The effect of temperature and dietary fat level on tissue lipid composition in Atlantic salmon (Salmo salar) fed wax ester-rich oil from Calanus finmarchicus

Copepod oil (CO) from the marine zooplankton, Calanus finmarchicus, is a potential alternative to fish oils for inclusion in aquafeeds. The oil is composed mainly of wax esters containing high levels of saturated fatty acids and monounsaturated fatty alcohols that are poorly digested by fish at low temperatures. Consequently, tissue lipid compositions may be adversely affected in salmon fed CO at low temperatures. The present study examined the lipid and fatty acid compositions of muscle and liver of Atlantic salmon reared at two temperatures (3 °C and 12 °C) and fed diets containing either fish oil (FO) or CO, supplying 50 % of dietary lipid as wax esters, at two fat levels (~33 %, high; ~18 %, low). Fish were acclimatised to rearing temperature for one month and then fed one of four diets: high fat fish oil (HFFO), high fat Calanus oil (HFCO), low fat fish oil (LFFO) and low fat Calanus oil (LFCO). The fish were grown to produce an approximate doubling of initial weight at harvest (220 days at 3 °C and 67 days at 12 °C), and lipid content, lipid class composition and fatty acid composition of liver and muscle determined. The differences in tissue lipid composition between dietary groups were relatively small. The majority of fatty acid in triacylglycerols (TAG) in both tissues were monounsaturated and their levels were generally higher at 3oC than 12oC. Polyunsaturated fatty acids (PUFA), particularly (n-3) PUFA, predominated in the polar lipids and their level was not significantly affected by temperature. The PUFA content of TAG was highest (approximately 26%) in the muscle of fish fed the HFCO diet at both temperatures. Tissue levels of saturated fatty acids were lower in fish fed diets containing HFCO than those fed HFFO, LFFO or LFCO, particularly at 3oC. The results are consistent with Atlantic salmon being able to incorporate both the fatty acid and fatty alcohol components of wax esters into tissue lipids when CO is supplied at a level of 33 % in the diet at an environmental temperature of 3 oC without a major change in tissue lipid composition.