Baba et al.: An environmentally based growth model for |uvenile Corbicula japonica 



15 



physiological processes as a black box; we constructed the 

 model directly from fluctuations in environmental factors 

 and growth rates. Our approach is reasonable for animals 

 for which ecophysiological knowledge is limited, especially 

 when the main purpose of investigation is to derive the 

 relationships between environment and growth. 



We applied the model to a single cohort of Corbicula 

 japonica juveniles spawned in August 1997. We did not 

 consider any bias caused by adjacent cohorts because C. 

 japonica failed to spawn in 1995, 1996, and 1998 in Lake 

 Abashiri owing to low water temperatures during the 

 spawning season (Baba et al., 1999). Such investigations 

 provide important basic information, such as the shape of 

 the distribution of a single cohort, and the relationship 

 between growth rate and expansion rate of size variation 

 in a single cohort. 



Corbicula spp. are harvested commercially in Japan. The 

 annual catch ranged from 19,000 to 27,000 metric tons in 

 1996 to 2000 ( Ministry of Agriculture, Forestry and Fisher- 

 ies 1 ), of which C. japonica was the main species. Corbicula 

 japonica is distributed in brackish lakes and tidal flats of 

 rivers from the south of Japan to the south of Sakhalin 

 (Kafanov, 1991), is a dominant macrozoobenthos in these 

 lakes, and has important roles in bioturbation and energy 

 flow (Nakamura et al., 1988; Yamamuro and Koike, 1993). 

 Juvenile C. japonica growth is fast in southern habitats. 

 Their spats collected in Lake Shinji, which lies in the south- 

 ern part of its range, grow to a mean shell length of around 

 6.7 mm in natural conditions by the first winter ( Yamane et 

 al. 2 ). In northern habitats, growth is also believed to be fast; 

 Utoh ( 1981 ) reported that mean shell length at the first an- 

 nual mark was around 5.7 mm in Lake Abashiri. In Utoh's 

 study differences between the shell lengths at the first an- 

 nual marks and the shell lengths of individuals aged to be 

 one year were also reported. The purposes of the present 

 study are to elucidate juvenile growth and its relationship 

 to environmental factors in Lake Abashiri. 



Materials and methods 



Overview of the model 



Our model expresses relative growth rate for C. japonica by 

 a sigmoid function with environmental factors and animal 

 ages as independent variables. Modeling processes in gen- 

 eral follow five steps: 1) Shell lengths of a single cohort are 

 summarized by an adequate probability density function, 

 which is expressed by a location parameter and a scale 

 parameter; 2 ) Daily relative increase rate of the location 



1 Ministry of Agriculture, Forestry and Fisheries. 1996- 

 2002. Statistics on fisheries and water culture production. 

 Association of Agriculture and Forestry. 1-2-1 Kasumigaseki, 

 Chiyoda, Tokyo 100-0013. Japan. 



2 Yamane, K., M. Nakamura, T. Kiyokawa, H. Fukui, and E. 

 Shigemoto. 1999. Experiment on the artificial spat collec- 

 tion. Bull. Shimane Pref. Fish. Exp. Stn., p. 232-234. Unpubl. 

 rep. Shimane Prefectural Fisheries Experimental Station, 

 25-1 Setogashima, Hamada, Shimane 697-0051, Japan. |In 

 Japanese.] 



parameter (dRIRL) is approximated by a sigmoid function 

 with environmental factors and animal ages as indepen- 

 dent variables; 3) Daily relative increase rate of the scale 

 parameter is approximated by a simple function with the 

 dRIRL as an independent variable; 4) The model is opti- 

 mized by a maximum likelihood method; and 5) The best 

 model is selected by Akaike information criterion (AIC). 

 The AIC is an information-theoretic criterion extended 

 from Fisher's likelihood theory and is useful for simulta- 

 neous comparison of models (Akaike, 1973; Burnham and 

 Anderson, 1998). 



Study site and sampling method 



To collect juveniles of C japonica spawned in August 1997, 

 sediments were sampled with a 0.05-m 2 Smith-Mclntyre 

 grab once or twice a month from September 1997 to July 

 1999 at a depth of 3.5-4.0 m in Lake Abashiri (Fig. 1). The 

 habitat of C. japonica is restricted to areas shallower than 

 6-m depth because the deeper area, the lower stratum of 

 the lake, is covered by anoxic polyhaline water. We assumed 

 that the selectivity of the sampling gear on C. japonica 

 juveniles was negligible because the gear grabs the juve- 

 niles with the sediment. Because the magnitude of spawn- 

 ing in 1997 was relatively small (Baba et al., 1999), we 

 selected a sampling site where we found abundant settled 

 juveniles in our preliminary investigations. Samples could 

 not be obtained during winter because of ice cover. Sedi- 

 ments were washed with tap water on 2- mm and 0.125- 

 mm mesh sieves from September 1997 to October 1998, 

 and on 4.75-mm and 0.125-mm mesh sieves from April 

 to July 1999. To separate the juveniles from the retained 

 sediments, we treated the sediments with zinc chloride 

 solution as described by Sellmer (1956). Then we sorted 

 the juveniles under a binocular microscope. Identification 

 of the cohort spawned in 1997 was quite easy because C. 

 japonica failed to spawn in 1995, 1996, and 1998 owing to 

 low water temperatures during the spawning season ( Baba 

 et al., 1999). We considered all the individuals that passed 

 through the larger-mesh sieves and that were retained on 

 the smaller-mesh sieve as the 1997 cohort. Shell lengths 

 were measured under a profile projector (V-12, Nikon Ltd., 

 Chiyoda, Tokyo) at 50x magnification with a digital caliper 

 (Digimatic caliper, Mitsutoyo Ltd., Kawasaki, Kanagawa), 

 which has a 0.02-mm precision. 



Environmental factors 



Values for water temperature (°C), water fluorescence 

 (fluorescence equivalent to uranin density, ug/L). salinity 

 (psu, practical salinity unit), and turbidity (equivalent to 

 kaolin density, ppm) were obtained for 0.1-m intervals 

 from unpublished data at the Abashiri Local Office of 

 the Hokkaido Development Bureau. 3 The variables were 

 measured by a submersible fluorometer (Memory Chloro- 

 tec, ACL-1180-OK, Alec Electronics Ltd., Kobe, Hyogo) at 

 four sites in Lake Abashiri at intervals of about one week 

 ( Fig. 1 ). The average values of each variable between the 

 depths of 1 m and 6 m were used for later analyses. Values 

 between the measured dates were interpolated linearly 



