Differences in scale characteristics taken from various parts of masu salmon, Oncorhynchus masou (Brevoort) and its scale development.

The scales of 17 adult masu salmon (Oncorhynchus masou), caught in coastal waters off south-western Hokkaido, were taken from 41 positions from each individual fish. This was done to compare their different characteristics with relation to their positions along the body. The characteristics investigated include radius of freshwater life (RF) and seawater life (RS), focus radius (F0) measurements, and the number count of circuli made during freshwater life (CF) and seawater life (CS). The percentage of normal scales (NSR) in each position was also examined. Those RS, RF, and CS were compared using values (%) relative to the value of scales (as 100%) taken from position 5 where scales were sampled for ordinary scale analysis. Scales of juvenile fish (10 wild and 10 reared masu juvenile) were sampled from 6 of the 41 positions to compare their focus radii. Of the masu salmon fry reared in an experimental tank, about 20 fry were sampled about every 10 days after emergence, in order to observe scale development. NSR were rather low for the entire body surface. The mean value of all 41 positions was 55.9%. The highest rate (85.5%) was recorded at the position of anterior-dorsal tissue. The rate in the lateral tissue was as low as 40% to 50%. F0 was largest at the lateral area below the adipose fin. The radius gradually decreased toward the anterior, dorsal, and ventral directions. CF, RF, and RS had their largest values at the lateral area below the adipose fin and displayed a tendency of becoming smaller in the anterior, ventral, or dorsal directions. One exception is CS which showed smaller differences among the various positions. Relative to position 5, the ratio RF/(RF+RS) values, which can help estimate the fork length at the time freshwater annuls were formed, ranged between 95 to 105% throughout the entire area along the lateral line. This suggested that the scales along the lateral line would be utilized for back-calculating the fork length if the scales of the defined part (position 5) were missing or deformed. Because circuli counts and scale radii varied between positions and few fish were examined, highly accurate analyses were not achieved. Though the pattern of scale development was similar to those mentioned before, the origin of the scale development began in the lateral tissue below the adipose fin, below the area between the adipose fin and caudal peduncle, or slightly posterior than the area between the dorsal and adipose fin as shown in previous reports.