A review and update of krill biomass trends in the South Shetland Islands, Antarctica, using the simplified stochastic wave born approximation

Using data collected from US AMLR surveys conducted in the South Shetland Islands we review the trends in biomass, size frequency distribution and proportional recruitment of Antarctic Krill (Euphausia superba) between 1997 and present. Over the last five years proportional recruitment as been extremely low, and the population has aged, suggesting there has been little recruitment since 2000-2001 season. Relative biomass, estimated from acoustic surveys, and calculated using both the Greene et al (1991) and the simplified Stochastic Wave Borne Approximation (SDWBA) target strength algorithms are compared. Biomass estimates from the two algorithms are highly correlated (>0.95), with identical CV’s. Use of acoustic windows based on the range of krill size increases variability in biomass estimates and CV’s. These data suggest that future development focus on better propagating error through levels of analysis, to better account for process and model error structure, now that a physically based krill target strength model has been developed.