CHANGES IN THE STRUCTURE AND FUNCTION OF THE EPAXIAL MUSCLE OF RAINBOW-TROUT (ONCORHYNCHUS-MYKISS) IN RELATION TO RATION AND AGE .1. GROWTH DYNAMICS

Growth of white and red muscle fibres was studied in a feeding ration experiment with rainbow trout, from hatching until sexual maturity (2.5 years). The trout were fed ration levels (RL) which ranged from appreciable underfeeding to overfeeding (RL = 25% and 200% of full ration, respectively). They were also subjected to changed rations in order to study adaptation effects and recovery growth. The ratio of carcass to body weight (CW%) indicated that in young fish (0.3 year) on a restricted ration, growth of internal organs was favoured compared with that of total muscle. In older fish all groups, irrespective of ration, increased their carcass to body ratio until saltwater adaptation (1.0 year) and thereafter a sharp decline was noted. No decrease was seen in fish remaining in freshwater. There was a high correlation between body length and the length (r2 = 0.99), width (r2 = 0.97) and height (r2 = 0.96) of the muscle, and also of the diameter (r2 = 0.86-0.93) and length (r2 = 0.94) of individual white muscle fibres. In fish with a body length less than 250 mm, red fibre diameter was more closely related to ration level than to body length and vice versa in larger fish. In large fish, recruitment of small fibres was seen in all muscles studied. However, periods of rapid growth favoured fibre hypertrophy and periods of slow growth fibre recruitment. Significant differences were found between the lateral and the dorsal parts of the white epaxial muscle, suggesting different growth rates in the two muscle areas.