North Pacific blue shark stock assessment

Abstract

A stock assessment of the blue shark (Prionace glauca) population in the North Pacific was conducted using catch and effort data from commercial longline and large mesh driftnet fisheries from the years 1971 through 2002 as well as small mesh driftnet fisheries operating primarily in the 1980s. Because reporting of shark catch has not been required in these fisheries, which target primarily tunas, a system for identifying the more reliable longline catch reports was utilized. Two different assessment models were utilized, a surplus production model, and an integrated age and spatial structured model tested with a variety of structural assumptions. The two models were found to be in general agreement. The trends in abundance in the production model and all alternate runs of the integrated model show the same pattern of decline in the 1980s followed by recovery to above the level at the start of the time series. The integrated model analyses indicated some probability (around 30%) that biomass at the end of the time series was less than BMSY (overfished) and that there was a lesser probability at that time that fishing mortality was greater than FMSY (overfishing occurring). There was an increasing trend in total effort expended by longline fisheries toward the end of the time series, and this trend may have continued thereafter. It would be prudent to assume that the population is at least close to MSY level and fishing mortality may be approaching to the MSY level in the future.