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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 rainbow trout

The first trials with rainbow trout (RT) in France involved feeding juvenile trout diets containing 100% FO, 100% LO, 100% RO or 50% OO to triplicate groups for a period of 12 weeks. In the second dietary trials, RT were fed 100% FO or 75% and 100% replacement with a blend of rapeseed/linseed and palm oils (75% VO blend and 100% blend) to triplicate 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 12 weeks (trial I) or 24 weeks (trial II). In RT, replacement of fish oil (FO) with vegetable oil (VO), up to 100%, did not significantly affect growth and feed conversion. Flesh lipid content was unaffected by dietary lipid and flesh fatty acid concentrations reflected diet fatty acid concentrations, with DHA tending to be highly conserved. In trout fed 100% VO, flesh DHA & EPA concentrations were reduced by ~50% 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 high in PUFA. DHA and EPA could be restored to > 90% of their values in FO fish, in RT, by feeding a FO finishing diet for 12-24 weeks. The duration of the finishing diet period was dependent on fish size, growth rate, and dietary DHA and EPA contents.

This data 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. High levels of dietary linseed oil increased beta-oxidation in RT white muscle. However, using blends of VO, no clear effects on beta-oxidation capacity were found in trout or any of the target species. Replacement of FO with VO did not affect tissue lipid uptake in RT. In addition, no differences were seen in lipoprotein lipase activity in muscle and adipose tissue of RT.

Delta-6 FAD was cloned and characterised from RT although there was no evidence for dietary VO increasing delta-6 FAD in trout liver.

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Institut National de la Recherche Agronomique
64310 St-Pee sur Nivelle
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