Chemosensory detection of predators by coho salmon (Oncorhynchus kisutch): behavioural reaction and the physiological stress response

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Abstract
Behavioral and physiological reactions were examined in juvenile coho salmon (Oncorhynchus kisutch) before and after the chemodetection of alarm substance and chemical stimuli released from predators and nonpredators. Chemical stimulus from northern squawfish (Ptychocheilus oregonensis) was avoided in a two-choice Y trough, whereas stimulus from the largescale sucker (Catostomus macrocheilus) was not. Paradoxically, both stimuli induced a stress response as indicated by elevations in plasma cortisol and glucose. Plasma thyroxine was not a sensitive indicator of stress. Extracts from the broken skin of squawfish or suckers induced a physiological stress response, thereby raising the possibility of a nonostariophysan fish recognizing an ostariophysan alarm substance. Rinses from human skin or L-serine were behaviorally avoided, but neither stimulus induced physiological stress responses. A conclusion from these results is that behavioral and physiological reactions to chemical stimuli from predators do not necessarily co-occur. These data also suggest that fright is not necessarily a sufficient condition for inducing a stress response of the general adaptation syndrome type in fish.

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