Contrasting behavioural responses of grazing mayflies and detritivorous caddisflies to predatory fish

Invertebrates living in streams where predatory fish are present are often able to detect them through water-borne chemical cues and respond with behavioural changes that lower predation risk. We hypothesised that behavioural responses to a predatory fish (brown trout, Salmo trutta) would be stronger in grazing mayflies (Baetis sp. and Epeorus sp.) than in detritivorous caddisflies (Potamophylax latipennis and Chaetopteryx sp.) in a montane stream in central Spain, because of differences in their foraging ecology and the presence of a protective case in the latter. Grazing mayflies reduced their rates of movement and entrance into the drift when trout were present, but this response disappeared shortly after trout removal by electrofishing. Mayflies also showed marked drift diel periodicity when trout were present. In contrast, detritivorous caddisflies responded to the potential predation threat by withdrawing into their case. However, their response was the same regardless of the presence of trout, which also had no influence on their movement patterns or drift activity. Our results suggest that effects of predatory fish on stream invertebrates vary with invertebrate traits such as foraging ecology and predator-avoidance strategies.