ASSESSMENT OF DRIFT AND SELECTION IN HATCHERY POPULATIONS OF OYSTERS (CRASSOSTREA-VIRGINICA)

Allozyme frequencies in three hatchery sublines of oysters (Crassostrea virginica) after one generation of selection for rapid juvenile growth were compared to their respective unselected control sublines. Using genotypic data for six enzyme-coding loci, we analyzed (a) differences in allelic frequencies between sublines, (b) departures from Hardy-Weinberg proportions within sublines. and (c) distributions of multilocus genotypes. Because gene frequencies in the parents and parental source populations were unknown, Monte Carlo simulations based on the census number of spawning adults were used to obtain expected genotypic distributions. Comparison of expected and observed distributions indicated that estimated effective breeding numbers were significantly lower than the census number of spawning adults in mass-spawned populations. The discrepancy between nominal and estimated N(e) was reduced in sublines formed by pooling the progeny of multiple small group spawns. Control over N(e) in hatchery populations of cultured aquatic species may be improved by following this simple expedient.