Estimates of mortality rates from mean length in the fully selected size range for porbeagle
The method of Beverton and Holt was used to estimate fishing mortality rates from mean lengths of fully selected porbeagle sharks in each year for the northwest, southwest, and southeast stocks. Confidence intervals were calculated by bootstrapping the length data and drawing values of the parameters from a multivariate normal distribution. The analysis was conducted first with all the length data combined for each stock and then by fleet within each stock. Fishing mortality rates were estimated to be higher than M and declining in the northwest Atlantic, high and variable in the southeast Atlantic, and low in the northeast Atlantic. These results imply that the length distributions in the northwest and southeast are consistent with a population experiencing overfishing, while the southwest is not experiencing overfishing. However, since this method makes an equilibrium assumption, the results reflect historical overfishing more than current fishing mortality rates.