Polyunsaturated fatty acid metabolism in Atlantic salmon (Salmo salar) undergoing parr-smolt transformation and the effects of dietary linseed and rapeseed oils

Duplicate groups of Atlantic salmon parr were fed diets containing either f ish oil (FO), rapeseed oil (RO), linseed oil (LO) or linseed oil supplement ed with arachidonic acid (20:4n-6; AA) (LOA) from October (week 0) to seawa ter transfer in March (week 19). From March to July (weeks 20-34) all fish were fed a fish oil-containing diet. Fatty acyl desaturation and elongation activity in isolated hepatocytes incubated with [1-C-14]18:3n-3 increased in all dietary groups, peaking in early March about one month prior to seaw ater transfer. Desaturation activities at their peak were significantly gre ater in fish fed the vegetable oils, particularly RO, compared to fish fed FO. Docosahexaenoic acid (22:6n-3:DHA) and AA in liver and gill polar lipid s (PL) increased in all dietary groups during the freshwater phase whereas eicosapentaenoic acid (20:5n-3; EPA) increased greatly in all groups after seawater transfer. The AA/EPA ratio in tissue PL increased up to seawater t ransfer and then decreased after transfer. AA levels and the AA/EPA ratio i n gill PL were generally higher in the LOA group. The levels of 18:3n-3 in muscle total lipid were increased significantly in the LO, LOA and, to a le sser extent, RO groups prior to transfer but were reduced to initial levels by the termination of the experiment (week 34). In contrast, 18:2n-6 in mu scle total lipid was significantly increased after 18 weeks in fish fed the diets supplemented with RO and LO, and was significantly greater in the FO and RO groups at the termination of the experiment. Gill PGF production sh owed a large peak about two months after transfer to seawater. The producti on of total PGF post-transfer was significantly lower in fish previously fe d the LOA diet. However, plasma chloride concentrations in fish subjected t o a seawater challenge at 18 weeks were all lower in fish fed the diets wit h vegetable oils. This effect was significant in the case of fish receiving the diet with LOA, compared to those fed the diet containing FO. The prese nt study showed that during parr-smolt transformation in Atlantic salmon th ere is a pre-adaptive increase in hepatocyte fatty acyl desaturation/elonga tion activities that is controlled primarily by environmental factors such as photoperiod and temperature but that can also be significantly modulated by diet. Feeding salmon parr diets supplemented with rapeseed or linseed o ils prevented inhibition of the desaturase activities that is induced by fe eding parr diets with fish oils and thus influenced the smoltification proc ess by altering tissue PL fatty acid compositions and eicosanoid production . These effects, in turn, had a beneficial effect on the ability of the fis h to osmoregulate and thus adapt to salinity changes.