Effects of dietary fatty acid profile and fat content on smolting and seawater performance in Atlantic salmon (Salmo salar L.)

An experiment was conducted to study the effects of dietary fat level and fatty acid composition on seawater acclimation and growth in Atlantic salmon. Marine fish oil or a blend of rapeseed and linseed oils were added to extruded pellets to produce four feeds differing in fat content (LF: 21% and HF: 34%) and fatty acid composition. The feeds were designated LFFO, LFVO, HFFO and HFVO according to fat level (LF--low fat; HF--high fat) and oil source (FO--fish oil; VO--vegetable oil). Each feed was fed to salmon parr (approximately 19 g) held at 2 °C on a 12L:12D regime for 6 months. Parr-smolt transformation was then induced by increasing the photoperiod from 12L:12D to 24L:0D, and water temperature to 8 °C. Fish fed the four feeds grew at similar rates both during the parr stage (SGR; 0.40±0.01) and during the smoltification period (SGR; 0.64±0.01). Fish fed the high-fat feeds had a higher percentage body fat than fish fed low-fat feeds, and fatty acid profiles resembled those of the feed. Parr-smolt transformation was accomplished within 3 weeks after change in photoperiod in all groups, as assessed by gill Na+,K+-ATPase activity, muscle water and plasma chloride following 24-h seawater tests. During the 42-days seawater period the fish were fed either LFFO or HFFO feed. Groups of 50-60 fish were subjected to one of eight feed treatments: no dietary shift, shift in feed oil type, shift in feed fat (energy) content or shift both in feed oil type and feed fat content at the time of seawater transfer. Fish in all groups lost weight during the first 3 weeks in seawater, but fish fed the LFVO feed (i.e. low-fat vegetable oil) during freshwater rearing gained weight during the total 6-week seawater period. Significantly better growth and a significantly higher proportion of fish with positive growth rates than in other treatments was, however, only seen in the group in which a shift in both lipid source (from VO to FO) and feed fat content (from LF to HF) had been applied. Whether this was an effect of increased supply of n-3 HUFAs or dietary energy, or a combination of both factors, is not clear. There were no significant differences in plasma chloride or plasma osmolality between groups during seawater residence or in gill Na+,K+-ATPase activity at the end of the seawater period.