Environmental enrichment for fish in captive environments: effects of physical structures and substrates

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.