R.T.M. Baker

Muscle fibre density in relation to the colour and texture of smoked Atlantic salmon (Salmo salar L.).

Muscle fibre cellularity was quantified during seawater growth in populations of predominantly early (strain X) and late maturing (strain Y) Atlantic salmon (Salmo salar L.). The fibre density (number mm−2 white muscle cross-sectional area) in the fresh fillet was related to pigment concentration, colour as determined with the Roche SalmoFan™, and lipid content. The relationship between fibre density and the textural characteristics of the smoked fillet, as assessed by trained taste panels, was also determined. There was no significant correlation between astaxanthin concentration and muscle fibre density. However, significant positive relationships were obtained between Roche SalmoFan™ score and fibre density, explaining 33% and 44% of the total variation in colour visualisation in strains X and Y, respectively. Significant positive correlations were observed between muscle fibre density and all four measures of texture assessed by the taste panels, “chewiness”, “firmness”, “mouth-feel” and “dryness”. A firm texture was therefore associated with a high muscle fibre density. At harvest, the lipid content of the fillet was significantly higher in strain X (11.2%) than strain Y (7.0%). There was, however, no significant correlation found between sensoric “oiliness” score and the percentage lipid content of the fillet. The results indicate that muscle fibre cellularity is an important factor in several key flesh quality traits. The potential for manipulating muscle cellularity to produce desirable flesh quality characteristics is briefly discussed.

Soybean meal alters autochthonous microbial populations, microvilli morphology and compromises intestinal enterocyte integrity of rainbow trout, Oncorhynchus mykiss (Walbaum).

Abstract Rainbow trout were fed either a diet containing fishmeal (FM) as the crude protein source or a diet containing 50% replacement with soybean meal (SBM) for 16 weeks. An enteritis-like effect was observed in the SBM group; villi, enterocytes and microvilli were noticeably damaged compared with the FM group. The posterior intestine microvilli of SBM-fed fish were significantly shorter and the anterior intestine microvilli significantly less dense than the FM-fed fish. Electron microscopy confirmed the presence of autochthonous bacterial populations associated with microvilli of both fish groups. Reduced density of microvilli consequently led to increased exposure of enterocyte tight junctions, which combined with necrotic enterocytes is likely to diminish the protective barrier of the intestinal epithelium. No significant differences in total viable counts of culturable microbial populations were found between the groups in any of the intestinal regions. A total of 1500 isolates were tentatively placed into groups or genera, according to standard methods. Subsequent partial 16S rRNA sequencing revealed species that have not been identified from the rainbow trout intestine previously. Compared with the FM group levels of Psychrobacter spp. and yeast were considerably higher in the SBM group; a reduction of Aeromonas spp. was also observed.

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

Abstract 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 5×5×5 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 μm in strain X and 171 μm 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.