Effects of Minute Water Temperature Changes on Downstream Movement and Migratory Behavior in Pacific Salmon

We suggest that downstream orientation and perhaps downstream migration in juvenile Pacific salmon is triggered or regulated by slight decreases in water temperature in the rivers.  Further, our data suggest that the relationship between temperature and downstream behavior is mediated by cortisol which is involved in the smoltification.  Over a three year period, small decreases in water temperature (< 2 C) were associated with elevations in plasma cortisol levels in masu salmon smolts (O. masou) in a Japanese river.  In an artificial raceway, cortisol implantation increased downstream orientation, and a slight decrease in water temperature (< 1 C) increased plasma cortisol levels.  In Oregon, Chinook salmon (O. tschawytscha) juveniles, exposed to a similar decrease in water temperature also exhibited an increase of cortisol levels.  And, coho salmon (O. kisutch) parr in an artificial stream exhibited downstream orientation when exposed to a 1 C drop in water temperature.  On the other hand, coho salmon parr given cortisol implants showed constant downstream movement independent of water temperature.  Therefore, we conclude that a slight decrease in water temperature is one of the important environmental cues resulting in modification of downstream migratory behavior in Pacific salmons.