1Nagano Prefectural Fishries Experimental Station Saku Branch, Nagano 385-0042, 2Department of Population Dynamics of Marine Organisms, Ocean Research Institute, University of Tokyo, Tokyo 164-8639, Japan
The initial population number of ayu fish at the beginning of the fishing season and catchability coefficient estimated using DeLury's method and the stocking effect was examined in the Chikuma River. The accumulative catch and the CPUE of the Tomozuri angling fishery observed by hourly sighting were used for estimation. Since the number of the released fingerlings was known, the survival rates until the beginning of the fishing seasonwere calculated to be 13.0% in 1997 and 36.9% in 1998. The results showed that DeLury's method was applica-ble to the assessment of the ayu population.