The ability of an animal to perform a task successfully is limited by the amount of attention being simultaneously focused on other activities. One way in which individuals might reduce the cost of divided attention is by preferentially focusing on the most beneficial tasks. In territorial animals where aggression is lower among familiar individuals, the decision to associate preferentially with familiar conspecifics may therefore confer advantages by allowing attention to be switched from aggression to predator vigilance and feeding. Wild juvenile brown trout were used to test the prediction that familiar fishes respond more quickly than unfamiliar fishes to a simulated predator attack. Our results confirm this prediction by demonstrating that familiar trout respond 14% faster than unfamiliar individuals to a predator attack. The results also show that familiar fishes consume a greater number of food items, foraging at more than twice the rate of unfamiliar conspecifics. To the best of our knowledge, these results provide the first evidence that familiarity-biased association confers advantages through the immediate fitness benefits afforded by faster predator-evasion responses and the long-term benefits provided by increased feeding opportunities.

Why do captive-reared fishes generally have lower fitness in natural environments than wild conspecifics, even when the hatchery fishes are derived from wild parents from the local population? A thorough understanding of this question is the key to design artificial rearing environments that optimize post-release performance, as well as to recognize the limitations of what can be achieved by modifying hatchery rearing methods. Fishes are generally very plastic in their development and through gene-environment interactions, epigenetic and maternal effects their phenotypes will develop differently depending on their rearing environment. This suggests that there is scope for modifying conventional rearing environments to better prepare fishes for release into the wild. The complexity of the natural environment is impossible to mimic in full-scale rearing facilities. So, in reality, the challenge is to identify key modifications of the artificial rearing environment that are practically and economically feasible and that efficiently promote development towards a more wild-like phenotype. Do such key modifications really exist? Here, attempts to use physical enrichment and density reduction to improve the performance of hatchery fishes are discussed and evaluated. These manipulations show potential to increase the fitness of hatchery fishes released into natural environments, but the success is strongly dependent on adequately adapting methods to species and life stage-specific conditions.

Hatchery fish released for supplementation purposes often have difficulties adapting to wild conditions and, therefore, perform poorly in the wild. This can, at least partly, be explained by differences between hatchery and wild conditions. Our objective was to evaluate the effects of rearing density and structural complexity on the culture and postrelease performance of age‐0 Atlantic salmon Salmo salar. By using a 2 × 2 factorial design, we manipulated density (standard density and one‐third of standard density) and structure (standard rearing tanks with and without added rocks and plastic plants) in a conventional hatchery. After 3 months of rearing, 300 fish/treatment were individually tagged with passive integrated transponders, released in a nearby stream, and recaptured in late autumn (November) and summer (June). Fish not released were retained in the hatchery until smolting occurred during the next spring. In the stream, Atlantic salmon reared at reduced density grew faster during the first period after release, but there was no difference in June. The treatments had no effect on postrelease survival estimated by the recapture rates. In the hatchery, fish kept at low density with structure grew faster than conventionally reared fish. At smoltification, fish kept at low density had higher levels of insulin‐like growth factor I than those reared at standard density. Independent of size, fish kept at low density were more silvery (smolt‐like) and had a lower mortality rate than fish reared at high density. There was also a density effect on dorsal fin damage; Atlantic salmon at reduced density had less‐damaged fins than those at standard density. These results collectively indicate that reduced rearing density may be more important than structural complexity for improving postrelease performance of juvenile Atlantic salmon.

Theory suggests that habitat structure and population density profoundly influence the phenotypic development of animals. Here, we predicted that reduced rearing density and increased structural complexity promote food search ability, anti-predator response and the ability to forage on novel prey, all behavioural skills important for surviving in the wild. Brown trout were reared at three densities (conventional hatchery density, a fourth of conventional hatchery density and natural density) in tanks with or without structure. Treatment effects on behaviour were studied on trout fry and parr, whereupon 20 trout from each of the six treatment groups were released in an enclosed natural stream and recaptured after 36 days. Fry reared at natural density were faster to find prey in a maze. Moreover, parr reared at natural density were faster to eat novel prey, and showed more efficient anti-predator behaviour than fish reared at higher densities. Furthermore, parr reared at reduced densities were twice as likely to survive in the stream as trout reared at high density. In contrast, we found no clear treatment effects of structure. These novel results suggest that reduced rearing densities can facilitate the development of behavioural life skills in captive animals, thereby increasing their contribution to natural production.

Hatchery fish reared for conservation or supplementation often have difficulties adapting to natural conditions, resulting in poor performance in the wild. In a standard hatchery, fish are confined at high densities, which creates a social environment different from that experienced after release. Here we investigated how rearing density influences social dominance, postrelease growth, and survival in brown trout ( Salmo trutta ). Fish were reared at three density treatments: conventional hatchery density, half of conventional hatchery density, and natural density. Four months after hatching, dominance status was determined, and 36 fish from each treatment were released into an enclosed stream and recaptured after 36 days. Trout reared at natural density had higher dominance status and grew faster, both in the hatchery and in the natural stream, than trout from higher densities. Moreover, trout reared at natural density were twice as likely to survive in the stream as trout from higher densities. These novel results suggest that more natural rearing densities would facilitate the development of adaptive behaviour in hatchery salmonids and, thereby, their contribution to natural production.