AGE STRUCTURE AND MORTALITY OF THE SUNRAY SURF CLAM MACTRA-CHINENSIS OFF TOMAKOMAI, SOUTHWEST HOKKAIDO

The age structure and seasonal changes in mortality of the sunray surf clam Mactra chinensis older than age at recruitment (3 years) were st udied using data collected from April 1991 to February 1992 off Tomako mai, southwest Hokkaido, Japan. Wide fluctuations in the number of rec ruits of various ages were clearly observed and the estimated density of the recruits was ranged from 0.6 to 4.6 ind./m(2). The highest mont hly mortality of 3-7-year-old clams occurred in the fall, whereas the mortality of those aged 8 years and older was constant throughout the year. The estimated mean annual mortality rates of 3-9- and 10-year-ol d clams were 42.9 and 76.0%, respectively. It was considered that the mortality would appear to be due to the digging out from the surface o f the bottom and subsequent dispersion of the clam as a result of wave -sweep action in the fall, whereas the high mortality of clams aged 8 years and older was a result of physiological longevity. The most effe ctive utilization of the clam stock in this area is discussed.