Blackchin shiner exposed to warming and predation risk exhibit changes in behavioural and physiological metrics of...

Anthropogenic stressors that increasingly threaten freshwater biodiversity rarely exist in isolation, and their interacting effects on aquatic organisms must be evaluated. One example is the potential interaction between warming waters and increased predation pressure by range-shifting or invading piscivores. We investigated how concurrent stress from warming and predation risk (via non-lethally produced chemical cues; conspecific disturbance cues or predator odors) affected behavioural and physiological metrics of thermal tolerance in a common minnow, blackchin shiner (Miniellus heterodon). Shoals of blackchin shiner were acclimated to either 18 °C or 25 °C and underwent five critical thermal maxima (CT_Max) trials: after chronic predation risk exposure via disturbance cues and low predation risk exposure via water; after an undisturbed period; after chronic predation risk exposure via predator odor and low predation risk via water; with acute exposure to predator odor; and with acute exposure to disturbance cues. We quantified CT_Max as the temperature at loss of equilibrium, agitation temperature (T_Ag) as the first sign of behavioural stress, and the agitation window as the difference between CT_Max and T_Ag. Acclimation to 25 °C increased CT_Max in blackchin shiner, but there was no effect of chronic predation risk exposure. However, in fish acclimated to18 °C, acute exposure to disturbance cues elicited a 0.91 °C higher CT_Max and significantly larger agitation window, suggesting that blackchin shiner are experiencing signs of thermal stress over a larger range of temperatures and for a longer duration relative to fish acclimated to 25 °C.