Proceedings of the Third World Fisheries Congress: Feeding the World with Fish in the Next Millenium—The Balance between Production and Environment

Application and Improvement of Virtual Population Analysis in Stock Assessment of Filefish Thamnaconus septentrionalis (Günther) in the East China Sea

Weizhong Chen, Yuanjia Zheng, Changsong Li


Filefish Thamnaconus septentrionalis (Günther) is one of the most important commercial fish species in the East China Sea. Fishing in the East China Sea began in 1974, and filefish was the main target species for most of the large fishing companies. Maximum yield reached more than 300,000 metric tons in China and was second overall in the marine capture fishery. However, mainly as a result of overfishing, the filefish stock collapsed and almost disappeared in the early 1990s. In the late 1990s, the stock recovered slightly. The current stock condition of filefish is of great concern to the fishing companies, fisheries management departments, and fisheries scientists. Xu and Pu (1987) analyze the dynamics of the filefish resource and calculate the maximum sustainable yield using a Beverton–Holt model. Zhan and Zhong (1986) estimate the stock size of filefish by virtual population analysis (VPA) and give some suggestion on the rational use of filefish. We began assessing the filefish stock status and estimating yield in the early 1980s.

In 1987, the Fisheries Resources Monitoring and Surveillance System in the East China Sea was established. The primary focuses of this system are the stock assessment and yield estimation of filefish, which has been done every year since 1987 and has been well received by the fishing companies and governmental management departments. However, conducting VPA requires mathematical models, and complicated calculation must be performed.

We pioneered computerized assessment work in the mid-1980s. With increased computer speed, we made some progress and improved filefish assessment procedures. In this paper, we describe the application and improvement of VPA to the stock assessment of filefish.

Data used for assessing filefish stock and estimating yields were age compositions from catch sampling, catch from 1976 to 1999, average body weight for different age groups, and natural mortality rate (M) (Tables 1 and 2).

The VPA methods are recursive algorithms that estimate stock size on the basis of historical catches (Hilborn and Walters 1992). The number of fish alive in each cohort are calculated for each of the past fishing years. Cohort analysis (Pope 1972) is an approximation of VPA in which the catch is assumed to be taken midyear (Rivard 1989).