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.

Basal or standard metabolic rate ( SMR) has been found to exhibit substantial intraspecific variation in a range of taxa, but the consequences of this variation are little understood. Here we explore how SMR is related to the energy cost of processing food, known as apparent specific dynamic action or the heat increment of feeding. Using juvenile Atlantic salmon Salmo salar, we show that fishes with a higher SMR had a higher peak and a greater total energy expenditure when digesting a given size of meal. However, the duration over which their metabolism was elevated after consuming the meal was shorter. The greater energy costs they incur for processing food may be related to their assimilation efficiency. These relationships are likely to have implications for feeding strategies and growth rates, since individuals with a higher SMR have higher routine costs of living but recover more quickly following feeding and so may have a greater potential for processing food.

Bioenergetics studies of free-living animals have long been hampered by limitations on our abilities to measure the energy costs of different activities. Here we evaluate whether it is possible to use the opercular ventilatory beat rate of a fish to estimate its rate of energy expenditure. Changes in metabolic rate (MR) and ventilation rate (VR) were recorded in yearling Atlantic salmon (Salmo salar, weight range 1.8-12.64 g) engaged in different activities at different temperatures while within a respirometer. MR was found to correlate strongly with VR in all fish. The relationship was linear, and both the slope and corresponding intercept of the regression equation were strongly dependent on the fish's body weight and the test temperature. From these relationships, a general equation was generated to predict MR at a range of temperatures from knowledge of a fish's weight and its VR; this proved to be highly accurate (correlation between predicted and observed MRs: r = 0.95), although calibration of individual fish is recommended in studies that compare performance of individuals. Visual measurements of VR may therefore provide a highly accurate, cheap, and noninvasive method of measuring the energy consumption of fish engaged in natural behaviours in more natural settings. Résumé :Les études bioénergétiques d'animaux en liberté ont longtemps été restreintes par l'impossibilité de mesurer les coûts énergétiques des différentes activités. Nous vérifions ici s'il est possible d'utiliser le taux de battement de ventilation des opercules d'un poisson pour mesurer son taux de dépense d'énergie. Nous avons enregistré dans un respiromètre les changements de taux métabolique (MR) et de taux de ventilation (VR) de saumons atlantiques (Salmo salar, étendue des masses 1,8 -12,64 g) d'un an durant diverses activités à différentes températures. Il y a une forte corrélation entre MR et VR chez tous les poissons. La relation est linéaire et la pente et l'ordonnée à l'origine correspondante de l'équation de ré-gression sont toutes deux fortement dépendantes de la masse corporelle du poisson et de la température expérimentale. À partir de ces relations, nous avons élaboré une équation générale pour prédire MR sur une gamme de températures d'après la masse du poisson et son VR. L'équation est très précise (corrélation entre les MRs prédits et observés : r = 0,95), bien nous recommandions de calibrer chacun des poissons lors d'études qui comparent les performances individuelles. Les mesures visuelles de VR peuvent ainsi constituer une méthode très précise, peu coûteuse et non effractive de déterminer la consommation d'énergie chez des poissons en train d'accomplir des comportements normaux dans des environnements plus naturels.[Traduit par la Rédaction]

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