Fig. 5. The mean rate of hypertrophy of fast muscle fibres between successive sample points plotted against seawater growth for the ambient (solid circles) and photoperiod-manipulated (open circles) fish. The period of continuous lighting in the photoperiod manipulated treatment is illustrated by the grey box. The rate of hypertrophy has been plotted at the midpoint of the time period over which it was calculated. Hypertrophy was calculated as the mean of the difference between the observed fibre diameter and the mean of the fibre diameter in the preceding sample.