Diurnal changes of some biological characteristics of Euphausia superba Dana in swarms (westward of the South Orkney Islands, 24 March to 18 June 1990 - based on data reported by biologist-observer)

Investigations of diurnal changeability of size composition of E. superba was carried out during the working period of commercial trawler ‘Grigory Kovtun’ in the fishing area near the South Orkney Islands during March-June 1990. observations at 6 time stations were fulfilled. Every station consisted of series of catches by mean the usual commercial trawl (9-12 tows per every station). Increasing of average size of animals during light time was observed at several stations. These changes are related and were determined by increasing of males proportion as well as by difference between average length of males and females. It were namely males, which first deign diurnal vertical migrations inside of swarms. When diurnal migration are absent (particularly at most later season) the diurnal changes in E. superba size composition became feebler expresses or stop. Significance of these observations in relation to data on E. superba’s size composition accepted during usual surveys (collecting E. superba from every point of survey only once) is considered.
Gradual decreasing of average size of E. superba from end March to June was observed in operation region. As may suppose, these changes are related with drift of swarm, but not with progress of life cycle at winter postspawning non-feeding season. They hardly can be related also with selective stress of fishery activity upon larger specimens of E. superba.