Fig. 4. The mean probability density function (pdf) of fibre diameter in the fast muscle of Atlantic salmon in June 2001 at the end of the period of photoperiod manipulation. Ambient photoperiod (solid line; N=10) and 24 h continuous lighting regime (dashed line; N=10). The dotted line represents the average probability of the combined population, and the grey shaded area represents 100 bootstrap estimates of the probability density. Areas where the mean pdf of the ambient and photoperiod-manipulated treatments fall outside the shaded area provide a graphical representation of the parts of the distribution that are significantly different. The position of data points is shown on the abscissa.