Environmental effects on lipid nutrition of farmed Atlantic salmon (Salmo salar L.) parr and smolts

The aim of this study was to investigate the effects of water temperature and salinity on lipid nutrition of farmed Atlantic salmon (Salmo salar L.) parr and smolt. Salmon parr were held at low water temperature (2°C) for six months while being fed feeds that differed in oil source (i.e. marine fish oil or vegetable oil blend) and concentration (low, 21% and high, 34%). The responses at low temperature were compared with those of fish held at 8°C using full-factorial design. Feeding and growth were maintained at 2°C, although at lower rates than at 8°C. Growth and feed utilisation improved over time, suggestive for a long-term acclimation response in fish held at low temperature. Overall feed efficiency was better at the lower temperature. A gradual decrease in growth rate and feed utilisation was seen at the higher temperature as the fish grew larger. The fish compensated for reduced energy density by increasing feed intake. At the higher temperature, better growth was found for fish fed the low-fat feeds, and there was also a tendency for improved growth when vegetable oil was used. Thus, there were no signs that vegetable oils are inferior to marine fish oil in promoting growth of Atlantic salmon parr in fresh water. Fish fed high fat feed were fatter than fish fed low fat feed, suggestive of lipostatic regulation of feed intake. Fat and protein digestibility were high at both 2°C and 8°C, although both fat and protein digestibility were lower at 2°C. At the lower temperature, increased dietary fat level increased the fat digestibility, and improved protein digestibility were seen when vegetable oil was included in the feed. Protein retention was higher at the higher temperature irrespective of feed treatment, indicating that proteins were both readily digested and converted into new tissues. The effects of feed treatment on low temperature acclimation responses were assessed from deposition of dietary fatty acids in fish tissues and from n-3 and n-6 essential fatty acid (EFA) budgets. Fatty acid composition of polar (membrane) and non-polar (storage) lipids in muscle, viscera and carcass were markedly influenced by the dietary oil, and non-polar lipids were more influenced than polar lipids. The retention n-6 EFAs was lower than for n- 3 EFAs, and was independent of temperature. The retention of n-3 EFAs retention was higher at the 2°C, especially amongst fish given the fish oil based diets. This may be a reflection of the importance of n-3 HUFAs during low temperature acclimation. However, the unsaturation (UFA:SFA ratio) of polar lipids was higher in fish fed the vegetable oils than for fish fed fish oil based feed. This may imply that vegetable oils produced fish that were better able to withstand exposure to low temperature, while having membrane lipids less susceptible to oxidative damage, due to the lower contents of n-3 HUFAs (mainly EPA and DHA). The six months feeding period in freshwater was followed by parr-smolt transformation, and a subsequent 42-days on-growing in seawater. Feed history during freshwater rearing influenced on-growth of smolts. A positive effect of using a vegetable oil was indicated, but this effect was only seen when there was a shift to a high-lipid fish oil based feed at the time of transfer to seawater. As such, it was evident that use of vegetable oils in freshwater feed did not interfere with low temperature acclimation or parr-smolt transformation of juvenile salmon, and subsequent on-growing in seawater was better when vegetable oil had been used in the feed. This indicates that fatty acid (lipid) requirement of Atlantic salmon are probably different in fresh water and seawater, and that these changes are linked to parr-smolt transformation. It could be speculated that that salinity may be more important than temperature as an environmental influence on the fatty acid requirements of Atlantic salmon