Effects of dietary α-linolenic acid (18:3n−3)/linoleic acid (18:2n−6) ratio on growth performance, fillet fatty acid profile and finishing efficiency in Murray cod

The objective of the present study was to evaluate the effect of different dietary α-linolenic acid (ALA, 18:3n−3) to linoleic acid (LA, 18:2n−6) ratios, while employing 100% fish oil deprived diets, on growth performance and flesh fatty acid composition of Murray cod, and subsequent finishing (wash-out) efficiency. The ALA/LA ratios of the five experimental diets were gradually increased from 0.3 to 2.9, with a constant total content of ALA + LA of 51%. Fish oil was used for the sixth diet (control), which was also used during the finishing period. It was shown that an increased dietary ALA/LA ratio does not impair growth performance or the tissue lipid concentration of Murray cod. However, the dietary ALA/LA ratio significantly impacts on the final fatty acid make-up and nutritional quality of the fish fillet. In particular, the fillets of fish fed with higher ALA/LA ratios (hence receiving more dietary ALA) recorded significantly higher concentrations of EPA and DHA (eicosapentaenoic acid, 20:5n−3, and docosahexaenoic acid, 22:6n−3, respectively). High dietary LA content, however, was shown to have significant negative impacts on the efficiency of a finishing strategy. Interestingly, it was observed that, on average, only 10% of the total EPA and DHA provided during the finishing period via the fish oil-based diet was retained and deposited into the fish fillet. Additionally, the deposition of EPA and DHA during the finishing period was shown to be affected by previous feeding history, with fish previously fed high LA diets depositing significantly lower amounts of these fatty acids in comparison to fish previously fed a diet rich in ALA.