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.

中文翻译：

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首次测量野生幼鱼的田间代谢率显示出强烈的热敏感性，但同胞生态型之间存在差异

生理与温度之间的关系对人口对气候变化的反应影响很大。在自然环境中，对新陈代谢的直接热效应可能会因影响个体能量平衡的行为反应而被夸大或抵消。利用新开发的代理，我们提供了野鱼田间代谢热性能曲线的第一个估计值。我们调查了来自挪威南部Skagerrak海岸的两种同伴，遗传上不同的大西洋鳕鱼的田间代谢率的热敏感性。田间代谢速率的综合生态型中位数随温度升高而升高，直至约16°C，这与大西洋鳕鱼幼体生长的热最适温度相吻合。单个鳕鱼所经历的温度超过了田间代谢速率的热最适温度，这表明在复杂的自然环境中，田间代谢具有一定程度的热限制，具有热避难所的潜力。两种鳕鱼的生态型对田间代谢速率显示出不同的热性能曲线，表明遗传成分对温度的敏感性持续超过驯化效应。与冷适应的峡湾生态型相比，冷适应的峡湾生态型在较低的温度下能保持较高的田间代谢率，北海生态型在热最适温度附近表现出对较温暖水域的明显偏好。两种生态型的田间代谢率受采样年份和位置的强烈影响，这意味着对环境条件的行为响应更加复杂。