Antarctic krill (Euphausia superba) are considered to be one of the key species in the Antarctic marine food web, being prey to a wide variety of dependent species as well as being commercially harvested. The commercial exploitation of krill is managed under the direction of CCAMLR (Commission for the Conservation of Antarctic Marine Living Resources), utilising results derived from the CCAMLR generalised yield model and krill yield model (Constable and de la Mare, 1996; Butterworth et al. 1994). A key parameter of the krill yield model is an estimate of the pre-exploitation biomass of krill (B0). The current estimate of B0 which is used by CCAMLR in the model is derived from the CCAMLR-2000 synoptic acoustic survey (hereafter CCAMLR-2000) in the Food and Agriculture Organisation (FAO) statistical subareas 48.1, 48.2, 48.3 and 48.4 (Hewitt et al. 2004; Watkins et al. 2004). The design, planning, implementation and subsequent first analysis of the CCAMLR-2000 B0 estimate were initiated in 1995 and documented within the CCAMLR WG-EMM reports (SC-CAMLR-XIV, Annex 4, Paragraph 4.61; SC-CAMLR-XV, Annex 4, Paragraphs 3.72 to 3.75, 8.8; SC-CAMLR-XVI, Annex 4, Paragraphs 8.109 to 8.129; SC-CAMLR-XVII, Annex 4, Paragraphs 9.49 to 9.90; SC-CAMLR-XVIII, Annex 4, paragraphs 8.41 to 8.49) as well as papers tabled at those meetings (CCAMLR 1995; 1996; 1997; 1998; 1999). In particular two workshops, one planning (SC-CAMLR-XVIII, Annex 4, Appendix D) and one analysis (SC-CAMLR-XIX, Annex 4, Appendix G), provide background and detail to the survey design, implementation and original analysis (CCAMLR 1999; 2000). Since 2000 a number of re-analyses, improvements and corrections to the acoustic protocol to determine krill density have been tabled at the CCAMLR WG-EMM and SG-ASAM meetings. This document aims to summarise those changes and their rationale.
For ease of reading, this document has been separated into nine sections: Survey design; Acoustic data collection; Acoustic data processing; Target strength (TS) model and implementation; Target identification; Echo integration; Conversion of acoustic backscatter to areal biomass; Estimation of total biomass from areal biomass; and Estimation of uncertainty. Key methods and their derivations are highlighted in bold. This document does not describe a re-design of the CCAMLR synoptic survey, but details changes to the initial analysis, and the procedures (Table 1) which lead to the current estimate of B0.