Optimum growth temperature declines with body size within fish species

According to the temperature-size rule, warming of aquatic ecosystems is generally predicted to increase individual growth rates but reduce asymptotic body sizes of ectotherms. However, we lack a comprehensive understanding of how growth and key processes affecting it, such as consumption and metabolism, depend on both temperature and body mass within species. This limits our ability to inform growth models, link experimental data to observed growth patterns, and advance mechanistic food web models. To examine the combined effects of body size and temperature on individual growth, as well as the link between maximum consumption, metabolism and body growth, we conducted a systematic review and compiled experimental data on fishes from 59 studies that combined body mass and temperature treatments. By fitting hierarchical models accounting for variation between species, we estimated how these three processes scale jointly with temperature and body mass within species. We found that whole-organism maximum consumption increases more slowly with body mass than metabolism, and is unimodal over the full temperature range, which leads to the prediction that optimum growth temperatures decline with body size. Using an independent dataset, we confirmed this negative relationship between optimum growth temperature and size within fish species. Small individuals may therefore exhibit increased growth with initial warming, whereas larger conspecifics could be the first to experience negative impacts of warming on growth. These findings help advance mechanistic models of individual growth and food web dynamics and improve our understanding of how climate warming affects the growth and size structure of aquatic ectotherms.Significance statementPredicting organism responses to a warming climate requires understanding how physiological processes such as feeding, metabolism, and growth depend on body size and temperature. Common growth models predict declining optimum growth temperatures with body size if energetic costs (metabolism) increase faster than gains (feeding) with body size. However, the generality of these features has not been evaluated at the within-species level. By collating data on fish through a systematic literature review, we find support for both declining net energy gain and declining optimum growth temperatures with body size. This implies large individuals within populations may be the first to suffer poor growth due to warming, with consequences for fisheries yield and food web structure in warmer climates.