Structural environmental enrichment, that is, a deliberate addition of physical complexity to the rearing environment, is sometimes utilized to reduce the expression of the undesirable traits that fish develop in captivity. Increasing demands and regulations regarding usage of enrichment to promote fish welfare also make investigations on the effects of enrichment important. Here, we sythesize the current state‐of‐the‐art knowledge about the effects of structural environmental enrichment for fish in captive environments. We find that enrichment can affect several aspects of the biology of captive fish, for example, aggression, stress, energy expenditure, injury and disease susceptibility. Importantly, these effects are often varying in direction and magnitude, and it is clear that just addition of structure is not a solution to all problems in fish rearing. Each species and life stage needs special consideration with respect to its natural history and preferences. A multitude of different enrichment types has been investigated and many studies investigate several enrichment components at the same time, making comparisons among studies difficult. To the present date, the majority of efforts have been directed to investigate salmonid fish in stock‐fish hatcheries and cichlids from a basic research perspective. Some contexts are under‐studied with respect to environmental enrichment, for instance effects on results in basic research and welfare effects in display aquaria. There are many research opportunities left within this field. However, future studies could utilize experimental designs which make it possible to discriminate between effects of different environmental manipulations to a higher degree than what has been performed to this date.

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.

No abstract

No abstract

Selection programs for fish frequently target growth rate as a breeding goal, yet surprisingly little is known about which mechanisms underlying the growth process are being targeted. The aim of this study was thus to examine whether the process of artificial selection of Atlantic salmon (Salmo salar) that has resulted in higher growth rate resulted in underlying changes in the growth hormone (GH) – insulin-like growth factor I (IGF-I) axis of endocrine growth regulation. This was tested by comparing similarly reared seventh-generation farm salmon with wild salmon from the principal founder population of the farm strain at three life stages. Not unexpectedly, the domesticated fish outgrew their wild counterparts; this was most evident in salt water, where they averaged three times the weight by the end. Pituitary GH content was positively correlated with growth rate and correspondingly was significantly higher in the faster growing domesticated fish than in the wild fish. Plasma GH levels were also significantly higher in the domesticated fish, whereas IGF-I levels did not differ. These findings provide some of the first direct evidence indicating a link between domestication selection for growth and its endocrine regulation, whereby individuals with more active endocrine growth regulatory components are targeted.