By W. R. Poole, D. T. Nolan, T. Wevers, M. Dillane, D. Cotter, O. Tully.
Aquaculture, Volume 222, Issues 1-4, Salmonid Smoltification, 26 May 2003, Pages 301-314, ISSN 0044-8486, DOI: 10.1016/S0044-8486(03)00129-7.

Total fish trapping facilities that enable monitoring of all migrating fish have been operating in the Burrishoole river system, west of Ireland, since 1970. Ocean ranching of reared salmon smolts has been used since the early 1960s to supplement grilse returns to the rod fishery and as a research tool for assessing marine survival and exploitation. Wild Burrishoole salmon (Salmo salar) smolts have, on average, a fivefold higher return rate to freshwater than hatchery-reared smolts derived from the same stock. To help explain these differences in return rates, histological and physiological data were collected in 2000 from two groups of hatchery-reared salmon smolts just before release and from wild smolts migrating about the same time. The data show striking differences (size, condition factor (CF), blood cortisol and glucose, gill mucous cells) between both smolt groups. Wild smolt were significantly smaller (mean length: 14.8 and 13.9 cm) and thinner (Mean CF: 0.91 on both dates) than hatchery reared smolt (mean length: 20.3 and 19.1 cm; Mean CF: 1.01 and 1.09). Wild smolt had higher basal cortisol levels in April than in May and did not display 'typical' quadratic responses to capture stress, while hatchery-reared smolt did display quadratic cortisol stress responses. Similar differences were observed in serum glucose levels. There was considerable variation in gill and skin histology between fish sampled from the same group, even on the same day, although these were generally present at low levels and were not related to catching order or confinement. Hatchery-reared smolt had significantly higher concentrations of mucous cells in both skin and secondary gill lamellae. This preliminary look at Burrishoole wild and reared salmon smolts indicated striking differences between the two groups which may be linked to subsequent marine survival.
Keywords: Salmon; Salmo salar; Cortisol; Histology; Gill; Skin; Physiology