Impact of accelerated smoltification on muscle structure and fillet firmness at harvest in Atlantic salmon (Salmo salar)

Mixed families of Atlantic salmon (Salmo salar L.) were subject to two photoperiods to produce accelerated and 1+ smolts in January and May 2001, respectively. The fish were reared in freshwater land-based tanks until October 2000 and then split into two groups: natural photoperiod (1+ smolts) and manipulated photoperiod (accelerated smolts). Fish were sorted into small and large grades and transferred to 12 sea cages at Gildeskål Research Station, Northern Norway (13°57′20ʺE, 67°04′10ʺN) in May-June 2001. The four groups were reared in triplicate cages until October 2002. The number and size distributions of fast myotomal muscle fibres were determined at the level of the first dorsal fin for all groups in May 2002 (selected fish of 2.4 kg) and October 2002 (randomly sampled fish). The presence of muscle fibres less than 10 μm diameter indicated active fibre recruitment in all groups in May 2002. Muscle fibre number had reached its maximum value (FNmax) in the October harvest. Analysis of variance revealed no significant difference in average values of FNmax (6.6×105±1.4×10 4), muscle fibre density (76.2±1.7 fibres mm−2 muscle) and fibre diameter (121.3±1.5 μm) between different photoperiod treatments (mean±S.E., n=66). Smoothed curves were fitted to the measurements of fibre diameter using a kernel function. Nonparametric Kolmogorov-Smirnov tests revealed no significant difference in the distribution of fibre size between accelerated and 1+ post-smolts. There were, however, significantly more muscle fibres in the large than small grade fish, probably reflecting genetic differences in FNmax. Flesh quality was assessed as fillet firmness using an instrumental texture method. The total work (J) required to shear muscle samples of standardized thickness showed no significant difference between photoperiod treatments. It was concluded that the manipulation of photoperiod required to produce accelerated smolts did not affect muscle cellularity and flesh firmness in post-smolts of a commercial harvest weight.