Distribution patterns and biomasses of Antarctic krill (Euphausia superba) and ice krill (E. crystallorophias) with note on distribution of Antarctic minke whales (Balaenoptera bonaerensis) in the Ross Sea in 2005

Distribution patterns and biomasses of Antarctic and ice krill in the Ross Sea in austral summer in 2005 were studied using a multi-disciplinary survey data set combining cetacean, krill and oceanography data. Two research vessels, KM and KS2, conducted the hydroacoustic surveys independently in the same area. Distribution patterns and length frequency information for two species were obtained from samples of RMT hauls and stomach contents of Antarctic minke whales. Ice krill was distributed on the continental shelf region (shallower than 1000m water depth). In contrast, Antarctic krill was distributed mainly in the oceanic waters where water depth is deeper than 1000m though it distributed on the continental shelf where the mean water temperature between 0-200m was higher than -1°C. The Ross Sea was stratified into two strata based on the distribution patterns of two krill species to estimate their biomasses. Biomass densities of Antarctic krill using KM and KS2 data were estimated as 5.13±7.14 and 2.53±2.25 g/m2, respectively. Biomass densities of ice krill using KM and KS2 data were estimated as 2.58±1.47 and 1.13±0.65 g/m2, respectively. Because there was no significant difference between the biomass density estimates from both vessels, two data sets were combined to estimate the biomass. The biomasses of Antarctic and ice krill in this study were estimated as 1.40 (CV=0.32) and 0.60 (CV=0.18) million t, respectively. School sizes of Antarctic minke whales were large where the densities of Antarctic krill were high. Distribution pattern of Antarctic minke whales in the Ross Sea could be regulated by distribution patterns of Antarctic krill.