Enhanced conspicuousness of prey in warmer water mitigates the constraint of turbidity for predators.

Changes in environmental conditions impact predator-prey interactions by altering behavior through sensory and non-sensory (eg metabolic or cognitive) pathways. Elevated water temperature and turbidity are known to alter activity levels and anti-predator responses in prey fish, and are increasing globally as a result of anthropogenic activities. Less is known about how temperature and turbidity impact predators' ability to detect prey directly, or indirectly via changes to prey behavior. We quantified the detectability of Trinidadian guppies (Poecilia reticulata) free-swimming in a large arena from the perspective of a stationary visual predator (simulated as an underwater camera). We used a fully factorial experimental design testing the independent and combined effects of increased temperature and turbidity. We found that both stressors had a strong influence on the appearance of prey (objectively quantified as the mean magnitude of the optical flow in the videos). As expected, turbidity reduced the frequency of detection between the guppies and the simulated predator, ie the magnitude of optical flow exceeded the threshold for a "detection event" more often in clear water. Events were also shorter in duration in turbid water, reducing the time available for a predator to detect the prey. However, during an event, prey were more detectable in warmer water (ie the mean magnitude was greater). Although we found no evidence of interactive effects of turbidity and temperature on the response variables, their cumulative main effects suggest an antagonistic effect between the two stressors on the predator-prey dynamic overall.