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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 gilthead sea bream

The first trials with gilthead sea bream in Gran Canaria involved feeding juvenile bream diets containing 100% FO, 60% LO, 60% RO or 80% LO to triplicate groups for a period of 29 weeks. In the second dietary trials, bream were fed 100% FO or two different 60% blends and a 100% blend of rapeseed/linseed and palm oils (60% VO blends A & B and 100% VO blend) to triplicate groups for 40 weeks from initial weight of 24g. In both trials fish were grown to market size and were then fed a FO finishing diet for a further 14 weeks (trial I) or 20 weeks (trial II).

In sea bream, replacement of up to 60% of FO with VO had no detrimental effects on growth or feed conversion. Replacement with 80% linseed oil or 100% of VO blend significantly reduced growth rates. However, in fish > 250-300g this reduction was not observed suggesting that in larger/older fish requirements for essential fatty acids may be lower than in smaller fish. Flesh lipid content was unaffected by dietary lipid although liver lipid was increased in sea bream fed 80% LO and the 100% VO blend. Flesh fatty acid concentrations reflected diet fatty acid concentrations, with DHA tending to be conserved, as was the case in other target species.

In sea bream fed 60% VO, DHA & EPA were reduced by ~50% while in fish fed 100% VO the reduction was ~65%. Reduction of DHA and EPA was less in fish fed diets with low PUFA contents, e.g. the VO blend than in fish fed diets high in PUFA, such as LO and RO. DHA and EPA could be restored to > 90% of their values in FO fed fish, sea bream, by feeding a FO diet for 14-21 weeks. The duration of the finishing diet period was dependent on fish size, growth rate, and dietary DHA and EPA contents. These trials suggest that oils most suited as FO substitutes were 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 sea bream white muscle but when using blends of VO, no clear effects on beta-oxidation capacity were found. There were no effects of replacing FO with VO on hepatocyte beta-oxidation activity or substrate specificity. There were no differences in NADPH producing enzyme activities in sea bream.

Changes of plasma lipoprotein fatty acid compositions in sea bream 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.
In sea bream larvae there was evidence of C18-20 elongase activity but not for delta-6 or delta-5 desaturase activities.
Delta-6 FAD was cloned and characterised from sea bream and PUFA ELOs were cloned and characterised from salmon and sea bream. Trends were observed for dietary VO to increase delta-6 FAD in sea bream liver but these were not significant.
Sea bream showed reduced macrophage phagocytic activity when fed 60% VO compared to fish fed FO.
Arachidonic acid - derived prostaglandin production was not significantly affected by feeding VO diets but EPA-derived prostaglandin production was reduced in sea bream fed 100% VO. Sea bream fed linseed oil had significantly increased basal cortisol levels in plasma and levels were also elevated in sea bream fed linseed and rapeseed oil when subjected to a crowding stress.

Reported by

Instituto Canario de Ciencias Marinas
Grupo de Investigacion en Acuicultura
35200 Telde
Spain
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