Sustainable yield estimates of the Barents sea capelig stock

Abstract

The Barents Sea capelin suffer mass mortality after spawning. A model is developed to calculate the biomass of springspawners using previous autumn acoustic survey estimates of the total stock as input data. It is assumed that the age composition of the March catches reflects the age composition of the spawning stock, and on this basis a maturing length separating spawners from non-spawners has been calculated for the years 1973 - 1980, Based upon the stock estimation of non-spawners on year and total stock estimates next year the nakural mortality has then been calculated. Assuming constant maturing length and natural mortality for the whole period, estimated by fitting the forward calculated juveniles to the total stock next autumn, a value of the maturing length close ts the smallest of the values obtained by using the March catches is found. This is consistent with a significant late spawning of young fish, as the age composition of this component is not being reflected in the catches. Using the estimated values of maturing length and natural mortality, models for growth and reproduction are developed and combined into a long-term simulation model. Varying the exploitation pattern a series of equilibrium states of stock and yield are evaluated and the MSY and corresponding spawningstock level are calculated.