Critical thermal maxima of three small-bodied fish species (Cypriniformes) of different origin and protection status

Environmental changes related to global warming are both a threat to cold-water fishes and simultaneously create favourable conditions for the spread of eurythermic or warm-water species. In both cases, it is necessary to develop conservation strategies supported by precise ecological data, including thermal requirements. In this study, we determined the upper thermal tolerance thresholds and the critical maxima for three small, non-commercial Eurasian freshwater fish species; i.e. lake minnow Eupallasella (=Rhynchocypris) percnurus, sunbleak Leucaspius delineatus and topmouth gudgeon Pseudorasbora parva using the Critical Thermal Method at different acclimation temperatures, i.e. 18, 23, 28, and 33°C (highest treatment for topmouth gudgeon only). We hypothesized that lake minnow would have the lowest thermal tolerance and the topmouth gudgeon the highest. The response to temperature rise and the upper thermal limits were species-specific and correlated with the acclimation temperature, but not with fish length. Sunbleak showed the lowest thermal tolerance, though at 28°C both E. percnurus and L. delineatus reached a similar critical thermal limit. Topmouth gudgeon showed distinctly higher upper thresholds of thermal tolerance and at the highest acclimation temperature the critical upper limit for the species was close to 42°C. The results obtained for L. delineatus were surprising as we predicted that this leuciscid fish would be more tolerant of high temperatures than E. percnurus. We discuss the results in relation to the threat of extinction and the risk of species spreading beyond their natural range in the context of a warming environment.