抄録
An index is proposed to evaluate stock overfishing using catch and effort data. The overfishing index is defined as the slope of the parabola curve between stock size and surplus yield at a given stock size. The value of the index is calculated from estimates of parameters of dynamic production models and observed catch per unit effort. The sign of the index indicates the stock state: overfishing if positive, MSY level if zero, and non-overfishing if negative. The index is useful in displaying changes in the stock state over time. Monte Carlo simulation experiments are carried out to suggest that the sign of the index has robustness against stochastic errors in the catch data as well as difference in the models. Limitations of the present method are discussed after applying it to the actual data.
