Stress and saltwater-entry behavior of juvenile chinook salmon (Oncorhynchus tshawytscha):: conflicts in physiological motivation

Two experiments were conducted to determine the effects of a mild or severe stressor on the saltwater preference of juvenile spring chinook salmon (Oncorhynchus tshawytscha). To observe the response of fish to an overhead threat, we presented stressed fish with an avian predator model in the second experiment. Experiments were conducted in 757-L tanks containing a stable vertical salinity gradient. Only 69% of fish stressed by being chased for 2 min before saltwater introduction (mild stressor) held in salt water, whereas 95% of unchased control fish preferred the saltwater layer. After the more severe handling and confinement stressor, only 20% of fish entered and remained in salt water compared with 100% of unstressed controls. After the presentation of the avian model, stressed fish holding in fresh water moved into the saltwater layer, but this behavioral response was transient. Fish began returning to fresh water within 10 min, and after 1 h, only 26% of stressed fish remained in the saltwater layer. Stress significantly decreases the saltwater preference of chinook salmon that would otherwise select full-strength salt water and may affect behavior in the estuary. Although smoltification primes these fish for seawater residence, stress apparently induced a conflicting physiological motivation.