Plasticity of muscle fibre number in seawater stages of Atlantic salmon in response to photoperiod manipulation.

Atlantic salmon (Salmo salar L.) were fed to satiety and reared from approximately 60 g to 5000 g at ambient seawater temperatures. The effect of photoperiod manipulation on muscle growth was investigated from the start of the first sea winter. Continuous light treatment in winter/spring (1 November to 18 June) improved growth performance in fish, resulting in a 30% increase in mean body mass relative to the ambient photoperiod fish by 12 August, but had no effect on sexual maturation. Significant increases in body mass in the continuous light groups were observed after 126 days (P<0.01). The number of fast muscle fibres per trunk cross-section was determined in a subset of the fish and was 28.5% higher in the continuous light (799 x 10(3)) than the natural day length (644 x 10(3)) groups after only 40 days, corresponding to the period of decreasing natural day length. Subsequent rates of fibre recruitment were similar between treatments. At the end of the fibre recruitment phase of growth (combined June and August samples), the maximum number of fast muscle fibres was 23% higher in fish from the cages receiving continuous light (881 x 10(3)+/-32 x 10(3); N=19) than in the ambient photoperiod cages (717 x 10(3)+/-15 x 10(3); N=20) (P<0.001). Continuous light treatment was associated with a shift in the distribution of fibre diameters, reflecting the altered patterns of fibre recruitment. However, the mean rate of fibre hypertrophy showed no consistent difference between treatments. There was a linear relationship between the myonuclear content of isolated single fibres and fibre diameter. On average, there were 27% more myonuclei in 150 microm-diameter fibres in the continuous light (3118 myonuclei cm(-1)) than the ambient photoperiod (2448 myonuclei cm(-1)) fish. After 40 days, continuous light treatment resulted in a transient increase in the density of myogenic progenitor cells, identified using a c-met antibody, to a level 70% above that of fish exposed to natural light. It is suggested that short days inhibited the proliferation of myogenic progenitor cells and that this was overcome by transferring fish to continuous light, causing an increase in the number of times the myogenic precursor cells divided and/or a decrease in cell cycle time. The net increase in myogenic progenitor cells resulted in proportional increases in the number and myonuclear content of fibres. The subsequent hypertrophy of these additional fibres can explain the delayed increase in body mass observed with continuous light treatment.