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The effects of substitution of dietary fish oil with vegetable oils on the growth, tissue lipid composition, metabolism and health of Atlantic salmon (Norway)

The first salmon trials in Norway involved feeding post-smolts diets containing FO, and where FO were replaced by 25, 50, 75 & 100% rapeseed oil (RO) plus a single replacement with 50% olive oil (OO). In the second dietary trials, salmon were fed FO or a 100% replacement of a blend of rapeseed, linseed and palm oils (100% VO blend) to replicate groups over the whole production cycle from first feeding. In both trials fish were grown to market size and were then fed a FO finishing diet for a further 24 weeks. In salmon, replacement of fish oil (FO) with vegetable oil (VO), up to 100%, did not affect growth and feed conversion. Salmon fed a 100% VO blend had significantly higher final weights than fish fed FO.

Flesh lipid content was unaffected by dietary lipid in all trial groups but flesh fatty acid concentrations reflected diet fatty acid concentrations, with DHA tending to be highly conserved. In salmon fed 100% VO, flesh DHA & EPA concentrations were reduced by ~65% although reduction of DHA and EPA was less in fish fed diets with low PUFA contents, e.g. olive oil, and the VO blend than in fish fed diets higher in PUFA such as RO. However, DHA and EPA values could be restored to >70% of the values in salmon fed FO by feeding a 100% FO finishing diet for 16-24 weeks and the duration of the finishing diet period was dependent on fish size, growth rate, and dietary DHA and EPA contents. Therefore, we would suggest that oils most suited as FO substitutes should be high in monoenes, contain saturate levels similar to those in the fish being fed and be low in C18 PUFA, especially 18:2n-6, as this fatty acid is poorly oxidised and difficult to remove using finishing diets.

Dietary rapeseed oil added at up to 75% replacement increased- oxidation in salmon white and red muscle.
- Hepatic-oxidation increased during smoltification of Atlantic salmon, followed by a decreased
--oxidation capacity in red muscle although there were no effects of replacing FO with VO on hepatocyte
- -oxidation activity or substrate specificity.

There were no differences in NADPH producing enzyme activities in salmon nor were any differences observed in fatty acid synthetase activity. The cholesterol level in salmon plasma during the freshwater stage decreased with VO substitution and the level of plasma LDL lipid decreased with VO substitution. However, VO substitution caused no changes in plasma triglycerides. Changes of plasma lipoprotein fatty acid compositions in Atlantic salmon and caused by VO substitution included: increased FA characteristic of VO (18:1,18:2 n-6 and 18:3 n-3); decreased FA characteristic of FO (EPA, DHA). These changes occured in plasma lipoprotein fractions in the order: VLDL> LDL> HDL.

When using a single VO to replace FO, a number of immune parameters (haematocrit, leucocyte numbers, macrophage respiratory burst) were altered by dietary treatment, in salmon.
Cataract incidences in salmon fed the 100% VO blend was 5 times higher compared to fish fed FO. However, vaccination efficacy in salmon was not affected by the dietary VO.

Contact

Bente TORSTENSEN, (Project leader)
Tel.: +47-55-238262/5
Fax: +30-210-7722050
E-mail
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