Understanding the extent to which captivity generates maladaptation in wild species can inform species recovery programs and elucidate wild population responses to novel environmental change. Though rarely quantified, effective population size (Ne) and genetic diversity should influence the magnitude of plastic and genetic changes manifested in captivity that reduce wild fitness. Sexually-dimorphic traits might also mediate consequences of captivity. To evaluate these relationships, we generated >600 full- and half-sibling families from nine wild brook trout populations, reared them for one generation under common, captive environmental conditions, and contrasted several fitness-related traits in wild vs. captive lines. We found substantial variation in lifetime success (lifetime survival and reproductive success) and life history traits among wild populations after just one captive generation (fourteen- and three-fold ranges across populations, respectively). Populations with lower heterozygosity showed lower captive lifetime success, suggesting that captivity generates maladaptation within one generation. Greater male-biased mortality in captivity occurred in populations having disproportionately higher growth rates in males than females. Wild population Ne and allelic diversity had little or no influence on captive trait expression and lifetime success. Our results have four conservation implications: (i) Trait values and lifetime success were highly variable across populations following one generation of captivity. (ii) Maladaptation induced by captive breeding might be particularly intense for the very populations practitioners are most interested in conserving, such as those with low heterozygosity. (iii) Maladaptive sex differences in captivity might be associated with population-dependent growth costs of reproduction. (iv) Heterozygosity can be a good indicator of short-term, intraspecific responses to novel environmental change.