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 (δ¹³C_oto). We describe the relationship between δ¹³C_otovalues and oxygen consumption rate, and report results from laboratory experiments relating individual-level measurements of oxygen consumption rates to δ¹³C_otovalues in Atlantic cod (Gadus morhua). We apply our new δ¹³C_otometabolic proxy to existing δ¹³C_otodata from wild cod and four deepwater fish species to test the validity of inferred FMR estimates. The δ¹³C_otometabolic 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.