Patterns of muscle growth in early and late maturing populations of Atlantic salmon (Salmo salar L.)

Muscle growth was investigated in two populations of Atlantic salmon (Salmo salar L) derived from an early maturing stock of West Coast Scottish origin (strain X) and a late maturing stock (strain Y) of Norwegian origin. Fish from six families per population were PIT-tagged and reared together in a 5x5x5 m sea cage between April 1997 and September 1998. The distribution of muscle fibre diameters in different age classes was investigated using non-parametric smoothing and bootstrapping techniques. Body mass and the total cross-sectional area of white muscle at the level of the first dorsal fin ray increased at a significantly faster rate in strain X than in strain Y after the first 6 months in seawater (April-October). The relative contributions of fibre recruitment and hypertrophy to muscle growth were very different for the two populations. The number of white muscle fibres per trunk cross-section was around 150,000 in June and 250,000 in July for both populations. The majority of fish sampled in June, July and August 1997 showed a unimodal distribution of fibre diameter. Between July and August fish from strain Y recruited two times more fibres for each square millimeter increase in white muscle cross-sectional area than fish from strain X. Fibre number in strain X had caught up with strain Y by January with 545,000 fibres per cross-section. There was no significant increase in the number of white fibres in strain Y between January and September 1998, such that growth was entirely by fibre hypertrophy. In contrast, fibre number continued to increase in strain X until the final sample in June, reaching 718,000 per trunk cross-section. All fish showed a bimodal distribution of fibre diameter in the October and subsequent samples with significant differences between strains. The right-hand peak of the distribution was shifted towards higher diameters in strain X than in strain Y, indicating superior hypertrophic growth performance in the early maturing population. For example, in the May 1998 sample, the 95th percentile of fibre diameter was at 215 mu m in strain X and 171 mu m in strain Y. In March and subsequent samples, the left-hand peak of the distribution was shifted to higher fibre diameters in strain Y relative to strain X, reflecting the cessation of new fibre recruitment in strain Y. Immature fish within strain X had a lower density of small diameter fibres and the right-tail of the distribution was shifted to higher diameters relative to fish that had begun to sexually mature. Thus greater muscle fibre hypertrophy was observed in immature than maturing fish of the same strain. It was concluded that the superior growth performance of the early maturing population was associated with a longer period of fibre recruitment and greater fibre hypertrophy than in the late maturing population, although these differences were not directly related to sexual maturation. (C) 2000 Elsevier Science B.V. All rights reserved.