344 
Abstract .“Annual fluctuation in 
the number of newly settled juveniles 
and in the stock size of the sunray surf 
clam, Mactra chinensis, were examined 
from 1987 to 1994 off the coast of 
Tomakomai, southwest Hokkaido. Stock 
size was estimated with a model based 
on two known population parameters, 
juvenile density and age composition. 
Juvenile density and the stock size (or 
mass) ranges were 1.3 to 157.0 ind./m 2 
and 249.8 to 1,127.4 metric tons, re- 
spectively; a time lag between the rela- 
tive fluctuations of both agreed with 
age-at-recruitment to the stock. Our 
results suggest that population dynam- 
ics are directly influenced by the num- 
ber of juveniles. The predicted stock 
size approximated the measured value 
and, consequently, a long-term predic- 
tion of stock size was deemed possible, 
provided age composition and juvenile 
density are determined. 
Manuscript accepted 16 June 1997. 
Fishery Bulletin 96:344-351 (1998). 
Population dynamics and stock size 
prediction for the sunray surfclam, 
Mactra chinensis, at Tomakomai, 
southwest Hokkaido, Japan 
Izumi Sakurai 
Hokkaido Central Fisheries Experimental Station 
238, Hamanaka, Yoichi, Hokkaido 046, Japan 
E-mail address, kb6i-skri@asahi-net.or.jp 
Takashi Horii 
Muroran Branch, Hakodate Fisheries Experimental Station 
1-133-31, Funami. Muroran, Hokkaido 05 1 , Japan 
Osamu Murakami 
Hokkaido Abashiri Fisheries Experimental Station 
3 1 , Masuura, Abashiri, Hokkaido 099-3 1 , Japan 
Shigeru Nakao 
Faculty of Fisheries, Hokkaido University 
3-1-1, Minato, Hakodate, Hokkaido 041, Japan 
The sunray surfclam, Mactra chi- 
nensis , is a commercially important 
bivalve belonging to the family 
Mactridae and is widely distributed 
on the upper subtidal sandy bottom 
off the coast of Japan, China, Ko- 
rea, Sakhalin, and the maritime 
province of Siberia (Habe, 1977). 
This clam has been harvested from 
marine waters off Hokkaido as in- 
cidental catch in the fishery for 
Japanese surfclam, Pseudocardium 
sachalinensis, which is the most 
commercially important bivalve in 
northern Japan (Kinoshita and Terai, 
1954). 
Recently, the demand for M. chin- 
ensis caught off Hokkaido has in- 
creased owing to decreasing catches 
on Honshu Island (Kurata, 1991). 
However, from 1985 to 1994, the 
annual Hokkaido catch fluctuated 
between 705 and 1,310 metric tons. 
In response to this wide fluctuation, 
stock management plans are now 
required. Although there are some 
studies of the breeding season 
(Miyazaki, 1957; Tomita, 1974), 
growth (Hanaoka and Shimazu, 
1949), spatial distribution (Hayashi 
et al., 1965, 1967), increase in the 
number of juveniles (Inoue and Ozu, 
1960; Yoshimatsu, 1977), and catch 
fluctuation (Saito et al., 1982), these 
studies have provided only frag- 
mentary information. Of necessity, 
several temporary stock manage- 
ment measures, similar to those 
used forP. sachalinensis, have been 
implemented by several fisheries co- 
operative associations in Hokkaido 
to regulate the fishery for M. 
chinensis . These measures include 
limiting allowable catch and impos- 
ing a minimum harvestable shell 
length and period or area for fish- 
ing. We have systematically stud- 
ied the life history of M. chinensis 
and have already reported on its re- 
productive cycle (Sakurai et al., 
1992), age at first maturation (Sa- 
kurai et al., 1992), relation between 
