Nowadays, global warming is a real issue and scientists are trying to find ways to anticipate its effects on wildlife, especially ectotherms as there are the first in line to be impacted by temperature increase. Indeed, their body temperature directly depends on environmental temperature, which is a factor that drastically influences biochemical and physiological reaction rates and, in fine, body condition and behaviour. We chose to study two different populations of a pelagic fish, Alburnoides bipunctatus, that were caught mid-February 2024 upstream or downstream of the Bugey nuclear power plant, located along the Rhône River. Indeed, as the water is warmer downstream from the power plant, this river section can be regarded as a “future condition” considering global warming. So, we wondered whether downstream fish had different adaptations regarding thermal tolerance, thermal preference and swim performance compared to the upstream ones. Both populations were kept at 12°C before entering experimentation. 12 fish from each population were put through 3 experiments which were randomised in order for 3 weeks, so each fish had a week of recovery between each experiment.

Our aim was to characterize the phenotypes of the two populations through different physiological and behavioural experimental protocols. We used CTmax (critical thermal maximum), which is a commonly used metric to measure an organism's upper thermal tolerance limit and a Ucrit protocol, which is the most known method to measure swimming performance in fish and can be considered as a proxy of the fish's general fitness and well-being. This protocol was also linked to MO2 measurement. The fish were also put in a shuttle box with dynamic temperature change to elucidate their thermal preference.

We supposed that upstream fish would be in better condition than downstream ones, but that the latter would probably have a higher metabolism, higher CTmax and higher thermal preference as they live in a warmer environment.

Indeed, downstream fish had a greater CTmax than upstream ones, indicating that their upper thermal limit is higher. Interestingly, both groups had a similar thermal preference but the variance among the downstream fish was high. Upstream fish had a greater Ucrit, indicating that they may be in better condition, and their metabolic rates were also higher. However, upstream fish were the smallest, so there is an effect of size on our results. Consequently, even though most of the fish are juveniles, our results could be driven by the metabolic theory of ecology.