Effects of feed, feeding regime and growth rate on flesh quality, connective tissue and plasma hormones in farmed Atlantic salmon (Salmo salar L.)

Maintaining control of the harvest quality of the edible product is becoming increasingly important for successful fish farming operations. 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 the mechanism underpinning quantitative and qualitative aspects of Atlantic salmon (Salmo salar L.) farming. Two isonitrogenous feeds (340g proteinkg⁻¹ feed) comprising either high energy (HE; 22.1MJkg⁻¹) or low energy (LE; 19.3MJkg⁻¹) were fed to 2.4kg fish differing in whole body energy status until harvest. Over this period both HE and LE feeds were fed to satiation (Sat) or by moderately reduced (Res) ration to create four dietary treatments; HE-Sat, HE-Res, LE-Sat, and LE-Res. Satiation feeding induced higher energy intake, faster growth rates, elevated red colour intensity (a*-value), softer flesh, and higher liquid loss compared to restricted feeding. At satiation feeding, increased digestible feed energy lowered the feed intake without compromising growth. LE diet resulted in reduced fat accumulation without compromising growth, and feeding LE feed restrictively had the largest effect (~2% unit) on fat accumulation. Individual growth rates (TGC₃) correlated positively to fat deposition (r²=0.38), but less evident to fillet firmness (r²=0.16). There was a significant positive correlation between the hydroxylysyl pyridinoline (PYD) cross-link concentration and fillet firmness, where PYD explained 28% of the total variation. A multiple linear regression (MLR) analysis revealed PYD as the only factor influencing fillet firmness significantly, whilst TGC₃, pre-rigor muscle pH, fillet fat, alkaline-soluble and -insoluble hydroxyproline (HYP), and plasma leptin and ghrelin had no significant effect on fillet firmness. The incidence of gaping was low across feed treatments, and gaping scores were not correlated to fillet firmness nor influenced by the applied feed or feeding regime over the experiment. The decrease in plasma leptin levels following long-term HE diet suggests that leptin is linked to energy balance in salmon, but may not act as an adiposity signal. From the present experiment it is concluded that both feed energy and feeding regime can be used to manage important quality attributes of Atlantic salmon, and that cross-linking of collagen fibres is an important factor to maintain desired fillet firmness.