1Fisheries Research Division, Mie Prefectural Science and Technology Promotion Center, Suzuka, Mie 510-0243, 2Faculty of Bioresources, Mie University, Tsu, Mie 514-8507, Japan
Based on monthly catch-effort data of the brackish water clam Corbicula japonica in Kiso estuaries, central Japan, for 7 years from 1996 to 2002, monthly and yearly averaged daily CPUE (kg/boat/day) of the clam was calculated. Then, fluctuations of yearly averaged CPUE of the clam, which was regarded as an index of the annual quantity of recruits, was examined. Daily CPUE of the clam increased from spring to summer and then decreased toward winter, every year, in both the Ibi/Nagara and Kiso Rivers. For the Ibi/Nagara River, annual catch of the clam has tended to decrease recently while this was not the case for stock size and its recruits. This suggests that the recent decrease of annual catch of the clam may be driven by some factors operating not before but after the recruitment. On the other hand, for the Kiso River, annual catch of the clam did not tend to decrease, although its fluctuations were marked. There were high significant relationships between annual catch and yearly averaged daily CPUE (an index of the quantity of recruits) of the clam. This suggests that catch fluctuations of the clam may be driven by annual quantity of recruits.