Otolith δ ¹³ C values as a metabolic proxy: Approaches and mechanical underpinnings

Knowledge of metabolic costs associated with maintenance, foraging, activity and growth under natural conditions is important for understanding fish behaviours and the bioenergetic consequences of a changing environment. Fish performance in the wild and within a complex environment can be investigated by analysing individual-level field metabolic rate and, at present, the natural stable carbon isotope tracer in otoliths offers the possibility to reconstruct field metabolic rate. The isotopic composition of carbon in fish otoliths is linked to oxygen consumption through metabolic oxidation of dietary carbon. The proportion of metabolically derived carbon can be estimated with knowledge of δ 13 C values of diet and dissolved inorganic carbon in the water. Over the past 10 years, new techniques to study fish ecology have been developed, and these can be used to strengthen the application of otolith δ 13 C values as a metabolic proxy. Here, we illustrate the great potential of the otolith δ 13 C metabolic proxy in combination with other valuable and well-established approaches. The novel approach of the otolith δ 13 C metabolic proxy allows us to track the effects of ontogenetic and environmental drivers on individual fish physiology, and removes a major obstacle to understanding and predicting the performance of free-ranging wild fish.