Growth dynamics of white and red muscle fibres in fast- and slow-growing strains of rainbow trout

Compared with fish of a slow-growing strain, fast-growing rainbow trout exh ibited significantly smaller white fibre diameters, throughout development from hatching to 24 cm body length, although possessing similar total numbe r of fibres. In contrast, in red muscle, no differences were observed in fi bre diameter between the two strains, but the fast growing fish showed a si gnificantly higher number of red fibres. The differences in growth rate bet ween the two strains were related to the mean white fibre diameter and were found to be matched by proportional adjustments in recruitment of new fibr es to the growing muscle. Thus, the largest and fastest-growing strain show ed evidence of sustained higher recruitment of muscle fibres that endowed t his strain with the potential to maintain rapid somatic growth for longer a nd accomplish greater muscle growth. (C) 1999 The Fisheries Society of the British Isles.