The migration patterns of Electrona carlsbergi (Tåning, 1932)

On the basis of analyses and review of published data on the biology and distribution of Electrona carlsbergi, as well as on the hydrological patterns of the Southern Ocean - especially concerning the structure and spatial variation of the Antarctic Circumpolar Current (ACC) - this paper describes possible ways and means (mechanisms) by which this species carries out meridional migration in various frontal zones. The yearly transport of E. carlsbergi into the waters of the Southern Polar Frontal Zone (SPFZ) occurs regularly although its intensity changes in time. The densest concentrations are observed here in the spring-summer period (November to February) when zooplankton. the main dietary component for E. carlsbergi, is undergoing rapid development. In various areas of the SPFZ and in all sectors of Antarctica E. carlsbergi is represented by immature specimens 7 to 8 cm in length and 2 years of age, indicating dominance by a single cohort. Moreover, a very similar size composition in the SPFZ is evident from both a seasonal and interannual point of view. The proportion of immature specimens in the Subantarctic zone, classified as a breeding area, decreases to 20-40%, while the number of mature specimens 8.5-11 cm in length and 3-5 years old increases. Based on the dynamic processes of the Southern Ocean and taking account of the locations of spawning areas and distribution of E. carlsbergi in the early stages of its life cycle, a theory has been put forward concerning the presence in the notal region and various sectors of Antarctica of several reproductive zones coinciding with areas of large scale disruptions to the zonality of transport by the ACC under the influence of topogenic factors. This paper examines the fate of large swarms (in terms of biomass) of E. carlsbergi transported beyond the notal region and the possibility of a part of the population returning to the reproductive zone. There is a strong likelihood that E. carlsbergi migrates from the SPFZ into the Subantarctic zone by means of eddy formations which occur periodically in the ACC system. These eddies play an important role in the meridional exchange of waters between frontal zones. The paper also discusses questions of the within-species structure of E. carlsbergi and an ecological assessment of the impact of fishing on the ecosystem of the open waters of Antarctica.