Associations between metabolic traits and growth rate in brown trout (Salmo trutta) depend on thermal regime

Metabolism defines the energetic cost of life, yet we still know
relatively little about why intraspecific variation in metabolic rate
arises and persists. Spatiotemporal variation in selection potentially
maintains differences, but relationships between metabolic traits
(standard metabolic rate (SMR), maximum metabolic rate (MMR), and aerobic
scope) and fitness across contexts are unresolved. We show that
associations between SMR, MMR, and growth rate (a key fitness-related
trait) vary depending on thermal regime (a potential selective agent) in
offspring of wild-sampled brown trout from two populations reared for ~15
months in either a cool or warm (+ 1.8°C) regime. SMR was
positively related to growth in the cool, but negatively related in the
warm regime. The opposite patterns were found for MMR and growth
associations (positive in warm, negative in cool regime). Mean SMR, but
not MMR, was lower in warm regimes within both populations (i.e., basal
metabolic costs were reduced at higher temperatures), consistent with an
adaptive acclimation response that optimises growth. Metabolic
phenotypes thus exhibited a thermally sensitive metabolic ‘floor’
and a less flexible metabolic ‘ceiling’. Our findings suggest a
role for growth-related fluctuating selection in shaping patterns of
metabolic variation that is likely important in adapting to climate
change.