Estimated standard metabolic rate interacts with territory quality and density to determine the growth rates of juvenile Atlantic salmon

1. Physiological traits can vary greatly within a species and consequently have a significant impact on other aspects of performance. Many species exhibit substantial variation in basal or standard metabolic rate (SMR), even after controlling for body size and age, yet the ecological consequences of this are little known. 2. We examined the relationships between mass-specific SMR of yearling salmon (estimated from their ventilation rate) and their feeding and growth rates across a range of natural population densities within a semi-natural stream environment. 3. Standard metabolic rate was strongly correlated with dominance rank, and higher ranking fish were more likely to acquire good feeding territories. Despite this, there was no overall relationship between SMR and growth. We show for the first time that this paradox can be explained because within territories of a given quality, there was a negative correlation between SMR and growth rate, presumably owing to the costs of metabolism. 4. These effects were also influenced by density: lower densities led to reduced aggression and competition and hence higher average feeding and growth rates. Moreover, at low densities, where availability of good feeding locations was not limiting, there was no relationship between SMR and growth. 5. As a result of these processes, there was a context-dependent trade-off in energy budgets: the fish achieving the greatest growth were those with the lowest SMR that was necessary to achieve dominance over conspecifics (and hence acquire a good territory), but this minimum threshold SMR increased with population density. These relationships and trade-offs can explain the persistence of variation in SMR within populations.