Fisheries Science 62(2), 168-172 (1996) Age Structure and Mortality of the Sunray Surf Clam Mactra chinensis off Tomakomai, Southwest Hokkaido Izumi Sakurai,*1 Mamoru Kurata,*2 and Eiji Abe*3 *1Hokkaido Central Fisheries Experimental Station , Yoichi, Hokkaido 046, Japan *2Hokkaido Abashiri Fisheries Experimental Station , Abashiri, Hokkaido 099-31, Japan *3 Hokkaido Kushiro Fisheries Experimental Station, Kushiro, Hokkaido 085, Japan (Received January 27, 1995) The age structure and seasonal changes in mortality of the sunray surf clam Mactra chinensis older than age at recruitment (3 years) were studied using data collected from April 1991 to February 1992 off Tomakomai, southwest Hokkaido, Japan. Wide fluctuations in the number of recruits of various ages were clearly observed and the estimated density of the recruits was ranged from 0.6 to 4.6 ind./m2. The highest monthly 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-old 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 of 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 effective utilization of the clam stock in this area is discussed. Key words: age structure, mortality, recruitment, Mactra chinensis, Tomakomai The sunray surf clam Mactra chinensis is distributed gust and November 1991 and February 1992 using a widely on the upper subtidal sandy bottom in Sakhalin, Japanese surf clam Hydraulic Jet Dredge, shown in Fig. 2 Japan, China, Korea and the Maritime Province of Siber (width: 1.2m, mesh size of net bag: 70mm, teeth spaces: ia.1) This clam is commercially harvested in Japan and the 36mm, teeth angles: 60 degrees), towed for a distance of annual catches in Hokkaido ranged from 800 to 1,300 tons 20-60m. during 1985-1992. Almost all the clams of the 1989 year-class and younger The spatial distribution of adult and young M. chinensis were washed through the mesh of the net bag and, there clams,2,3) spawning season,4) shell length at first sexual fore, only clams of the 1988 year-class and older were sort maturity4) and the relationships between sedimentary con ed as the samples. The age of each clam was determined by ditions and the macrobenthos community containing the counting the external growth rings on the shell.7) The shell young clams5) in Hokkaido has been studied. Saito et a1.6) length was measured to the nearest 0.1 mm with a caliper reported that the annual catch fluctuated widely and a and the number of individuals at 5-mm shell-length inter large catch continuing for a few years depended upon the vals in each year-class was counted. dominant year-class. Using the estimates of the catch efficiencies of the dredge We have already studied the life cycle and population dy for each 5-mm-interval length class, obtained from the namics of M. chinensis, including developing a method of selectivity curve,"' the number of individuals (ind.) was determining age and growth,7) age at first sexual maturity and reproductive cycle,8) the relationships between distribu tion of the young clams and sedimentary conditions and macrobenthos communities 9) and mortality of the popula tion aged 0-2 years.10) The aim of the present study was to obtain information on the age structure and seasonal changes in mortality of the M. chinensis population older than the age at recruit ment (3 years) and the annual fluctuations in such recruits in the waters off Tomakomai, southwest Hokkaido. Materials and Methods Four surveys on sandy bottom off Tomakomai, southwest Hokkaido were carried out (Fig. 1). The sam pling station was located at a depth of 9 m in an area where M. chinensis was mainly distributed.10) Samples of 224-341 individuals (mean 276) were taken in April, Au Fig. 1. Map showing the sampling station . Age Structure and Mortality of the Surf Clam 169 Fig. 2. Japanese surf clam Hydraulic Jet Dredge. A, net bag; B, teeth; C, nozzle; D, coupling; E, sliding plate. converted into density (ind./m2). The spawning season of M. chinensis in this area was es timated to be from July to September!) Therefore, we con sidered the age renewal month for the clams to be August. The wave height, water temperature and oxygen satura Fig. 3. Mactra chinensis. Seasonal growth of mean shell length of each tion data were quoted from the data collected by the Hok year-class. kaido Development Agency in Tomakomai from April Vertical bars represent standard deviations. 1991 to March 1992. Results Growth and Age Structure The seasonal changes in the mean shell length of every year-class from April 1991 to February 1992 are shown in Fig. 3. No clams older than the 1980 year-class (aged 11 years) were caught during this study. The mean shell lengths of the 1988, 1987 and 1986 year-classes (aged 2-3, 3-4, and 4-5 years) increased from 62.8, 68.5 and 71.3mm in April to 71.3, 73.4 and 75.6mm in November, respec tively, and remained approximately steady from Novem ber-February. The shells of the 1985 year-class (aged 5-6 years) grew from 74.5 to 78.6mm in April-August and remained steady thereafter, whereas almost no incremen tal growth of clams older than the 1984 year-class (aged 6 10 years) occurred during this study period. The year-class compositions from April 1991 to Febru ary 1992, all of which were similar, are shown in Fig. 4. The 1988 year-class (aged 2-3 years) predominated with a share of 33.9-34.5%, the 1984, 1983 and 1987 year-classes (aged 6-7, 7-8 and 3-4 years) accounted for 17.3-19.0, 11.5-12.2 and 10.6-11.1%, respectively, and the other year-classes comprised less than 10% of the whole popula tion. Mortality Rate The monthly mortality rates for each season are present ed in Table 1, from which, the following points are evi dent. First, the mortality rates in April-August and Novem ber-February depended on the age. In particular, the rate Fig. 4. Mactra chinensis. Frequency of year-class from April 1991 to for clams aged 8 years and older (1983-1981 year-classes) February 1992. increased with age (chi-squared test, p < 0.01). Second, the highest mortality rates for ages 3-7 years (1984-1988 year classes) occurred in August-November (chi-squared test, 170 Sakurai et al. Table 1. Seasonal changes in monthly mortality rate of each year class for the clams Mactra chinensis Fig. 5. Mactra chinensis. Annual fluctuation of densities in recruit *1 Significant at p <0 ment. .05. *2 Significant at p <0 .01. Table 2. Survivorship equations of each age for clams Mactra chinensis Discussion It is well known that marine invertebrates with a planktonic larval stage show considerable fluctuations in the numbers of recruits and outbreaks of juveniles oc casionally occur. Such outbreaks of M. chinensis in vari ous parts of Japan have been reported. 12,13)In the present study, a widely fluctuating age structure of recruits off Tomakomai was clearly observed and the range of densi ties at recruitment was estimated to be 0.6-4.6 ind./m2. It is generally considered that the number of recruits is deter * Significant at p<0 .05 (F-test). mined by the number of settlements and mortality of Log N,5 and m represent intercept and slope (instantaneous mortality rate) of the equation, respectively. Juveniles.14) We think that it is necessary to examine the fac tors affecting the fluctuations in recruits to gain a full un derstanding of the population dynamics of M. chinensis. p<0.05). Third, high mortality rates for those aged 8+ The results we obtained showed that the mortality of (1981-1982 year-classes) were constant throughout the clams aged 3-7 years was highest in the fall, whereas that year. of those aged 8+ years was constant throughout the year. Next, the least squares estimation was applied to the fol The mortality rates were not influenced by fishing because lowing equation: clam fishing at a depth of 5-10m has been prohibited in all areas, including that of our survey, controlled by the Tomakomai Fisheries Cooperative Association where a: year-class, t: day, N: density (ind. /m2) and m: in (T.F.C.A.) since 1985. stantaneous mortality rate (ind. /m2•Eday). The results are Fujimoto15) and Fujimoto et al.16) reported that several tabulated in Table 2. The application of the analysis of factors, such as dispersion or washing ashore due to wave covariance to the regression lines indicated that there were sweep action, high water temperatures in summer, decline no statistically significant differences among the slopes for in dissolved oxygen levels and predation affect the mortali ages 3-9 years, but the slope for age 10 years significantly ty of M. chinensis. differed from that for the other ages (F-test, p <0.05). The The monthly changes in the water temperatures and annual mortality rates of 3-9-year-old clams ranged from oxygen saturation of the 10 m deep layer off Tomakomai 38.0 to 54.4% (mean 42.9) and increased to 76.0% by 10 from April 1991 to March 1992 are shown in Fig.