Fig. 2. The number of fast muscle fibres per trunk cross-section at the level of the first dorsal fin ray in a subset of Atlantic salmon (Salmo salar L.) reared under conditions of extended winter day length (yellow symbols, dashed line; period of continuous lighting shown by yellow box) or at ambient photoperiod (blue symbols, solid line). The broken blue line shows sunrise and sunset (Greenwich Mean Time, GMT) at Fort William, and the green line illustrates daily recordings of sea temperature. The results are means ± S.E.M. The number of fish sampled from each cage is shown in Table 2.