Summary 1.Although it is not clear to what extent density dependence acts on the survival, emigration or growth of organisms, experiments testing alternative explanations are rare. A field experiment on 1-year-old brown trout (Salmo trutta L.) was undertaken to address the following questions: are the mortality, movement and growth of wild stream-living trout affected by population density? If so, are the density-dependent effects of released hatchery trout different from the effects of wild fish? 2. In each of two small streams, two replicate treatment blocks were used, each with four treatments assigned to stream sections 50-70 m in length: (1) control, no fish was introduced and population density was kept at its original level. (2) Trout biomass was doubled by introducing additional wild fish. (3) Trout biomass was doubled by introducing additional hatchery fish. (4) Hatchery trout were introduced, but biomass was kept at its original level by the removal of some resident wild fish. 3. We found no treatment effects on the recapture rates of resident trout, which suggests that survival was not strongly affected by competition. They were also remarkably stationary, regardless of treatment. However, trout growth rate was reduced to the same extent in both treatments with increased density, suggesting that growth was negatively density-dependent, and that the density-dependent effects of hatchery trout and introduced wild fish were similar. 4. Wild resident fish grew faster than introduced wild trout, which in turn grew faster than hatchery trout. Hatchery fish and introduced wild fish moved more than wild resident fish. 5. The results show that population density affected growth in trout parr. We conclude that competition is not limited to the underyearlings, as has previously been suggested, and that density-dependent growth is the main density-dependent response in yearling trout. Furthermore, this effect was the same for wild and hatchery-reared competitors, suggesting that stocking of hatchery fish may affect natural populations negatively through density dependence.

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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.

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Artificial culture may relax the selective pressures from predators, thereby altering behavioural and heart rate responses to predation risk. Culture may also impose new selection that adapts fish to confinement. Predictions from these hypotheses were tested by comparing seventh-generation farm Atlantic salmon (Salmo salar) with wild Atlantic salmon from the principal founder population of the farm strain. Wild age 1+ salmon had higher standard heart rates and showed a more pronounced flight and heart rate response to a simulated predator attack than did farmed salmon. However, wild fish were closer to the model predator at attack, which may have accentuated these differences. Both strains habituated to the threat, showing less response to the second attack than to the first. In contrast with age 1+ fish, wild age 2+ salmon had lower standard heart rates than farmed fish. Moreover, in age 2+ salmon, domestication effects were less clear and the response to predation threat considerably weaker, suggesting that environmental effects of culture override genetic effects as time in captivity increases. Domestication selection may thus alter reaction norms of farmed animals over environmental gradients and time. This should be considered when attempting to predict the consequences of release or escape of domesticated animals in the wild.Résumé : L'élevage en milieu artificiel diminue les pressions des prédateurs et change ainsi les réactions comportementales et physiologiques (rythme cardiaque) aux risques de prédation. L'élevage peut aussi imposer un nouveau type de sélection qui favorise l'adaptation des poissons à la restriction d'espace. La comparaison de Saumons de l'Atlantique (Salmo salar) élevés depuis 7 générations en pisciculture et de saumons sauvages provenant de la population souche principale de la pisciculture a permis de vérifier certaines prédictions générées à partir de ces hypothèses. Les saumons sauvages d'âge 1+ ont des rythmes cardiaques standard plus élevés et exhibent des réactions plus fortes de fuite et de modification du rythme cardiaque à l'attaque simulée d'un prédateur que les saumons de pisciculture. Cependant, les poissons sauvages étaient plus près du modèle de prédateur lors de l'attaque, ce qui peut avoir accentué ces différences. Les deux groupes développent une habituation aux attaques et réagissent moins fortement à une seconde attaque qu'à la première. Contrairement aux poissons d'âge 1+, les saumons sauvages d'âge 2+ ont des rythmes cardiaques standard plus bas que les saumons de pisciculture. De plus, chez les saumon d'âge 2+, les effets de la domestication sont moins apparents et les réactions à la menace de prédation sont considérablement diminuées, ce qui laisse croire que les effets environnementaux de l'élevage masquent les effets génétiques à mesure que la durée de la captivité augmente. La sélection opérée par la domestication peut ainsi modifier les normes des réactions des animaux élevés en milieu artificiel sur une gamme de gradients environnementaux et temporels. Ce s...

This paper reviews the life history of brown trout and factors influencing decisions to migrate. Decisions that maximize fitness appear dependent on size at age. In partly anadromous populations, individuals that attain maturity at the parr stage typically become freshwater resident. For individual fish, the life history is not genetically fixed and can be modified by the previous growth history and energetic state in early life. This phenotypic plasticity may be influenced by epigenetic modifications of the genome. Thus, factors influencing survival and growth determine life‐history decisions. These are intra‐ and interspecific competition, feeding and shelter opportunities in freshwater and salt water, temperature in alternative habitats and flow conditions in running water. Male trout exhibit alternative mating strategies and can spawn as a subordinate sneaker or a dominant competitor. Females do not exhibit alternative mating behaviour. The relationship between growth, size and reproductive success differs between sexes in that females exhibit a higher tendency to migrate than males. Southern populations are sensitive to global warming. In addition, fisheries, aquaculture with increased spreading of salmon lice, introduction of new species, weirs and river regulation, poor water quality and coastal developments all threaten trout populations. The paper summarizes life‐history data from six populations across Europe and ends by presenting new research questions and directions for future research.