Sakurai et al. : Population dynamics and stock size of Mactra chinensis 
347 
N p' = A | X W '+i + n o,i-3 S o-, ( W ,c I x 10 6 ( 3 ) 
where n ( = density (ind./m 2 ) at age t; 
t = age at first capture; and 
t x = longevity. 
Although M. chinensis attains a commercial size at 
age 2.3 yr (i.e. in November of the second year) at 
Tomakomai (Sakurai, 1993), we assumed t to be 2.7 yr 
(i.e. in April of the second year) because the real stock 
size and the age composition in this study were inves- 
tigated during April. S t represents the survival rate at 
age t and was calculated from the following equation: 
S t = l-100 -1 m ( , (4) 
where m f is the annual mortality rate at age t (age 0 
yr: 94.0%; 1 yr: 53.1%; 2 yr: 44.8%; 3-9 yr: 42.7%) 
calculated with data from Sakurai (1996) with the 
assumption that annual mortality is constant. In 
April 1992-97, n t was estimated in turn by multi- 
plying the previous year by each S f and by adding 
density of recruitment which was estimated by mul- 
tiplying each n 0 ,._ 3 by S Q tc . Density at age 10 yr and 
older (^>10) was not considered in the estimated age 
composition because the longevity (t x ) of M. chinensis 
is 10 yr (Sakurai, 1993). The weight at age [f+0.7] 
yr, W t , was calculated according to Equation 1 and 
the following equation from Sakurai (1993) : 
L = 78.3ljl-e' 067u ~ 034) }, (5) 
where L = shell length (mm); and 
t = age (years) of M, chinensis. 
