Brook char (Salvelinus fontinalis) can differentiate chemical alarm cues produced by different age/size classes of conspecifics.

A wide diversity of aquatic organisms release chemical alarm cues when captured by a predator. For most animals, it is not known whether the specific chemicals that comprise the alarm cue are conserved as prey animals age. In this study, we tested whether brook char (Salvelinus fontinalis) can differentiate alarm cues produced by individuals of different ages/sizes. In separate laboratory experiments we exposed small brook char and large brook char to chemical alarm cues from small brook char, large brook char, and a control of swordtails (Xiphophorus helleri). Both small and large brook char responded with antipredator behavior to chemical alarm cues from both small and large char, but not to those from swordtails. Small char responded with a greater response intensity to cues of small char than to cues of large char. In contrast, large char responded with a greater response intensity to cues of large char than to cues of small char. These results suggest that chemical(s) that act as the alarm cue for fish of different age/size classes may be: (1) identical and that there may be other chemical(s) that allow the test fish to distinguish between cues from fish of different ages/sizes, or (2) the cues are not identical, but similar enough to be recognized.