Field metabolic rates of teleost fishes are recorded in otolith carbonate.

Field metabolic rate (FMR) is key to understanding individual and population-level responses to environmental changes, but is challenging to measure in field conditions, particularly in aquatic environments. Here we show that FMR can be estimated directly from the isotopic composition of carbon in fish otoliths (δ ¹³ C _oto ). We describe the relationship between δ ¹³ C _oto values and oxygen consumption rate, and report results from laboratory experiments relating individual-level measurements of oxygen consumption rates to δ ¹³ C _oto values in Atlantic cod ( Gadus morhua ). We apply our new δ ¹³ C _oto metabolic proxy to existing δ ¹³ C _oto data from wild cod and four deepwater fish species to test the validity of inferred FMR estimates. The δ ¹³ C _oto metabolic proxy offers a new approach to study physiological ecology in free-ranging wild fishes. Otolith-based proxies for FMR are particularly promising as they allow retrospective assessment of time-integrated, individual-level FMR throughout an individual fish's life history.
Competing Interests: The authors declare no competing interests.