Results of the mark–recapture experiment in Subarea 48.3, 2005

The mark-recapture experiment continued at South Georgia in 2005. In total some 8000 fish have now been tagged in 48.3, and both tagging effort, fishing effort and recaptures were well – distributed over the whole of the fishable grounds in 48.3 this year. The Petersen mark-recapture estimator, which has been applied previously and described in detail by Agnew et al (2004) and Payne et al (2005), was recalculated for Subarea 48.3 based on updated parameter estimates and four different selectivity regimes: the historically used Kirkwood shallow selectivity, the single- and double- fleet selectivities estimates by CASAL (see WG-FSA-05/16), and a selectivity estimated directly from the tagging data.
Estimates from 2004 and 2005 were very similar to each other, whichever selectivity was used: about 62,000 t of vulnerable biomass using the Kirkwood shallow selectivity; 52-55,000 t using the CASAL derived selectivity functions; and 41,000 t using the tag-derived selectivity function. Current vulnerable biomass (BV2005) was (naturally) lower when CASAL determined selectivity and tag-derived selectivity were used, since these selectivity functions are more peaked than the Kirkwood shallow selectivity used in 2004. Estimates of sustainable yield, made adjusting mean recruitment in GYM so that BV2005 corresponded to the current estimates of vulnerable biomass, resulted in consistent estimates of sustainable yield of about 4,800 t, irrespective of the selectivity used.