CHANGES IN THE STRUCTURE AND FUNCTION OF THE EPAXIAL MUSCLE OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) IN RELATION TO RATION AND AGE .3. CHEMICAL-COMPOSITION

The chemical composition of white and red muscle has been studied in rainbow trout in relation to age and ration level (RL), together with the deposition and mobilization of fat from the three main adipose tissues (viscera, dorsal and abdominal wall). Protein in red and white muscle increased with age up to 1.4 years and remained practically constant from then on, irrespective of ration level. In all tissues studied, fat content increased with age until a final steady level was reached, dependent on ration level and tissue. In white muscle and viscera, however, this increase was preceded by a decrease in fat content (0.3-1.1 years). Subjecting fish to a new RL resulted in a change in the fat content to the same level as that in fish fed at this RL from the start. The total amount of fat was regulated in all tissues, except visceral adipose tissue, by altering the relative fat content rather than by increasing or decreasing the relative size of the tissue. Overall changes in dry matter content, from hatching to sexual maturation, correlated mainly with protein in white and with fat in red muscle. However, when young and older fish were studied separately it became apparent that a change in protein content was closely related to changes in dry matter in both muscle types early in life (fish < 1.1 years) and fat later on (> 1.1 years). The glycogen level varied in parallel in red and white muscle, except for the large increase in white versus red muscle after the glycogen depletion in connection with saltwater transfer. Subjecting fish to a new RL caused rapid changes in glycogen content in young specimens. Underfed fish produced markedly elevated levels of muscle glycogen. Season, sex or sexual maturation had no effect on the chemical composition of the muscle.