In the plankton samples collected with a Bongo net besides Euphausia superba the following species of the Euphausiacea were found:. £. frigida, crystallorophias, E. triacantha and T. macrura. The most aboundant and most frequently caught species was T. macrura equally distributed throughout the research area. The species E. frigida also occurred regularly though in much smaller numbers. E. triacantha occurred sporadically, mainly in the western part of the research region, and E. crystallorophias only at two stations in the Bransfield Strait. Some aspects of the biology and ecology of the mentioned above species are presented in this study.
In the plankton samples, collected with a Nansen net in three water layers downwards to a depth of 500 m, larvae and eggs of Euphausia superba were found as well as the larvae of E. crystallorophias and E. frigida. Eggs of the species E. superba predominated and among the larvae most numerous were calyptopis I and metanauplius stages. Eggs and larvae of E. superba occurred in small quantities, mainly in Bransfield Strait. Larvae of E. crystallorophias were found in the southeastern part of Bransfield Strait. Metanauplius, calyptopis I and II stages were predominant. Larvae of E. frigida, mostly calyptopis I stage, were very scarce and occurred only at the stations located in Drake Passage.
In the planktonie material collected using a Nansen net (vertical hauls) larvae of two euphausiid species were found. The dominant and occurring in all stations were larvae of Thysanoessa macrura. Following larval stages were encountered: nauplius, metanauplius, calyptopis I and calyptopis II. The most numerous and occurring in the widest depth spectrum were calyptopes I. Only twice furcilia VI of Euphausia superba were found. The distribution of euphausiid larvae was influenced by the stratification and circulation of water masses in the investigated area.
In the investigated area krill occured in low abundance. It was recorded mainly above the shelf and above the continental slope close to the Palmer Archipelago and near the northern shores of Elephant Island. In the central part of the Bransfield Strait E. superba was caught in especially small quantities. In general krill of small size occurred, the size decreasing from the west to the east. Mature krill was dominan in the western part of the investigated area, whereas juveniles in the eastern part. Gravid females were caught very rarely.
Krill population structure was studied in Western Antarctic in austral spring and summer 1986/87. At the end of October and the beginning of November in the waters around Elephant Island the mean krill length was 44 mm and sexually mature specimens dominated. Juveniles were absent. In the Bransfield Strait the mean E. superba length was 43 mm. and males slightly overdominated females. The share of females with spermatophores and of immature females was higher than in the Elephant Island area. Juveniles were also not recorded. Krill was most diversified around Elephant Island in January; juveniles and females with ovaries filled with eggs were recorded. It was found that 84% of krill population was infested by ciliate protozoans (Apostomatida).