CHANGES IN CONCENTRATIONS OF SERUM THYROXINE, SKIN AND SERUM GUANINE, AND TOTAL SERUM-PROTEIN IN THE MASU SALMON DURING SMOLTIFICATION

Changes in concentrations of serum thyroxine, skin and serum guanine, and total serum protein were measured in hatchery-reared masu salmon Oncorhynchus masou during smoltification. In June, the condition factor was at its lowest value while the serum thyroxine was at its highest level, showing the occurrence of the peak of smoltification in the stock used in the present study. Total serum protein decreased significantly in May, gradually increasing thereafter. Serum guanine increased from April and reached a maximum in May. In contrast, skin guanine dropped to a minimum in May but rose in June to reach a peak in July, suggesting that guanine was transported to the skin from the circulation. By measuring the serum guanine concentrations, it is possible to analogize the degree of the development of smolting.