Abstract
The present study proposes a method for calculating and estimating relative biomass of recruits of fish stock using catch and effort data. The relative biomass of the recruits is defined by the product of the absolute biomass of recruits and the catchability coefficient. Assumptions are that the catch per unit of effort (CPUE) is proportional to the total biomass of the recruited stock, that the recruitment suddenly occurs just before the beginning of the fishing season, and that such biological parameters as natural mortality coefficient and growth coefficient are small in magnitude. From these assumptions, an approximation equation for calculating the relative biomass of recruits, which uses only the CPUE and the effort, was derived. The maximum likelihood procedure was proposed for the parameter estimation. The accuracy of the equation was examined by the simulations. The method was applied to the data of the setline fishery exploiting the Pacific halibut, Hippoglossus stenolepis. The results showed that the extent of the discrepancy of the equation was small and that the annual changes in the recruitment biomass of the halibut was periodical. The interpretation of the results and the issues to be solved in the future are discussed.
