Maladaptation and phenotypic mismatch in cultured Atlantic salmon used for stocking

Compared to wild populations, cultured Atlantic salmon often sustain higher mortality rates and lower adult return rates when stocked as juveniles into natural streams. The ultimate causes for such differences in fitness, however, are not clear. Here we tested if relaxed levels of natural selection and improved survival in the hatchery environment could account for the observed degree of maladaptation among stocked fish. To do this, we assessed the degree of phenotypic mismatch between wild and cultured fish in three populations over five consecutive years. Significant differences were found in several phenotypic traits that are likely to have fitness implications. Thus, if the objective is to mimic wild individuals for restoration purposes, current hatchery practices aimed at maximising juvenile survival and enhancing growth may need to be revised.