Increased protein-to-lipid ratio in energy dense diets improves slaughter yields and muscle thickness of different weight classes of farmed Atlantic salmon (Salmo salar L.)

Abstract

Two isoenergetic feeding strategies, with emphasis on the grow-out stage (> 1 kg), with dietary high (HP > 1.1) and low protein-to-lipid ratio (LP < 1.0) were tested by using year-old smolt (S1) and under-year-old smolt (S0) Atlantic salmon in two large-scale trials, respectively, and in one small-scale experiment using S1 smolt. This was done to investigate the dietary influence on slaughter yield, muscle fat content, condition factor and thickness of the hypaxial anterior muscle (HAM) in all three trials. In addition, effects on the viscerosomatic index (VSI) was included in the small-scale trial. The initial body weights in the three trials were 100 g, 319 g and 978 g, respectively. At harvest, fish for analyzes were sampled into three weight classes of 4.5 kg, 5.5 kg, 6.5 kg in the large-scale trials, and 2.4 kg, 3.2 kg and 4.0 kg in the small-scale trial. In all three trials, the dietary HP strategy significantly improved slaughter yield (p < 0.01). In the large-scale trials, fish of the HP groups had lower muscle fat (p < 0.05), higher condition factor (significant in the S1 group: p < 0.01) and a trend towards a thicker HAM (p ≤ 0.10) than the LP groups. In all three trials, there were a significant positive relation between condition factor and HAM. The small-scale trial verified the large-scale trials revealing significantly lower VSI (p < 0.001) among the HP groups, partly explaining the high increase in slaughter yield (1.1%) for the HP groups compared to LP groups in the large-scale study. Except for slaughter yield and VSI, weight class significantly influenced all quality traits. Overall, this study indicates that the salmon farming industry, which generally prefers using lipid dense grower feeds, can improve product yields by using isoenergetic feeds with dietary high protein-to-lipid ratio. Keywords: Atlantic salmon, Dietary protein-to-lipid ratio, Quality, Slaughter yield, Muscle thickness