A sampling method is described to determine accurately the number of fast myotomal muscle fibres (NF) in a large flatfish species, the Atlantic halibut Hippoglossus hippoglossus. An unusual feature of the fast myotomal muscle is the presence of internalized strips of slow muscle fibres. In fish of 1·5–3·5 kg (n = 24), the total cross-sectional area (ATC) of fast muscle was 18% greater in the dorsal than ventral myotomal compartments (P < 0·05), whereas there was no significant difference between left- and right-hand sides of the body. Due the bilateral asymmetry, muscle blocks (5 × 5 × 5 mm) were prepared to systematically sample each myotomal quadrant (dorsal, ventral, left- and right-side) and the diameters of 150 fast fibres measured per block. Smooth non-parametric probability functions were fitted to a minimum of 800 measurements of fibre diameter per quadrant (n = 5). There were no significant differences in the distribution of muscle fibre diameters between myotomal compartments and therefore NF could be estimated from a single quadrant. The number of blocks required to estimate NF with a repeatability of ±2·5% increased from six at 300 g body mass to 17 at 96·5 kg, caused by variation within and between blocks. Gompertz curves were fitted to measurements of fibre number and fork length (LF). The estimated final fibre number was 8·96 × 105 (7·99–9·94 × 105, 95% CI) for males and 1·73 × 106 (1·56–1·90 × 106, 95% CI) for female fish. The estimated LF for cessation of fibre recruitment in the fast muscle of female fish (1775 mm) was almost twice that in males (810 mm), reflecting their greater ultimate body size.
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