Do juvenile Atlantic salmon (Salmo salar) use chemosensory cues to detect and avoid risky habitats in the wild?

We examined whether juvenile Atlantic salmon (Salmo solar) in the wild adjust their behaviour in response to chemical cues of predator activity during a 4-week period after emergence from gravel nests. In each of seven 75 m(2) sites in Catamaran Brook, New Brunswick, Canada, we established three contiguous sections differing in perceived predator activity by releasing stream water in control sections, conspecific alarm cues in risky sections, and nothing in buffer sections in both 2006 and 2007. As predicted, the density of young-of-the-year (YOY) salmon tended to decrease in alarm cue sections, while it increased in control and buffer sections. After the 2-week manipulation in 2006, we switched treatments so that buffer sections became alarm cue sections and alarm cue sections became buffer sections for an additional 2-week period. After the switch, the number of YOY increased least in the new alarm cue sections and most in control and new buffer sections. In contrast with YOY, the density of age 1 + parr was not affected by the experimental treatments. Our results suggest that YOY salmon can use chemical alarm cues to assess the predator activity of habitats in the wild.