Magnus Huss

Size-specific body growth responses to warming are common among animal taxa, but sex-specific responses are poorly known. Here we ask if body growth responses to warming are sex-dependent, and if such sex-specific responses vary with size and age. This was tested with sex-specific data of back-calculated individual growth trajectories, in European perch ( Perca fluviatilis) from a long-term whole-ecosystem warming experiment (6.3 °C above the surrounding sea). Warming led to both size- and sex-specific differences in growth responses. Warming had a consistent positive effect on body growth of females, but negative effects on male growth at size > 10 cm and age > 2 years. These sex-specific growth responses translate to an increased degree of female-biased sexual size dimorphism (in length-at-age) with warming. Although the exact temperature-mediated effects underlying differential growth responses could not be resolved, results imply global warming may have highly different effects during ontogeny of male and female perch. Such effects should be considered in climate warming scenarios concerning fish growth, population size-structure, and dynamics of aquatic food webs that include fish exhibiting sexual size dimorphism.

How does warming affect maturation and reproductive investment in ectotherms? Younger age and smaller size at maturation, as well as altered reproduction processes, have been found in a few species subjected to elevated temperatures. These observations, however, come from studies that do not distinguish effects of warming on maturation from those on growth, are also restricted to single generation responses to warming, or have additional stressors besides warming in the study system. Here, we study warming effects on maturation and reproductive investment in wild, unexploited fish populations using a whole‐ecosystem heating experiment. The experiment is conducted on Eurasian perch ( Perca fluviatilis ) in a heated and control area (with >5°C temperature difference) in the Baltic Sea. We compare female perch size at maturation using estimated probabilistic maturation reaction norms (PMRNs) and the gonado‐somatic index over 17 years of heating, spanning approximately five to eight perch generations. Using the PMRN approach, we show that warming has substantial effects on maturation size independent of warming‐induced changes in body growth. We found that young fish mature at a smaller size and invest more in developing their gonads in the heated population than in the unheated population. Our findings suggest that warming effects on reproductive investment may initially compensate for the cost of warming‐induced decrease in maturation size caused by the trade‐off between early maturation and size‐dependent fecundity. After multiple additional generations of warming, maturation and reproduction traits in perch differed from those in the first generations following the onset of warming, which suggests that warming‐induced evolution may have occurred. Our study is particularly relevant in the context of climate change because of the unusually large temperature difference between the areas and the fact that the heating occurred on an ecosystem level. We call for experimental studies resolving mechanisms of trait responses to warming across generations, complemented with genomic analyses, to aid understanding of organisms' long‐term responses to climate change.