-enhancement programs is the disruption of local adaptation that may occur when native gene pools undergo introgression with alien genotypes. Evidence for local adaptation in fishes, however, comes almost entirely from freshwater and anadromous
species. In marine species, where geographic barriers to gene flow are usually absent, certain life stages are highly dispersive and/or migratory, and populations are few, the opportunity for local adaptation would appear to be comparatively low. I argue that such a conclusion is premature
because few marine species have been rigorously tested for local adaptation. Patterns of variation observed in nature, whether based on phenotypic or neutral molecular characters, are not predictors of the pattern in fitness-related traits across environmental gradients, which may contain
hidden (e.g., countergradient) genetic variation. The Atlantic silverside, Menidia menidia, for example, shows extensive evidence of local adaptation even though life-history and allozyme analyses would predict otherwise. Knowledge of local adaptation does not necessarily preclude stock
enhancement and may, in fact, improve the chances for its success, but care to avoid introgression is necessary. Hatcheries can be important tools for establishing the existence of adaptive differences in life-history and other traits among stocks, because they provide controlled environments
where the genetic contribution to phenotypic variation can be measured. If viewed as both research and production facilities, marine hatcheries could contribute greatly to our understanding of natural populations.
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