Effects of early rearing history on selected endocrine and immune functions in juvenile Pacific salmonids

The effects of different early rearing conditions on plasma Cortisol concentration, immune function, hematological profile, and disease resistance were examined in populations of hatchery-reared and wild Chinook salmon (Oncorhynchus tshawytscha) and hatchery-reared, wild, and colonized coho salmon (O. kisutch). These features were examined during smoltification and after an acute stress. The effect of initiating feeding with a wild-type diet as compared to a commercially prepared diet was also examined in chinook salmon fry as one aspect of rearing history that is different between fish reared in the hatchery and those in the wild. During smoltification and following periods of acute stress, wild chinook salmon and wild and colonized coho salmon had significantly higher concentrations of plasma Cortisol. Hatchery-reared juveniles showed less sensitivity to stress and lower concentrations of plasma Cortisol during the smoltification period and after an acute stress. Antibody producing cell (APC) number and disease resistance to Vibrio anguillarum were not significantly different between the hatchery and wild chinook salmon. These features were also similar among the hatchery, wild and colonized coho salmon smolts, despite significantly higher levels of circulating Cortisol in the wild and colonized smolts. White blood cell to red blood cell (wbc/rbc) ratios were slightly higher in wild fish than in their hatchery-reared counterparts in chinook and coho salmon juveniles. Significant elevations in plasma Cortisol concentration after an acute stress was still evident retained in the wild and colonized coho salmon juveniles even after holding them for 6 months in an artificial rearing environment. Disease resistance in the wild fish significantly decreased over that time. Following the 6-month rearing period, the initial numbers of APC in the wild fish were significantly higher than those in their hatchery counterparts. This difference precludes the ability to conclude that a cause-effect relationship exists between a high Cortisol response and decreased specific immune function. The use of a live diet to initiate feeding in chinook salmon fry compares favorably to feeding a commercial diet in that the activity of lysozyme and wbc/rbc ratios were higher in this group. Specific immune function was correlated with body weight while non-specific immune defense was not. There appear to be physiological differences between hatchery-reared, wild and colonized coho and chinook salmon. Rearing history may be a determinant in the survival of hatchery-reared salmonids released into the natural environment.Land and Food Systems, Faculty ofGraduat