Decades of warming alters maturation and reproductive investment in fish

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 degrees 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.

Automated summary

How does warming affect maturation and reproductive investment in ectotherms? This study investigates the effects of warming on maturation and reproductive investment in wild fish populations through a whole-ecosystem heating experiment in the Baltic Sea. The results show that warming has substantial effects on maturation size independent of body growth changes, with young fish maturing at a smaller size and investing more in gonad development in the heated population. Furthermore, after multiple generations of warming, maturation and reproduction traits differed from the initial generations, suggesting warming-induced evolution may have occurred.