THE EFFECT OF DIETARY-LIPID ON POLYUNSATURATED FATTY-ACID METABOLISM IN ATLANTIC SALMON (SALMO-SALAR) UNDERGOING PARR-SMOLT TRANSFORMATION

The aim of this study was to measure the changes in lipid metabolism w hich occur during smoltification and seawater transfer in Atlantic sal mon (Salmo salad. Duplicate groups of Atlantic salmon parr were fed di ets containing either fish oil (FO) or a blend of linseed and rapeseed oils, vegetable oil (VO), from October (week 0) to seawater transfer in May (week 26). From May to August (weeks 26-43), all fish were fed a fish oil-containing diet. Fatty acyl desaturation and elongation act ivity were followed in isolated hepatocytes incubated with radioactive 18:3n-3 and 18:2n-6. Metabolism of 18:3n-3 was consistently around 5- fold greater than metabolism of 18:2n-6, and total metabolism of both substrate polyunsaturated fatty acids (PUFA) was increased in fish fed both VO and FO up to seawater transfer after which desaturation activ ities were reduced. Desaturation activities with both 18:3n-3 and 18:2 n-6 were significantly greater in fish fed VO, compared to fish fed FO , at 22 and 26 wk. Arachidonic acid (20:4n-6; AA) in liver polar lipid s (PL) of fish fed VO increased consistently from weeks 0-22 but varie d after seawater transfer. In fish fed FO, AA in liver PL remained con stant up to week 17 before increasing at seawater transfer and levelin g off thereafter. Eicosapentaenoic acid (20:5n-3; EPA) in liver PL of fish fed VO decreased significantly from week 0-22 before rising at se awater transfer and increasing rapidly posttransfer. EPA in liver PL o f fish fed FO showed a similar trend except EPA was always greater in the freshwater phase compared to fish fed VO. Docosahexaenoic acid (DH A) levels in liver FL of fish fed VO remained constant in the freshwat er phase before increasing following seawater transfer. In fish fed FO , DHA in liver PL increased from weeks 0-17 reducing and leveling off postseawater transfer. The levels of PGF(2 alpha) and PGF(3 alpha) wer e measured in isolated gill cells stimulated with calcium ionophore A2 3187. PGF(2 alpha) production in fish fed VO increased significantly b etween 0-7 wk before decreasing toward seawater transfer. After transf er, PGF(2 alpha) production increased to a peak at 35 wk. PGF(2 alpha) production in fish fed FO was not significantly altered during the tr ial period. The changes in PGF(3 alpha) production were broadly simila r to those occurring with PGF(2 alpha), but the latter was always in e xcess of the former (2- to 4-fold). Plasma chloride concentrations in fish subjected to seawater challenge at 20 wk were significantly lower in fish fed VO compared to those fed FO. This study has provided new information on the changes in lipid metabolism which accompany parr-sm elt transformation and suggests that diets which have a fatty acid com position more similar to that in aquatic invertebrates may be benefici al in effecting successful seawater adaptation.