Growth, feeding and metabolism in juvenile Atlantic salmon (Salmo salar L.)

Growth, feeding and metabolism in juvenile Atlantic salmon Salmo salar L. Growth, feeding and metabolism were studied during the juvenile growth phase of Atlantic salmon. Comparisons were made between the faster and slower growing members of sibling populations which formed the upper and lower modes of a bimodal growth distribution. Several new techniques were developed in order to investigate these parameters in small fish. Specific growth rates of marked individuals within the bimodal distribution followed the same pattern of development as the whole population, and appeared to be more closely related to changes in daylength than ambient water temperature. Morphometric assessment of the growth of the swimming musculature showed that both processes of cell enlargement (hypertrophy) and cellular proliferation (hyperplasia) were important. The relative importance of the two processes was dependent on developmental stage and season, although hyperplasia appeared to be more a characteristic of rapid growth than hypertrophy. Histochemical studies demonstrated the presence of "red", "white", and between them a diffuse band of "pink" muscle which appeared to become less evident during the freshwater growth period. Modal differences in food intake were found between October and March, with the upper mode fish feeding optimally whilst the lower mode fish fed at only maintenance levels. Although no differences were found in food turnover rate, upper mode fish demonstrated an elevated temperature specific food intake and gross conversion efficiency. Photoperiod was an important influence on feeding, affecting both the daily feeding pattern, which was suppressed in darkness; and evacuation rate, which was more rapid for fish experiencing an increasing rather than decreasing photoperiod. Upper mode fish and smolts had higher resting rates of oxygen consumption than lower mode fish at two of the experimental temperatures used. Smolts had larger gills than either modal group, although no differences were found between fish in the upper and lower modes. However, upper mode fish had larger hearts than those measured in the lower mode. The higher levels of metabolism usually associated with smolting appeared to be a normal characteristic of the upper growth mode. The parameters studied are discussed in relation to the energy budget, and it is concluded that the two growth modes comprise two distinct "physiological populations" within the sibling group. Photoperiod is considered to be a primary influence on growth, and it is suggested that differential responses to seasonal and daily changes in photoperiod by individuals may account for both the development of bimodality, and provide the means (through an increased daily and seasonal feeding opportunity) by which higher growth rates are sustained by the upper mode fish.