Researching Alternatives to Fish Oils in Aquaculture
Project Results
The major findings of the RAFOA project can be summarised as follows;
Growth and fatty acid compositions
- In salmon and trout, 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
- In sea bass and 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 in sea bream significantly reduced growth rates, although in fish > 250-300g this reduction was not observed
- Flesh lipid content was unaffected by dietary lipid in all species. However, liver lipid was increased in salmon fed > 50% LO, in sea bream fed 80% LO and in sea bream fed the 100% VO blend.
- In all species flesh fatty acid concentrations reflected diet fatty acid concentrations, with DHA tending to be conserved in all species
- In salmon fed 100% VO, flesh DHA & EPA concentrations were reduced by ~65%, while in trout the reduction was ~50%
- In sea bass and sea bream fed 60% VO, DHA & EPA were reduced by ~50% while in sea bream fed 100% VO the reduction was ~65%
- 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 values could be restored to 70-80% of the values in salmon fed FO by feeding a 100% FO finishing diet for 16-24 weeks
- DHA and EPA could be restored to > 90% of their values in FO fish, in trout, sea bass and sea bream, by feeding a FO diet for 14-24 weeks.
- The duration of the finishing diet period was dependent on fish size, growth rate, and dietary DHA and EPA contents.
- 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.
Lipid metabolism
- High levels of dietary linseed oil increased β-oxidation in trout and sea bream white muscle
- Dietary rapeseed oil added at up to 75% replacement increased β-oxidation in salmon white and red muscle
- Using blends of VO, no clear effects on β-oxidation capacity were found in any of the species
- Hepatic b-oxidation increased during smoltification of Atlantic salmon, followed by a decreased β-oxidation capacity in red muscle
- Substituting FO with VO increased β-oxidation of palmitic and oleic acids in trout muscle
- There were no effects of replacing FO with VO on hepatocyte β-oxidation activity or substrate specificity
- There were no differences in NADPH producing enzyme activities in trout and sea bream, or increased activity in salmon
- There were no differences in fatty acid synthetase activity in trout and salmon
- The cholesterol level in trout plasma and in salmon plasma during the freshwater stage decreased with VO substitution
- The level of plasma LDL lipid decreased in trout and salmon with VO substitution.
- VO substitution caused no changes in plasma triglycerides.
- Changes of plasma lipoprotein fatty acid compositions in Atlantic salmon and 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
- Replacement of FO with VO did not affect tissue lipid uptake in rainbow trout
- No differences were seen in lipoprotein lipase activity in muscle and adipose tissue of trout
- In salmon hepatocytes enzyme activities decrease along the pathway: Δ6 fatty acid desaturase (FAD) = C18-20 fatty acid elongase (ELO) > Δ5 FAD > C20-22 ELO > C22-24 ELO = Δ6*FAD
- In salmon hepatocytes HUFA synthesis was increased by dietary VO mainly at Δ6 FAD and C18-20 ELO
- In sea bass hepatocytes HUFA synthesis was very low, ~ 5- to 20-fold lower than in salmon
- In sea bass HUFA synthesis was increased by dietary VO in enterocytes but not hepatocytes
- In sea bream larvae there was evidence of C18-20 elongase activity but not for Δ6 or Δ5 desaturase activities
- DHA synthesis as measured by conversion of H5-18:3n-3 occurred at a low rate in vivo in trout and salmon
- DHA synthesis was increased by feeding VO, certainly in trout and possibly in salmon
- DHA synthesis was lower in Atlantic salmon parr than in equivalent sized rainbow trout
- DHA synthesis decreased as fish became larger/older
- Even in fully induced trout, DHA synthesis was insufficient to maintain carcass levels of DHA
- The majority of H5-18:3n-3 (>95%) fed to trout and salmon was oxidized
Molecular Biology: Cloning and characterisation of genes involved in HUFA synthesis
- Δ6 FAD was cloned and characterised from salmon, trout and sea bream
- Δ5 FAD was cloned and characterised from salmon
- PUFA ELOs were cloned and characterised from salmon and sea bream
- Hepatic Δ6 and Δ5 FAD gene expressions were increased by dietary VO in salmon
- ELO expression was increased in salmon only by LO
- There was no evidence for dietary VO increasing Δ6 FAD in trout liver
- Trends were observed for dietary VO to increase Δ6 FAD in sea bream and sea bass liver; however, this was only significant in sea bass for one of the vegetable oil blends tested
Fish health and immune function
- Both sea bass and sea bream showed reduced macrophage phagocytic activity when fed 60% VO
- Arachidonic acid - derived prostaglandin production was reduced in salmon and sea bass fed VO diets, although this was less affected feeding a VO blend than by single VO diets
- 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
- 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 and sea bass
- When a VO blend was chosen to replace FO, immune functions were not affected up to 75% replacement in salmon and up to 60% replacement in sea bass
- Cataract incidences in salmon fed 75% and 100% VO blend was 4 and 5 times higher, respectively, compared to fish fed FO
- Vaccination efficacy in salmon was not affected by the dietary VO
- Salmon challenged with Vibrio anguillarum did not show significant differences in mortality when FO was replaced by a VO blend
Organoleptic properties and fish quality
- High levels of VO in the diet had only minor effects on the organoleptic properties of:
- salmon, whether raw, cooked or smoked; in some cases fish fed VO diets were preferred to fish fed 100% FO,
- trout whether cooked or smoked,
- bass, although lower scores were observed in very fresh fish
- Electronic nose analyses discriminated between fish fed different oils with best separation in trout:
- a finishing FO diet reduced the discrimination substantially;
- Replacing FO with VO had no or only minor effects on product quality and storage stability characteristics with:
- time on ice being a more important factor for all species tested,
- feeding a 100% FO-finishing diet prior to harvest eliminating most earlier effects of VO diets