Myotube production in fast myotomal muscle is switched‐off at shorter body lengths in male than female Atlantic halibut Hippoglossus hippoglossus (L.) resulting in a lower final fibre number

A sampling method is described to determine accurately the number of fast myotomal muscle fibres ( N F ) 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 ( A TC ) 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 N F could be estimated from a single quadrant. The number of blocks required to estimate N F 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 ( L F ). The estimated final fibre number was 8·96 × 10 5 (7·99–9·94 × 10 5 , 95% CI) for males and 1·73 × 10 6 (1·56–1·90 × 10 6 , 95% CI) for female fish. The estimated L F 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.