New target-strength model indicates more krill in the Southern Ocean

Antarctic krill, Eaphausia superba, comprises the foundation of the foodweb in the Southern Ocean and is the target of a large fishery. Recently, 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 tonnes (Mt; CV 11.4%) (Hewitt et al., 2002). The new biomass estimate prompted 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). These survey results are based on the total echo energy attributed to krill, scaled by the Greene et al. (1991) model of krill acoustical reflectivity or target strength (TS). Presented here is a re-analysis of the CCAMLR 2000 data incorporating recent improvements in the characterization of krill TS. The results indicate that the estimated krill biomass in the Scotia Sea may be as high as 192.4 Mt (CV = 11.7%), or as low as 109.4 Mt (CV = 10.4%), depending solely on the expected distribution of krill orientations. The new Stochastic, Distorted-Wave, Born-Approximation (SDWBA) TS model solved with an empirically estimated distribution of in situ orientations leads to a krill-biomass estimate that is nearly 2.5 times the previous estimate. In consequence, revisions may be warranted of the standard krill TS model, the CCAMLR 2000 biomass estimate, and the associated precautionary catch level for krill in the Scotia Sea. (C) 2004 Published by Elsevier Ltd on behalf of International Council for the Exploration of the Sea.