A long-term feeding trial was implemented on rainbow trout (Oncorhynchus mykiss) to assess the effects of seven alternative oils on fish performance and fatty acid metabolism. The tested oils were as follows: monola (a high oleic acid canola cultivar; MO), canola (rapeseed; CO), poultry by-product (chicken fat; PbPO), palm (PO), sunflower (SFO), high oleic acid sunflower (HOSFO) and soybean (SBO). All tested oils were included at a 75% substitution level of fish oil (FO) and were compared with a control diet containing 100% FO. PO, and to a lesser extent PbPO, exhibited impaired performance and lower digestibility values. All treatments containing low levels of saturated fatty acids (namely MO, CO, SFO, HOSFO and SBO) recorded an apparent in vivo fatty acid de novo production. The apparent in vivo fatty acid b-oxidation was proportional to fatty acid dietary supply and limited apparent in vivo fatty acid bioconversion (elongation and desaturation) was recorded, primarily acting on n-6 PUFA. In all treatments, dietary 20:5n-3 was actively bioconverted into 22:6n-3. It was shown that when some FO is provided with the diet, the in vivo fatty acid metabolism plays a minor role in determining final fatty acid make-up of fish whole bodies.

The implementation of alternative lipid sources for use in aquaculture is of considerable interest globally. However, the possible benefit of using stearidonic acid (SDA)-rich fish oil (FO) alternatives has led to scientific confusion. Two hundred and forty rainbow trout (Oncorhynchus mykiss) were fed 1 of 4 diets (3 replicate tanks/treatment) containing either FO, linseed oil (LO), echium oil, or mixed vegetable oil (72% LO, 23% sunflower oil, and 6% canola oil) as the dietary lipid source (16.5%) for 73 d to investigate the competition and long-chain PUFA (LC-PUFA) biosynthesis between the fatty acid substrates α-linolenic acid (ALA) and SDA. SDA was more efficiently bioconverted to LC-PUFA compared with ALA. However, when the dietary lipid sources were directly compared, the increased provision of C18 PUFA within the LO diet resulted in no significant differences in (n-3) LC-PUFA content compared with fish fed the other diets. This study therefore shows that, rather than the previously speculated substrate competition, the limiting process in the apparent in vivo (n-3) LC-PUFA biosynthesis appears to be substrate availability. Rainbow trout fed the SDA- and ALA-rich dietary lipid sources subsequently had similar significant reductions in (n-3) LC-PUFA compared with fish fed the FO diet, therefore providing no additional dietary benefit on (n-3) LC-PUFA concentrations.

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