Atlantic salmon, Salmo salar L., smolt production strategy affects body composition and early seawater growth

The main objective of this study was to compare the development of seawater adaptation and changes in body composition of smolt produced using three different strategies, and to investigate growth during the first month in seawater. Smolt released at different times of the year experience different environmental conditions. In northern Norway there are large annual variations in seawater temperature and photoperiod, and both are known to influence growth rates of salmonids. Three groups of smolts were raised and released 15 April (Group 1, large 1+ smolt), 15 June (Group 2, medium sized 1+ smolt) and 15 September (Group 3, medium sized 0+ smolt). The 0+ out-of-season smolt (group 3) transferred to seawater in September bad the best hypo-osmoregulatory ability of the three groups after one month in seawater. These fish also had the highest growth rate (assessed using TGC), and deposited body fat during their first month in seawater. By contrast large 1+ salmon released in April bad poorer hypoosmoregulatory ability, failed to grow during their first month in the sea, and appeared to mobilise body fat stores to support metabolism during this critical phase. The medium-sized 1+ smolt, transferred to seawater in June, occupied an intermediate position. Although they appeared to experience some problems with hypo-osmotic regulation, the fish grew during their first month in seawater, and they were able to maintain their concentration of body fat. Our findings conclude that the 0+ out-of-season smolt were not disadvantaged during the early phase of seawater rearing compared to the smolt that were produced by a 'traditional' strategy and transferred to seawater in spring. Indications were that the 0+ smolt fared better following transfer to seawater than did the fish making up both the large 1+ (group 1) and the medium-sized 1+ (group 2) groups. (C) 2004 Elsevier B.V. All rights reserved.