Applications of rapeseed oil in ongrowing diets for Atlantic salmon (Salmo Salar)

Abstract

The overall aim of this project was to investigate the substitution of fish oil with rape seed oil in the diets of post-smolt (sea water) Atlantic salmon. Of primary importance was to investigate the effects of feeding rape seed oil on growth performance and flesh quality parameters important to retailers and consumers e.g. total oil content of the flesh, flesh fatty acid composition, particularly the content of n-3 highly unsaturated fatty acids (HUFA) and flesh colour. Duplicate groups of Atlantic salmon post-smolts were fed five practical-type diets in which the added lipid was either 100% fish oil [FO, 0% rape seed oil (0% RO)], 90% FO, 10% RO (10% RO), 75% FO, 25% RO (25% RO), 50% FO, 50% RO (50% RO) or 100% RO in two separate trials of 17 and 26 weeks. Most of the data described in this report refers to the former trial as growth rates were better compared to the second trial which used smaller fish and was performed when water temperatures were lower. There were no effects of diet on growth rate and feed conversion nor were any histopathological lesions found in liver, heart, muscle or kidney. Proximate analysis showed that the highest accumulation of muscle lipid was in fish fed 0% RO which correlated with significantly lower muscle protein in the same group, compared to all other treatments. Conversely, the highest lipid levels in liver were found in fish fed 100% RO. Fatty acid compositions of muscle were correlated with RO inclusion such that total monounsaturated fatty acids were increased, largely due to increased 18:1(n-9), along with 18:2(n-6) and 18:3(n-3). The concentration of muscle eicosapentenoic acid (20:5(n-3), EPA) and docosahexaenoic acid (22:6(n-3), DHA) were reduced, along with total saturated fatty acids, with increasing inclusion of dietary RO. Dietary-induced changes in liver fatty acid compositions were broadly similar. Hepatic fatty acid desaturation and elongation activity, i.e. the ability of the fish to synthesise long-chain HUFA from their 18-carbon dietary precursors, increased with addition of dietary RO and this appeared to compensate, at least in part, for the reduced levels of dietary EPA and DHA. In summary, RO can be used successfully as a substitute for fish oil in the culture of Atlantic salmon in sea water although, at levels of RO above 50% reductions in muscle EPA, DHA and (n-3)/(n-6) polyunsaturated fatty acid (PUFA) ratio will result which could reduce the availability of these fatty acids that are essential for human health.