Rapid growth of Atlantic salmon juveniles in captivity may indicate poor performance in nature

The hatchery environment often favours completely different traits than natural selection in the wild. Consequently, hatchery-reared fish are usually larger and more aggressive than their wild counterparts. Increased growth rate and aggression are predicted to be beneficial in feeding competition in hatcheries, but not necessarily in nature, where food resources are spatially and temporally more variable. We compared the growth, condition and mortality of landlocked Atlantic salmon (Salmo salar L.) juveniles in a common hatchery environment and when feeding on natural prey in semi-natural channels. We found that the growth and survival probability of the fish in the hatchery was negatively associated with their performance in the semi-natural channels. Furthermore, we found tendencies for directional selection (linear selection differential and gradient: P < 0.1, in both cases) against large body size in semi-natural channels, but not in the hatchery. Therefore, good performance during hatchery rearing may indicate reduced performance in food-limited natural conditions, where selection may favour smaller individuals that have a lower standard metabolic rate (higher growth efficiency). If our results are also valid in fully natural conditions, they suggest that selective stocking of the most successful hatchery phenotypes may not be an optimal strategy to conserve endangered natural salmonid populations.