Pacific Cod metabolism and swimming performance are similar across temperatures following prolonged thermal acclimation.

Warming ocean temperatures can increase the metabolic rates of fishes, potentially contributing to changes in their growth and survival to recruitment age. During prolonged marine heatwave conditions in the Gulf of Alaska between 2014 and 2019, Pacific Cod (Gadus macrocephalus) metabolic rates may have increased, but little is known about the relationship between metabolism and temperature for immature individuals of this species. We examined the effect of prolonged temperature exposure (~1 year) on the performance (standard, routine, and maximum metabolic rates; critical swimming speed; and aerobic scope) and swimming efficiency (cost of transport and optimal swimming speed) of age-1 Pacific Cod during two laboratory experiments across a range of temperatures (Expt. 1: 2°C, 4°C, 6°C and 8°C; Expt. 2: 6°C, 10°C and 14°C). We also explored relationships between performance and additional body state variables (e.g. condition and growth) and environmental variables (e.g. photoperiod and salinity). Temperature did not influence baseline metabolic performance (standard and routine metabolic rates) in either experiment. However, we observed significantly higher baseline metabolic rates in Expt. 2 compared to Expt. 1, even at the same temperatures. In contrast, maximum performance metrics (e.g. maximum metabolic rate and critical swimming speed) were significantly influenced by temperature. These patterns in performance were generally explained by differing costs of transport and rates of oxygen consumption during swimming trials between the two experiments. Further, body state variables and environmental variables were poorly correlated with performance, even when combined in a multivariate framework. Together, these findings suggest that other factors, such as season, oceanographic conditions early in life, year-class effects, or epigenetic effects, may influence Pacific Cod metabolism more than temperature or measured body state variables and environmental variables following prolonged thermal acclimation.