Fig. 3. Atlantic salmon (Salmo salar L.) reared under conditions of ambient photoperiod (closed circles, solid line; combined cages 1 and 2) or continuous lighting from 1 November 2000 to 18 June 2001 (open circles, broken line; combined cages 3 and 4). (A) The relationship between the number of fibres and the total cross-sectional area (TCA) of fast myotomal muscle at the level of the first dorsal fin ray. The arrows join common sample points. (B) The relationship between the densities of fast muscle fibres (fibres mm^-2 cross-sectional area) and the growth period in seawater. The results are means ± S.E.M. The number of fish sampled from each cage is shown in Table 2.