Data from: First measurements of field metabolic rate in wild juvenile fishes show strong thermal sensitivity but variations between sympatric ecotypes

The relationship between physiology and temperature has a large influence
on population-level responses to climate change. In natural settings,
direct thermal effects on metabolism may be exaggerated or offset by
behavioural responses influencing individual energy balance. Drawing on a
newly developed proxy, we provide the first estimates of the thermal
performance curve of field metabolism in a wild fish. We investigate the
thermal sensitivity of field metabolic rate in two sympatric, genetically
distinct ecotypes of Atlantic cod from the Skagerrak coast of southern
Norway. The combined ecotype median of field metabolic rate increased with
increasing temperature until around 16°C, coincident with the thermal
optimum for growth for juvenile Atlantic cod. Individual cod experienced
temperatures in excess of the thermal optimum for field metabolic rate,
indicating some degree of thermal limitation of field metabolism in a
complex natural environment with the potential for thermal refugia. The
two cod ecotypes showed different thermal performance curves for field
metabolic rate, revealing that genetic components to temperature
sensitivity persist beyond acclimation effects. The cold-adapted fjord
ecotype maintained higher field metabolic rates at cooler temperatures
than the warm-adapted North Sea ecotype, which showed clear preference for
warmer waters around the thermal optimum. Field metabolic rates of the two
ecotypes were strongly influenced by year and location of sampling,
implying more complex behavioural responses to environmental conditions.
We emphasise that the energy uses reflecting physiological conditions in
the field should be considered in the evaluation of the effect of climatic
variables on fish population dynamics and demonstrate that otolith
isotopes provide an analytical framework to answer this question.