Replacing fish oil with pre-extruded carbohydrate in diets for Atlantic salmon, Salmo salar, during their entire marine grow-out phase: Effects on growth, composition and colour

Abstract In an attempt to reduce fish-oil use in feeds for Atlantic salmon, graded amounts of fish oil were substituted with carbohydrate (pre-extruded wheat, corn or pea), such that feeds containing a low dietary crude fat content had a high level of carbohydrate inclusion and vice versa. These low, medium and high-energy feeds were reformulated 4 times, in response to increasing fish size (9 feeds in 3, 4 and 2 × 8.5 mm sizes, 36 feeds in total) and fed to S1 Atlantic salmon smolts throughout their 377 day marine grow-out phase. Growth performance, condition factor, yields and flesh quality attributes were documented. Fish grew from ∼53 to ∼2500 g, achieving SGRs ranging from 0.24 (over-winter) to 2.02% day − 1 (approaching their first summer after sea transfer). Growth rate showed expected seasonal variation, but was unaffected by carbohydrate source therefore, differences in growth rates and final harvest weights were attributed to dietary lipid content, being lowest for fish fed the low oil (high carbohydrate) feeds. Condition factor, carcass yield and hepatosomatic index also displayed seasonal variation and was again unaffected by carbohydrate source. However, condition factor was further influenced by fish size/dietary lipid content and correlated negatively with carcass yield, which increased from ∼89% to ∼93% throughout the study. At harvest, fillet fat content in the Scottish Quality Cut (SQC) ranged from 9.5% to 15.1% and Roche Salmofan™ scores were between 25 and 28. Differences in fillet fat content, pigmentation and visually/instrumentally measured flesh colour were also due to dietary lipid content and not carbohydrate source. Therefore, in economic terms, the low oil feeds (high carbohydrate inclusion) would be of little use, but only growth and not flesh quality was compromised. Thus, there is still scope for the use of such carbohydrate sources in Atlantic salmon feeds, but as a cost-effective means of substitution for lipids, their use is somewhat limited.