A biomass estimate of Antarctic krill (Euphausia superba) at the Balleny Islands

The biomass of Antarctic krill (Euphausia superba) at the Balleny Islands, Antarctica (67° S 163° E) was estimated from an acoustic transect survey carried out in February 2015. Acoustic data processing followed the recommendations of CCAMLR SG-ASAM.  Two parameterisations of the krill acoustic target strength model were used.  The first parameterisation assumed a vertical orientation of krill, θ, as a normal distribution, mean = 11°, standard deviation = 4°, N(11,4), the second parameterisation used θ = N(-20,28).  Krill biomass using estimated using θ = N(11,4) was 20.8 kilotonnes (coefficient of variation, CV = 0.37), whereas biomass using θ=N(-20,28) was 13.75 kilotonnes (CV = 0.14).  The reduced biomass of the θ=N(-20,28) estimate was caused by differing krill identification ‘dB difference’ windows as well as the conversion factor applied to scale acoustic observations to krill density.