Presence of shelter reduces maintenance metabolism of juvenile salmon

1. Shelter is of major importance to many animals in providing protection against both the physical environment and potential predators. 2. We hypothesized that animals without shelter suffer metabolic costs associated with a need for increased vigilance and preparedness to escape attacks from predators or competitors. This possibility was tested by comparing the standard metabolic rates of inactive postdigestive juvenile Atlantic salmon, Salmo salar, held either with or without a shelter, which took the form of a semicircular ledge under which the fish could fit comfortably. 3. The ledges were semitransparent (so did not substantially reduce light levels) and provided no protection against the minimal water velocities in the testing arena. Nonetheless, absence of ledge shelter resulted in a 30% higher rate of oxygen consumption. 4. Fish without a ledge shelter typically positioned themselves against vertical walls of the observation arena, which presumably afforded the best available sheltering option, and adopted a significantly darker coloration (indicative of greater stress) than those under ledges. Fish with ledges rested outside and adjacent to rather than beneath the shelter. Therefore, it seems that awareness that a shelter is readily available, rather than the act of sheltering, results in reduced metabolism. 5. We conclude that the presence of appropriate shelter not only reduces the risk of predation but also provides a metabolic benefit to fish that is likely to have implications for growth performance and activity budgets. Standard metabolic rate can be a function of habitat structure.