348 
Fishery Bulletin 96(2), 1998 
1987 1988 1989 1990 1991 1992 1993 1994 
Year 
Figure 3 
Annual fluctuation of the mean density of newly settled juvenile 
sunray surfclams. Vertical bars represent standard deviations. 
2000 
H 1500 
1 1000 
500 
0 
3 
(D 
CD 
Year 
Figure 4 
Annual fluctuation of the density (open circles) and the measured stock 
size (solid circles) of sunray surfclams. 
Results 
The densities of newly settled juveniles, as shown 
in Figure 3, ranged from 1.3 to 157.0 ind./m 2 in 
1987-94. The densities and the measured stock 
size of harvestable-size clams ranged from 0.7 to 
3.1 ind./m 2 and from 249.8 to 1127.4 tons in 1987- 
94, respectively (Fig. 4). 
The age composition of sunray surfclam in April 
1991 is shown in Figure 5. The 2-year age group 
predominated with a share of 33.9%; the 6-, 7- 
and 3-year groups accounted for 17.3%, 12.2%, and 
11.1%, respectively, and the other age groups rep- 
resented less than 10% of the whole composition. 
Clams older than 9 years were not found in the 
survey area. The predicted stock size at each age 
in April 1992-97 calculated from Equation 3 is 
shown in Figure 6. It was predicted that the age 
groups spawned in 1988 and 1991 would 
predominate from age 2 to 5 in the har- 
vestable-size clams. 
Although bivalve fishing was prohib- 
ited in 1985-91 at depths of 5 to 10 m off 
the coast of Tomakomai, including the 
survey area, in order to protect P. 
sack alinensis juveniles, fishing resumed 
there in July 1992. Therefore, the catch 
of M. chinensis from 1992 to 1994 in the 
survey area was estimated by multiply- 
ing the annual catch at Tomakomai 
(Hokkaido, 1994, 1995, 1996) by the ra- 
tio of the survey area to the whole fish- 
ery area (Table 2). Subsequently, stock 
size was estimated by subtracting the 
value of the survey area in Table 2 from 
the sum of the age composition in Figure 
6. The results are shown in Figure 7 to- 
gether with the real stock sizes from Fig- 
ure 4. The predicted values were within 
about 10% of the real values (Table 3). 
Discussion 
It is well known that marine invertebrates with 
planktonic larval stages show considerable fluctua- 
tions in numbers of recruits; sudden increases in 
numbers of juveniles occur occasionally (Hanaoka, 
1972). Nakaoka (1993) noted that it is necessary for 
these populations to experience such sudden in- 
creases in order to maintain their populations. Sud- 
den increases of M. chinensis have been reported from 
many of the fishing grounds in Japan. For example, 
a maximum density of about 980 ind./m 2 for clams 
with 60-mm-long shells was reported off the coast of 
Miura, Kanagawa Prefecture (Inoue and Ozu, 1960). 
In the present study, densities of newly settled juve- 
niles ranged from 1.3 to 157.0 ind/m 2 (Fig. 3); during 
our study we did not find such a remarkable fluctua- 
tion in recruitment off the coast of Tomakomai com- 
pared with that observed for the population off Miura. 
Additionally, there were no dominant or vacant age 
groups in the age composition (Fig. 5); thus, it ap- 
pears that the population of M. chinensis is main- 
tained by comparatively stable recruitment occurring 
off the coast of Tomakomai. 
Mactra chinensis grows to harvestable size at age 
2.3 yr at Tomakomai (Sakurai, 1993); consequently 
newly settled juveniles spawned in the years 1987 
through 1991 are presumed to have recruited to the 
