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Abstract

The use of alternate oils (AO) in aquafeeds is now a reality due to the rise in
the price of fish oil (FO). The lack of n-3 long-chain (>C20) polyunsaturated fatty
acid (n-3 LC-PUFA) in AO is a major constraint due to the resulting low levels of n-3
LC-PUFA obtained in farmed fish. The aim of this study was to understand the
metabolic basis underlying current and innovative strategies to maintain n-3 LCPUFA
levels in salmonids, particularly for Atlantic salmon, fed diets in which fish oil
(FO) is substituted with AO in a series of four independent experiments. Use of
Echium oil (EO) rich in stearidonic acid (SDA) has the potential to bypass the initial
Δ6 desaturase enzyme which is a rate limiting step for n-3 LC-PUFA biosynthesis.
This hypothesis was tested by growing Atlantic salmon in freshwater and seawater on
diets where FO was completely substituted by either EO or rapeseed oil (RO) with
comparison to a FO diet. The results indicated that SDA rich oil is a more suitable
candidate for FO replacement in aquafeeds for Atlantic salmon compared to
conventional vegetable oils due to enhanced n-3 LC-PUFA biosynthesis. The use of a
FO finishing diet (FOFD) is a suitable way to restore n-3 LC-PUFA in fish after a
growth period with AO. Preferential FA metabolism may contribute to n-3 LC-PUFA
restoration by favouring β-oxidation of saturated fatty acids and/or monosaturated
fatty acids. Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)
restoration was investigated in rainbow trout fed a diet where FO was substituted by
50% palm fatty acid distillate (PFAD) and 75% PFAD followed by a FOFD period.
There was no evidence for preferential FA metabolism occurring and the dilution
model was a good predictor of FA changes after dietary change. In aiming at
improving the n-3 LC-PUFA restoration by the FOFD strategy, a short term food
deprivation after growth on 75% PFAD prior to feeding the FOFD was undertaken for
Atlantic salmon smolts. Short term food deprivation reduced lipid content especially
in the fillet of fish which lead to an increase in n-3 LC-PUFA % composition. Subsequent feeding to satiation with a FOFD improved the restoration of n-3 LCPUFA
in the fillet of unfed fish. This thesis also examined the use of oil blends in
aquafeeds with emphasis on varied DHA and EPA ratio as a strategy for sustaining n-
3 LC-PUFA in Atlantic salmon. The DHA and EPA ratio of FO is typically 1:1.5 in
any blend of FO and AO. According to EPA and DHA metabolism in fish, higher
dietary DHA content to EPA might be more suitable for optimizing their deposition.
We tested whether altering the dietary DHA: EPA ratio in Atlantic salmon by
increasing dietary DHA inclusion in conjunction with low dietary n-3 LC-PUFA
content may enable efficient n-3 LC-PUFA deposition in fish. The results indicated
that a diet having a higher DHA: EPA ratio than usually encountered in FO or FO/AO
blend diets is better suited for optimizing n-3 LC-PUFA deposition in Atlantic
salmon.

Item Type:	Thesis - PhD
Authors/Creators:	Codabaccus, MB
Keywords:	fish oil, Atlantic salmon, fatty acid metabolism, n-3 Lc Pufa, biosynthesis, novel oils
Additional Information:	Copyright 2011 the Author
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