Abstract

Despite the wealth of laboratory studies, the effects of local predation risk on the behaviour and ecology of a prey species under natural conditions have rarely been examined. In Chapter 1, I tested the hypothesis that juvenile Atlantic salmon ( Salmo salar ) use both chemical and visual information to assess predation risk under natural conditions. Both young-of-the-year (YOY) and parr salmon exhibited antipredator responses when exposed to a chemical alarm cue and exposure to alarm cues influenced their response to a visual threat. While YOY and parr differed in the type and intensity of antipredator responses, both chemical and visual cues are used in an additive manner. In Chapter 2, I tested the prediction that YOY use chemical alarm cues to assess the predation risk of alternative habitats and decide where to settle over a two-week period following emergence. In seven similar reaches of Catamaran Brook, New Brunswick, I established three contiguous sections where I manipulated the perceived predation risk by releasing alarm cues versus a stream water control. The density of YOY decreased in risky sections and increased in control and untreated buffer sections, whereas the density of parr was not affected. Clearly, YOY salmon can assess and select habitats based on the perceived level of predation risk. If one assumes that predation risk is a cost, optimality models predict that territory size will decrease with increasing predation risk. In Chapter 3, I examined whether both acute and chronic predation risk influences the territorial behaviour of YOY. When exposed to a single dose of chemical alarm cue, YOY salmon decreased the number of switches between foraging stations, but did not change their territory size or foraging rate. When exposed to chemical alarm cue over a two week period, YOY salmon reduced the size of their territories, but did not change their foraging rate or number of switches. Clearly, YOY adjusted their territorial behaviour in response to both acute and chronic increases in perceived predation risk, but in different ways. Together, my results suggest that predation risk influences not only short-term (immediate) anti-predator behaviour, but is also an important component of habitat selection and shapes territorial behaviour over longer periods.