The effects of various dietary blends of menhaden oil (MO) with canola oil (CO) on the growth performance, whole body proximate composition, flesh quality (muscle proximate and lipid composition) and thyroidal status of immature Atlantic salmon in sea water were studied. Atlantic salmon (initial weight, 145.2-181.3 g), held on a natural photoperiod and in 1100 L fibreglass tanks that were supplied with running, aerated (D.O., 9-10.5 p.p.m.), ambient temperature (8-10.5 degrees C) sea water (salinity, 28-30 parts per thousand), were fed twice daily to satiation one of four isonitrogenous (similar to 36% digestible protein) and isoenergetic (similar to 18.8 MJ of digestible energy kg-l) extruded high-energy diets for 112 days. All diets contained omega -3 (n-3) fatty acids in excess of requirements and differed only with respect to the source of the supplemental lipid which was either, 25% MO; 20.75% MO and 4.25% CO; 16.5% MO and 8.5% CO; or 12.25% MO and 12.75% CO. Thus, CO comprised, respectively, 0, 15.5, 31.2, or 47.0% of the total dietary lipid content (similar to 28% on an air-dry basis). Dissimilar percentages of saturated fatty acids in the dietary lipids were not found to be consistently related to the apparent gross energy digestibility coefficients of the diets. Atlantic salmon growth, dry feed intake, feed and protein utilization, percent survival, thyroidal status, and whole body and muscle proximate compositions were generally not influenced by the different sources of supplemental lipid. Therefore, our results suggest that canola oil may comprise as much as 47% of the lipid in high-energy grower diets for Atlantic salmon without compromising performance. The muscle lipid compositions generally mirrored those of the dietary lipids which, in turn, were influenced strongly by the concentrations and compositions of the CO and MO in the diet. Hence, as the dietary CO level was increased there were attendant increases in percentages of oleic acid (18:1(n-9)), linoleic acid (18:2(n-6)), total omega-6 (n-6) fatty acid content, and ratios of (n-6) to (n-3) and decreases of eicosapentaenoic acid (EPA; 20:5(n-3)), docosahexaenoic acid (DHA; 22:6(n-3)) and n-3 HUFAs (EPA & DHA) in the flesh lipids. The ranges for percentages of saturated and unsaturated fatty acids in the flesh lipids were, however, much less than those noted respectively in the dietary lipids probably because of selective metabolism of many of the former acids and some of the 18 carbon unsaturates for energy purposes.
