The performance advantage of a high resting metabolic rate in juvenile salmon is habitat dependent

1. Basal levels of metabolism vary significantly among individuals in many taxa, but the effects of this on fitness are generally unknown. Resting metabolic rate (RMR) in juvenile salmon and trout is positively related to dominance status and ability to obtain a feeding territory, but it is not clear how this translates into performance in natural conditions. 2. The relationships between RMR, dominance, territoriality and growth rates of yearling Atlantic salmon Salmo salar were examined in relation to predictability in food supply and habitat complexity, using replicate sections of a large-scale controlled semi-natural stream. 3. Estimated RMR was a strong predictor of dominance, and under conditions of a predictable food supply in a structurally simple habitat, high estimated RMR fish obtained the best feeding territories and grew faster. 4. When the spatial distribution of food was made less predictable, dominant (high estimated RMR) fish were still able to occupy the most profitable feeding locations by periodically moving location to track the changes in food availability, but RMR was no longer a predictor of growth rate. Moreover, when a less predictable food supply was combined with a visually more complex (and realistic) habitat, fish were unable to track changes in food availability, grew more slowly and exhibited greater site fidelity, and there were no relationships between estimated RMR and quality of occupied territory or growth rate. 5. The relative benefit of RMR is thus context dependent, depending on both habitat complexity and the predictability of the food supply. Higher habitat complexity and lower food predictability decrease the performance advantages associated with a high RMR.