CCAMLR 2000 revisited

The total abundance of krill in the Scotia Sea was estimated from an international echosounder and net survey (CCAMLR 2000) to be 44.3 million metric tons (Mt; CV=11.4%), prompting the Antarctic Treaty's Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) to revise the precautionary catch level for krill in the area from 1.5 to 4 Mt (SC-CAMLR, 2000). By incorporating recent improvements in the remote identification and target strength (TS) of krill, a range of krill biomass was estimated, 108.0 Mt (CV=10.4%) to 192.4 Mt (CV=11.7%), depending solely on the expected distribution of krill orientations. The new methods were then reviewed by CCAMLR, and revised protocols based on the Stochastic Distorted Wave Born Approximation model (SDWBA) were adopted. Here, the protocols are applied to again re-analyze the CCAMLR 2000 data. Using the 120 kHz echosounder data, the resulting estimates of krill biomass in the Scotia Sea are 197.78 Mt (CV=11.06%) and 37.29 Mt (CV=21.20%), depending on whether two- or three-frequencies are used for krill identification, respectively. At 38 kHz, estimates of krill biomass range from 65.64 Mt (CV= 11.50%) to 10.39 Mt (CV= 15.25%); and at 200 kHz from 343.09 Mt (CV= 12.91%) to 38.73 Mt (CV= 14.86%). CCAMLR uses the estimates derived from the 120 kHz data. Results of the three-frequency method are likely less biased owing to better rejection of non-krill species; also the patchiness of krill is better elucidated, resulting in higher CVs. Thus, the revised estimates of krill biomass in the Scotia Sea during the CCAMLR 2000 survey are 37.29 Mt (CV=21.20%), or 15.8% lower than the original estimate, but with a larger CV.