Acute and chronic increases in predation risk affect the territorial behaviour of juvenile Atlantic salmon in the wild

Optimality models predict that territory size will decrease as the costs of defence increase. One poorly understood cost is predation risk, especially the relative influence of short- versus long-term increases in predation risk. Under natural conditions, we quantified the territorial behaviour of juvenile Atlantic salmon, Salmo salar, exposed to either acute or chronic increases in perceived predation risk. The effects of an acute increase in predation risk were assessed by exposing 18 young-of-the-year (YOY) Atlantic salmon to a control of stream water and to an alarm cue (i.e. conspecific skin extract) while monitoring their territorial behaviour. We investigated the effects of a chronic increase in perceived predation risk by quantifying the territorial behaviour of YOY salmon in control versus risky sections of seven sites, where we manipulated the perceived predation risk over a 4-week period by releasing stream water in control sections and alarm cue in risky sections. We found that salmon exposed to the alarm cue decreased the number of switches between foraging stations, but they did not change their territory size or foraging rate. As predicted, YOY salmon in risky sections had smaller territories than in control sections. However, their foraging rates and number of switches between foraging stations did not differ between treatments. Our study suggests that juvenile Atlantic salmon are sensitive to both acute and chronic increases in perceived predation risk under natural conditions, and support the predictions of optimality models that territory size decreases with increasing predation risk.