Swarm of euphausiids in the ocean

Abstract

Euphausiids often form dense swarm in the sea. The surface swarms of Euphausia superba, E. pacifica and some species of Thysanoessa are often observed in the polar waters. The size and density of swarms of euphausiids are various. The average size of round swarms of Euphausia superba in the Antarctic is about 30 metres in diameter sometimes amounting to several hundreds. The species composition of those swarms is simple. It was mostly of Euphausia superba in the research in the 1972/73 season. Five mixed swarms of E. superba and Thysanoessa macrura are observed among 317 swarms studied in the high Antarctic. The biomass of swarm sometimes attains to 30kg/m^3,generally ranging between 100g/m^3 and 200g/m^3. Smaller individuals form heavier dense swarms in individual number, but the biomass per unit volume of sea water is less than the swarm of larger individuals. The relation between the size of zooplankton and micronekton and the density of individual number and biomass of swarms is also discussed. The Japanese common names for the English names of different types of aggregation of euphausiids are proposed to clarify the nature of swarms in the future description and studies.