Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)




The period of smoltification is crucial to the maintenance of a self-supporting salmon population. This study addresses the physiological and behavioral changes that occur during smoltification and the environmental cues which induce this metamorphosis in Chinook salmon (Oncorhynchus tshawytscha). The timing of imprinting in this species is also investigated.

The effects of novel water chemistry, increased stream flow rate, photoperiod and age and size on plasma levels of thyroxine, triiodothyronine and cortisol, hematocrit, condition factor and hepatosomatic index were examined. Exposure to a novel water source was found to induce smoltification while doubling the stream flow rate had no effect. Smoltification was inhibited in salmon raised under 24 hour light or a constant 9L:15D photoperiod but was stimulated in salmon raised under an increasing photoperiod. Large age I salmon (L = 225.7 mm; W = 151.33 g) showed more signs of a complete, coordinated smoltification process than small age I (L = 148.3 mm; W = 26.74 g) or age 0 (L = 82.4 mm; W = 5.37 g) salmon.

Changes in behavior were studied by observation of salmon in an artificial stream at a hatchery and a release facility. Salmon observed at the release facility were more active and more likely to be found in the upstream, downstream and pelagic areas of the experimental tank than fish observed at the hatchery.

The timing of imprinting was studied by artificially imprinting groups of tagged salmon in the hatchery, releasing them into Lake Oahe, South Dakota, and recording the number of spawning adults returning to the release site. No differences were seen in return rates between imprinted and non-imprinted salmon even after accounting for higher mortality in the imprinted salmon. Potential reasons for this result include the lack of environmental changes during the imprinting period.

This study shows that appropriate environmental stimuli prior to and during the time of smoltification are necessary for Chinook salmon to undergo a complete and coordinated smoltification process. The results of this study underscore the importance of understanding the role of environmental cues in the induction and coordination of the behavioral, physiological and biochemical changes that occur during smoltification in salmon.