FEEDING REQUIREMENTS OF KRILL IN RELATION TO FOOD SOURCES

ABSTRACT During the Melville expedition to the Scotia Sea in January-March 1981, a multidisciplinary team of investigators studied the distribution and biomass of Euphausia superba, in addition to its food resources. The areas studied included five north-south transects of the Scotia Sea (33°–50°W) where krill were relatively sparse, and an area north of Elephant Island where a superswarm of krill was encountered. Krill biomass, determined on samples obtained by paired bongo nets (1 m2), has been used to calculate the minimal daily food requirements for all stages of krill from Calyptopis I to the adult stage. The mean krill biomass in the upper 200 m of the water column in the Scotia Sea (10.6 mg dry weight m–3) would require 0.105–-0.211 mg C m–3 d–1; the corresponding value for krill in the swarm area (270 mg dry weight m–3) was 2.4-5.4 mg C m–3 d-'. The phytoplankton productivity for the upper 200 m in the Scotia Sea and in the swarm area was estimated to be 4.8 and 4.2 mg C m–3 d-', respectively. Our calculations have been concerned mainly with the minimal daily carbon requirements for "maintenance" metabolism. Data of other investigators have been used to compare the total carbon requirement for growth of krill on a seasonal basis. In areas of dense krill aggregations (such as that north of Elephant Island) our calculations indicate that phytoplankton productivity can sustain "maintenance" metabolism by the krill population, but may not provide for full growth of the individuals. Other factors must be invoked here, such as dispersal of the swarm, horizontal movements of the krill along lines of increasing food concentrations, and advection of food resources into the swarm area. Floristic analyses of phytoplankton "in" and "out" of krill swarms suggest that the krill preferentially consume the larger phytoplankton, and leave most of the nannoplankton. Our data from the Scotia Sea indicate that this area is very high in primary production as compared to most other sections of the Antarctic Ocean, and this might be responsible for the high krill biomass reported for this area. The BIOMASS acoustic data set indicates, however, that the greatest krill concentrations exist in the East Indian Ocean (61 mg dry weight m–3), where data indicate that primary production is very low. This anomaly requires further investigation on the relationship of large-scale zones of high primary production in Antarctic waters and their relation to zooplankton biomass.