Innate abilities to distinguish between predator species and cue concentration in Atlantic salmon

Atlantic salmon, Salmo salar, increase their ventilation rate in response to predator odour cues. We measured the ventilation response of newly hatched Atlantic salmon to odours of high- and low-risk predators at five concentrations (10–200%). Concentration may be interpreted as predator proximity and/or number of predators present and thus represents a gradation of predation risk. Predator species (pike, Esox lucius, and minnow, Phoxinus phoxinus) and predator odour concentration affected the number of individuals responding and the strength of individual response. Fewer individuals responded to minnow odour than pike odour, and the proportion of individuals responding also declined with odour concentration. Salmon responded more strongly to high-risk predator (pike) odour than the low-risk (minnow) odour for a given concentration, and ventilation rate generally increased with increasing concentrations of both odours. This study shows that Atlantic salmon have the innate ability to evaluate risk in terms of odour concentration and predator species.