Heritability of fibre number and size parameters and their genetic relationship to flesh quality traits in Atlantic salmon (Salmo salar L.)

Two year classes of Atlantic salmon (Salmo salar L.) (Fanad–Mowi strain) were studied from the Marine Harvest family selection program. Fish were harvested at ∼ 3 years of age in 2004 and 2005 and comprised 149 and 157 full-sib families. All the fish in each year class were reared under the same conditions. Following paternity analysis using DNA microsatellites a sub-set of 142 fish in 2004 and 70 fish in 2005 were selected from 28 families and the number and size of fast myotomal muscle fibres determined at 0.7 fork length (FL). Muscle fat content, flesh colour and pigment concentration were also measured. There was a tendency for fish sampled in 2005 to be heavier for a given fork length than in 2004, and for males to be longer than females reflecting sexual maturation. It was established that muscle fibre recruitment had ceased in all the fish sampled and the final fibre number had been reached. The average muscle fibre number per family ranged from 3.48 × 105 ± 1.7 × 104 to 4.65 × 105 ± 1.9 × 104 (Mean ± SE, n = 5 and 8 fish respectively). Fibre number showed no significant differences between male and female fish or with year class. In contrast, muscle fibre density (fibres per mm2 muscle) was inversely related to FL, and higher in 2004 than 2005 reflecting a lower average fibre size. Myogenic progenitor cells were quantified using a specific marker, an antibody to paired box protein 7 (Pax-7). There were significant positive correlations between the density of Pax-7 positive cells and fibre number (R2 = 0.20; F1,59 = 15.0; P = 0.001) and muscle fibre density (R2 = 0.30; F1,59 = 25.8; P < 0.0001) standardized to a 4 kg fish. Flesh lipid content was positively correlated with FL, and was higher in 2005 than 2004. Multi-trait genetic models with sex nested within year were constructed with and without corrections for body length. Muscle fibre number and density showed medium heritabilities: 0.33 ± 0.05 (with body length as covariate) and 0.51 ± 0.09 and 0.26 ± 0.08 (without body length as covariate) (Mean ± SE). All phenotypic correlations between fibre number and density and quality traits were low or negative, but genetic correlation to visually assessed colour and pigment content were significantly positive (P < 0.05). There was also a strong and significant negative genetic correlation between fat content and both fibre number (− 0.85) and fibre density (− 0.76). The use of muscle fibre traits in breeding programs designed to improve flesh quality is discussed.