Sakurai et a I.: Population dynamics and stock size of Mactra chinensis 
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age and growth (Sakurai, 1993), spatial distribution 
pattern (Sakurai, 1994), and annual mortality rate 
(Sakurai, 1996) off the coast of Tomakomai, south- 
west Hokkaido, Japan. 
The aim of this study is first to examine the fluc- 
tuation in density of newly settled and harvestable- 
size M. chinensis at Tomakomai and to predict the 
stock size according to the population parameters 
that we have already reported. 
Materials and methods 
Study area 
The study was conducted on a subtidal sandy bot- 
tom off Tomakomai (42°36'N, 141°32'E), where M. 
chinensis with shells longer than 60 mm are har- 
vested commercially. Figure 1 shows the survey area 
in relation to the entire fishing ground, which is about 
32 km 2 in area. The survey area is situated on the 
fishing grounds within 6 km of shore and covers 
depth ranges from 3 to 11 m. The sediment in the 
area is fine or very fine sand with low organic car- 
bon content, less than 0.3% of dry weight (Sakurai 
et al., 1991). The bottom water temperature at 10-m 
depth ranges between 3.0°C in March and 20.2°C in 
September (Sakurai, 1993). 
Sampling and processing of clams 
The breeding season of M. chinensis occurs between 
July and September at Tomakomai (Sakurai et al., 
1992), the planktonic larval period lasts for 14-21 
days (Tsurui, 1980; Kobayashi and Ujima, 1983), and 
shells of newly settled juveniles grow to 1-3 mm long 
between September and November (Sakurai, 1994). 
The density of newly settled juveniles was therefore 
Figure 1 
Location of the survey area off the coast of Tomakomai, southwest Hokkaido, Japan. (A) Shaded and dotted 
portions represent the entire fishing ground of M. Chinensis', dotted portion represents the survey area. (B) 
Solid and open circles indicate stations sampled by a Smith-Mclntyre grab sampler and Japanese surfclam 
hydraulic jet dredge, respectively. 
