“Historically, it was thought that warming would affect ectotherms [animaux à «sang froid»], like fish, in a uniform way: it was thought that they would either all become bigger or all smaller. But our results prove that freshwater fish respond differently to warming depending on their preferred habitat,” relates Connor Warne, doctoral student at the University of Guelph and lead author of the study.
To provide this proof, the Ontario team used a huge database of data collected as part of the Large-Scale Monitoring Program for Ontario’s Inland Water Bodies. The researchers thus analyzed data (length at maturity) of more than 174,000 individuals of five species of predatory fish (lake trout, lake whitefish, walleye, smallmouth bass and yellow perch), chosen for their commercial, cultural and recreational.
Their study, published recently in the journal Canadian Journal of Fisheries and Aquatic Sciences, thus demonstrated that with warming of the lakes, the size of species adapted to the cold (such as lake whitefish) tends to decrease, while that of species adapted to cool waters (e.g. walleye) and warm waters (e.g. small bass mouth) increases.
Smallmouth bass are expected to increase in average size due to global warming. (commons.wikimedia.org/commons.wikimedia.org)
According to Emmanuelle Chrétien, professor of fish ecology at UQAR and member of the Interuniversity Research Group in Limnology (GRIL), the challenge when using such data sets is that they can include a certain uncertainty. However, she judges that the authors of the study took all the necessary precautions to avoid potential bias, in addition to having interpreted their results with caution.
The ideal temperature range
As ectotherms, fish rely heavily on their environment for the regulation of their internal heat. In order to optimize their physiological performance, they must therefore live in a habitat whose temperature is within a precise range, varying depending on the species.
And if the warming is such that they can no longer have access to their preferred temperature, then they will have to resign themselves to living in a sub-optimal habitat, in which their metabolism will not function at its full potential.
“Especially in freshwater, unlike in the marine environment, fish don’t have a lot of escape options — they’re stuck in the lake they’re in, since there’s not a lot of connectivity.”
— Emmanuelle Chrétien
Note, however, that if Connor Warne’s team succeeded in “detecting changes in body size” in freshwater fish in response to the warming of their habitat, the data from the study did not however make it possible to quantify these changes. Not knowing their order of magnitude, it is difficult to predict what the consequences of global warming will be on the ecology of fish, or even on fishing activities.
Regardless, the results of this study add to a growing list of evidence indicating that the response of fish to climate change will be highly variable and species dependent. “This type of study will potentially have an impact on mitigating the effects of climate change. For example, for fisheries management, it will be necessary to develop different mitigation strategies depending on the thermal preferences of the species concerned,” concludes Warne.
Understanding the impact of temperature on fish size is crucial since it is a key indicator of interactions between individuals, but also between fish species. “Adult size is a good determinant of reproductive capacity in fish. We know, for example, that a reduction in the size of females can lead to a reduction in the number of eggs produced which will be viable,” explains Ms. Chrétien.
