Ocean Research Institute, The University of Tokyo, Nakano, Tokyo 164-8639, Japan
The reliability of VPA used for stock assessment for the Pacific stock of Japanese sardine Sardinops melanostictus was evaluated using a profile likelihood confidence interval and by examining the effect of the assumption about selectivity used in the VPA. The 95% confidence interval for the stock size in 2005 was wide, ranging from 30 to 550 thousand tons. Although the VPA assumes that the selectivity in the latest year is the average of the recent three years, the actual selectivity varies from year to year and this assumption causes estimation errors. In order to improve the reliability of stock size estimates, it is necessary to improve the accuracy of the abundance indices and to develop VPA without the assumption of selectivity.