Chemical alarm signals and complex hatchery rearing habitats affect antipredator behavior and survival of chinook salmon (Oncorhynchus tshawytscha) juveniles

The present study examined the effects of chemical antipredator conditioning on antipredator behavior and the relative effects of antipredator conditioning and seminatural rearing environments on postrelease survival of chinook salmon (Onocrhynchus tshawytscha). Hatchery-reared juvenile chinook salmon were exposed to extracts from conspecific tissue or to comparable stimuli from green swordtail (Xiphophorus helleri). These "injured fish" stimuli were paired with water that contained the odour of predatory cutthroat trout (Oncorhynchus clarki). Chinook salmon receiving conspecific stimuli showed higher levels of several antipredator behaviors compared with chinook salmon receiving green swordtail extracts. When the two groups of chinook salmon were tested 2 days later with cutthroat trout stimulus alone, the chinook salmon that had originally received injured conspecific stimuli paired with cutthroat trout odour spent more time motionless than chinook salmon that had received green swordtail stimuli and cutthroat trout odour. In another experiment, complex rearing treatments had a negative effect on instream survival (contrary to previous studies) that was compensated for by the application of the chinook salmon extract and cutthroat trout odour prior to release. Chinook salmon, like rainbow trout (Oncorhynchus mykiss), show antipredator behavior in response to chemical stimuli from injured conspecifics and learn predator recognition when such stimuli are paired with predator odour, improving survival in the wild.