Rainbow trout (15.5 cm) fed "ad lib." rations grew a little faster at 12 °C than at 16 °C, and both groups grew much faster than a third group at 12 °C receiving "3–4%" rations. Growth of the mosaic muscle mass was characterized by increase in fibre mean diameter, though most bulk increase (up to 25 cm or more) resulted from increase in fibre number. Fibre diameter: fish length was least in the fastest growing trout (12 °C ad lib.), signifying their greater ability to add new fibres during growth compared with the slower growing trout; diameter range increased markedly among trout > 18 cm long, but small fibres (< 20 μm) persisted in diminishing numbers even in the largest trout. In the 12 °C ad lib. trout, condition (K) correlated with length and fibre diameter; in the slower growing groups K was more influenced by mean fibre diameter. A higher asymptote of K occurred at greater length in the fastest growing group than in the two slower growing groups. Percent dry weight: length was greater for 16 °C ad lib. than for the fastest or slowest growth groups; absolute dry weight: length ratios were similar for 12 °C and 16 °C ad lib. trout; both exceeded those for the 12 °C 3-4% group.
A radiotelemetry apparatus is described for sensing and transmitting electromyograms (EMGs) from free-swimming fish. EMGs are recorded from the epaxial muscles of adult rainbow trout during periods in spontaneous (= routine) activity, and forced-swim, respirometers. When such EMG records are integrated, subjected to spectral analysis, and computer-averaged, the EMG values (in pV) are highly correlated with the fish oxygen consumption during the activity periods. However, there is a marked difference between the regression slopes for oxygen v. EMG value for the data from the spontaneous, and forcedswim, respirometers; the former slope is the steeper. The probable explanation of this phenomenon is that whereas in forced swims the epaxial myomeres are responsible for most of the activity of the fish, in spontaneous activity other muscle systems ( e g of the lateral, dorsal and ventral fins) come to account for a greater relative proportion ofbody movement. The difference in slope, although great, is evidently a regular phenomenon. The shift from one regression to the other occurs at a fairly precise epaxial EMG value (c. 5 pV). This suggests that the laboratory calibration of EMG value in terms of oxygen consumption can be utilized in the wild so that EMG records from free-swimming fish, fitted with telemetry packages can be used to deduce oxygen consumption attributable to activity. It also appears that such records can be used as a guide to the type of activity of the fish, i.e. desultory movements or free cruising.
The dynamics of increase of the mosaic muscle in hatchery reared rainbow trout (2.3 to 6 1.3 cm fork length) are investigated. In trout < 5 cm, all fibres are < 40 pm in diameter;from 5 to 20 cm the diameter mode remains in the 0-39.9 pm class and there is some extension in range of diameter, thereby suggesting that mosaic muscle increase up to 20 cm is mainly by recruitment of new (small) fibres. When trout exceed 20 cm, mode of fibre diameter shifts to the 40-79.9 pm class and fibres of larger diameter (1 100 km) appear but the subsequent overall fibre diameter frequency distribution changes little until 50 cm. Increase in muscle during the phase 20-50 cm seems partly attributable to increase in fibre diameter, but remains largely the result of recruitment of small fibres, although the mechanism of the latter process appears less clear than in fish of <20 cm. The recruitment of new fibres stops at 55 cm and further increase in mosaic muscle evidently depends upon the ability of existing fibres to increase in diameter. Possible means by which increasing muscle fibre diameter may limit the ultimate size which trout may achieve are discussed. A phenomenon of apparent reduction in fibre diameter in winter among fish 20-39.9 cm long is noted and its significance is considered.
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