Estimate of exploitation rates and population size of skipjack tuna off the southeastern coast of Brazil

Abstract

Size eorrrpositions and total landings of skipjaek tuna eaught in the southeastern Brazilian waters by bait-boats are used as the basis of the assessment of the population for 1980-1983 period, employing the length eohort analysis and virtual population analysis. From monthly size frequeney data it is suggested that there is eonstant immigration and emigration of different modal groups in the fishing area with an ~nteY'Val of two to three months. The exploitation rate was very low for length elass between 35-45 em FL, then increased gradually. The weighted mean exploitation rates were 0.060 and 0.448, respectively for length classes smaller and larger than 55 em FL. The estima_te of the average number of fish attaining a size of 43 em FL (recruitJ was 11.0 x 10 6 fish for assumed values of M = 0.7 and K = 0.307. Increasing the fishing mortality rate by 30% and 50%, an estimated increment in yield is 8% and 12%, respectively. From virtual population analysis, we obtained the biomass estimate of skipjack population older than two years-old to be 70.3 thousand tonnes for assumed value of M = 0.7 and the estimated MSY was 24.6 thousand tonnes.