Effects of feed, feeding regime and growth rate on flesh quality in farmed Atlantic salmon: function of fast-muscle collagen cross-links and plasma hormones

The present experiment was designed to investigate interrelations, and possible conflicts between high rearing intensity and maintaining desired quality of the end product, and to explore biological mechanism underpinning qualitative and quantitative aspects of Atlantic salmon farming (Salmo salar L.). Two size-matched salmon groups (~2.4 kg), with high or low fillet fat content were created by feeding isonitrogenous feeds with high (HE) or low energy (LE) content from ~1 kg. From ~2.4 kg until harvest (~4.0 kg) both the HE and LE groups were fed to satiation (Sat; 68 days) or by moderately reduced rations (Res; 103 d), using the same feed as during the pre-conditioning period. Feeding to satiation induced higher energy intake, whereas increased digestible feed energy content lowered the feed intake without compromising growth. Although altered feed energy level or feeding regime did not result in poor flesh quality, restrictive feeding gave firmer flesh due to increased cross-link concentration (r2 = 0.28). Liquid loss (~1.7% vs. ~3.2%) was also improved with restrictive feeding, and the LE diet combined with restrictive feeding gave the lowest fillet fat content (~12.7% vs. HE-Sat; 14.5%). However, satiated fed salmon had a more intense red colour than those restrictively fed. Fillet gaping score was low across feed treatments and not influenced by the applied feed or feeding regime. Individual growth rates (i.e. TGC3) correlated positively to fat deposition (r2 = 0.38), but less evident to texture (r2 = 0.16). A multiple linear regression analysis revealed PYD as the only factor influencing fillet firmness significantly, while TGC3, pre-rigor muscle pH, fillet fat, alkaline soluble and insoluble HYP, and plasma leptin and ghrelin had no significant effect on fillet firmness. The increase in plasma leptin levels following long-term LE diet suggest that leptin is linked to energy balance in salmon, but may not act as an adiposity signal as there was no correlation to fillet fat content. From the present experiment it is concluded that both feed energy and feeding regime can be used to manage important flesh quality characteristics in Atlantic salmon, and that cross-linking of collagen fibres is an important factor to maintain desired fillet firmness.