LOCAL ADAPTATION IN MARINE FISHES - EVIDENCE AND IMPLICATIONS FOR STOCK ENHANCEMENT

One of the potential problems in stock-enhancement programs is the dis ruption of local adaptation that may occur when native gene pools unde rgo introgression with alien genotypes. Evidence for local adaptation in fishes, however, comes almost entirely from freshwater and anadromo us species. In marine species, where geographic barriers to gene flow are usually absent, certain life stages are highly dispersive and/or m igratory, 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 fo r local adaptation. Patterns of variation observed in nature, whether based on phenotypic or neutral molecular characters, are not predictor s of the pattern in fitness-related traits across environmental gradie nts, which may contain hidden (e.g., countergradient) genetic variatio n. The Atlantic silverside, Menidia menidia, for example, shows extens ive 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 t he chances for its success, but care to avoid introgression is necessa ry. Hatcheries can be important tools for establishing the existence o f adaptive differences in life-history and other traits among stocks, because they provide controlled environments where the genetic contrib ution to phenotypic variation can be measured. If viewed as both resea rch and production facilities, marine hatcheries could contribute grea tly to our understanding of natural populations.