Population age structure of Antarctic krill Euphausia superba off the northern Antarctic Peninsula based on fishery survey

Antarctic krill,Euphausia superba Dana,is an important species in the Southern Ocean ecosystem.In the waters around South Orkney Islands,Antarctic krill contribute to a majority of the zooplankton biomass,forming a direct link between primary production and large top predators of the Antarctic.Thus,knowledge of the ecology and population dynamics of krill is fundamental to our understanding of the dynamics of the Southern Ocean ecosystem.The Antarctic krill is currently the major subject of biological research in the Antarctic.Despite considerable efforts to elucidate its life history and ecology,many questions are still un-answered. As one of the most abundant living resources in the world,Antarctic krill has received increasing attention for its huge biomass,potential fishery resource and ecological roles in the Antarctic waters.Much of this attention has been focused on defining its longevity and growth characteristics,important parameters needed in the assessment and management of a fishery.However,the age of Antarctic krill has not been determined reliably,limiting our understanding of life history parameters that regulate the population dynamics of krill.Age structures of Antarctic krill sampled from the waters of the northern Antarctic Peninsula(South Shetland Islands and the South Orkney Islands) in austral summer of 2009/2010 were analyzed by the Bhattacharya method based on the length frequency data.The results showed that the standard length ranged from 38.2 to 64.0 mm in January and 33.2 to 59.0 mm in February,and there was no significant difference for the standard length distribution of Antarctic krill between January and February(K-S test,Z=1.061,P=0.2110.05).Antarctic krill population might consist of as many as five age-groups from 2+,3+,4+,5+ and 6+.Great differences can be found for the age structure of samples collected at different stations,age group 2+ only existed in the isolated area.The age-group 5+(57.23%) was dominant in the population and followed by age group 4+(23.42%) in January,the age-group 6+(53.29%) was significant higher than that of the other age groups and followed by 3+(25.85%). The limited sampling duration in this study may,to some extent,make this study result inconclusive.Antarctic krill is an oceanic species,and continuous sampling on Antarctic krill samples throughout the year is difficult.In situ experiments on the growth and variation after over-wintering are also difficult.New methodology needs to be developed to improve our understanding of key life history process,such as growth,of Antarctic krill.