Life history attributes influencing metabolic variation in an estuarine fish

Considering that energy availability and oxygen transport potential are limited in supplying metabolic requirements, all animals face trade‐offs between competing metabolic demands. A key interest of eco‐physiologists is to understand these trade‐offs and determine which strategies might prove the most efficient in the face of environmental challenges like climate warming. Here, we analysed three large datasets on a diadromous fish species, Galaxias maculatus, to assess metabolic rates of juveniles and reproductive adults across four acclimation temperatures (15, 18, 20 and 23°C). We used natural variations in growth rate and reproductive maturation level between and within individuals at different life history stages to test for potential relationships with different levels of aerobic metabolism from resting (standard metabolic rate, SMR) to maximal (maximum metabolic rate, MMR) We found that allometry of SMR differed predictably with warming temperature, whereby the intercept (scaling coefficient) increased and the slope (scaling exponent) decreased. This was driven by a trend in which the SMR of larger fish was less responsive to temperature relative to smaller fish. MMR allometry showed differences in intercept, but negligible differences in slope with temperature. Linear regression analyses on mass‐standardized variables showed individual growth phenotypes have a weak positive correlation with SMR at some, but not all, temperatures. This suggests that SMR may be a contributing determinant of growth, or the measurement of SMR could be impacted by residual growth even when fish have been fasted. No correlation between individual growth and MMR was seen at any temperature. We found a reduction in the number of individuals that reproductively matured at the lowest and highest temperatures. While there was no discernible correlation between the level of reproductive maturation and SMR, a paired t ‐test on a subset of fish showed that SMR was significantly lower in individuals when they were mature relative to when they were immature. There was also a negative relationship between maturation and MMR at 18°C, raising questions around the whole‐animal metabolic consequences of allocating energy to reproduction. We suggest that metabolism, growth and reproductive investment may not trade‐off as clearly as theory predicts, and we propose future research directions to untangle the complicated balance of these traits for maximizing lifetime fitness under a changing climate. Read the free Plain Language Summary for this article on the Journal blog.