Fig. 8. Influence of freshwater temperature regime on the recruitment of fast muscle fibres during the seawater stages of Atlantic salmon (Salmo salar L). (A) The number of fast muscle fibres per myotomal cross-section (at the level of the first dorsal fin ray) in relation to fish fork length (cm) during seawater growth. Offspring from a large number of families were reared at cool ambient temperatures or in water heated by 1–3°C during the freshwater stages and then reared together in the same 5 mx5 mx5 m sea cages. The number of fish sampled at each fork length is shown in parentheses. The dotted lines (cool groups, N=45) and the solid lines (heated groups, N=40) represent fish in which fibre recruitment had ceased and these fibre number values represent the maximum (FN_max) for each freshwater treatment. (B) The myonuclei content of isolated fast muscle fibres in relation to fibre diameter (D) for seawater stages of Atlantic salmon. The open circles represent fibres from fish exposed to cool ambient temperatures and the closed circles fish exposed to water heated by 1–3°C during the freshwater stages. Myonuclei content was determined using single fibres stained with the fluorescent DNA stain Sytox Green^TM (Molecular Probes, Leiden, Holland). The lines represent first order linear regressions fitted to the data. The regression equations were as follows: for ambient fish myonuclei=314+17.1D, and for heated fish myonuclei=237+14.3D. See the original publication (Johnston et al., 2003b) for further experimental details.