Growth performance, muscle structure and flesh quality in out-of-season Atlantic salmon (Salmo salar) smolts reared under two different photoperiod regimes

This experiment on farmed Atlantic salmon (Salmo salar L.) was designed to investigate the effect out-of-season produced smolts. The offspring from three pairs of half-sibling crosses were studied to provide information on the genetic variability of treatment effects. In both treatments, parr were initially held on 10-h light/14-h dark for 6 weeks from the 27 September until the 8 November 2000, and then transferred to continuous light. All fish were transferred from fresh to salt water on the 12 January. In treatment A (TA), smolts were transferred from continuous light to ambient short days (23 January 2001), whereas in treatment B, (TB) smolts were continued on 24-h light until natural day length approached the seasonal maximum (I June 2001). Fish were reared in outdoor saltwater tanks and transferred to 5 X 5 X 5 m sea cages at the Feed Trial Unit, Loch Duich, Scotland on the 21 April 2001. In the 6-9 months following smoltification, the growth performance of 835 fish held on continuous light (TB) was superior to that of 715 fish returned to short days (TA). Thereafter, in five out of the six families, growth performance was higher in TA than TB groups, such that the average bodyweight of families was no different between treatments over the production cycle. Bodyweight, however, showed significant family effects and a significant treatment x family interaction. The cellularity of the fast myotomal muscle was determined in a subset of fish sampled 2 weeks after the lights were switched-off in the TB groups (14 June 200 1) and when a commercial harvest weight was achieved (5 September 2002). In June 2001, the average bodyweight was 80% higher, and the number of muscle fibres per myotomal cross-section was 43.5% greater in TB than TA groups. Muscle fibre recruitment was subsequently higher in the TA than TB groups such that the maximum fibre number (5.88 x 105) was similar between treatments. However, the density of fast muscle fibres (fibres mm(-2) muscle cross-sectional area) remained 7.5% higher for TB (82.1 +/- 1.3, n = 46) than TA (76.4 +/- 1.1, n = 53) groups (mean +/- S.E.; P < 0.05). Firmness was assessed in fillets 3 days postmortem using an instrumental texture method (TA-Hdi Texture Analyser, Stable Microsystems). The total work done to shear tissue samples (mJ) was 8.3% higher in TB (432 +/- 11, n = 46) than TA (399 +/- 7, n = 53) groups (mean S.E.; P < 0.05). At the family level, there was a significant correlation between the total shear work (Sw) and muscle fibre density (Fd), explaining 49% of the total variation. Individual growth (a) rate was investigated using a statistical model. For the relatively modest growth rates achieved in the trial, there was no significant correlation between a and either Sw or Fd. The concentration of muscle lipid was 9.1% higher in the TA (14.7% dry mass) than TB groups (13.3% dry mass). Muscle astaxanthin concentration was independent of treatment but showed significant differences between families. It was concluded that continuous light treatment immediately following seawater transfer produced small but significant effects on muscle growth over the production cycle resulting in a higher fibre density and a firmer flesh. The magnitude of such effects is likely to be critically dependent on the timing of the harvest. (C) 2004 Elsevier B.V. All rights reserved.