Cod in a changing environment - Spatial variation in field metabolic rate of Atlantic cod from Greenland to Denmark

Ocean temperatures are projected to rise rapidly, with consequences for species and ecosystems. For many animals, cold and warm distribution limits are well defined, yet the physiological and ecological constraints remain poorly understood. In fish, elevated temperature can increase metabolic rates, which is thought to be beneficial at cold ranges, where the aerobic capacity increases, while disadvantageous at warm ranges, where maintenance costs rise without corresponding gains in aerobic capacity. We conducted novel time-integrated field metabolic rate (FMR) analysis on Atlantic cod (Gadus morhua) along their offshore cold (west Greenland; 59–72°N) and warm (North Sea; 54–62°N) margins and compared these to environmental conditions. Cod were sampled via bottom-trawl surveys late spring/early summer (Greenland) and late summer (North Sea). FMR integrated over ~2-3 months prior to capture and was estimated based on δ13C values of otolith aragonite obtained from surface drilling. Temperature sensitivity and spatial variability of FMR differed markedly between regions. For the Greenlandic cod, temperature had only minor effects and did not constrain the northern distributional margin, as metabolic rates never approached maximum metabolic rate (MMR), even at the lowest temperatures. Instead, spatial variability in FMR corresponded with food availability and quality, suggesting these as primary drivers of distribution. In contrast, the North Sea cod showed stronger temperature effects than expected from the temperature scaling effect on standard metabolic rate (SMR) across 5–14 °C. This indicates a higher energy budget among the highest observed temperatures, which aligned with tagging studies that found ~14 °C as the preferred habitat. The study also revealed that cod caught along the Norwegian trench had higher FMR, possibly driven by a different population structure and food intake. These findings demonstrate that temperature effects on cod physiology and distribution are strongly context dependent, with other drivers like food intake playing a key role. This challenges assumptions of many bioenergetic growth models, which assume fixed temperature effects on metabolism, which could lead to false growth projections.