Antarctic krill, Euphausia superba
Antarctic krill, Euphausia superba, (hereafter “krill”) shapes the structure of the marine Antarctic ecosystem, due to their central position within the Southern Ocean (SO) food web as prey of a wide range of higher trophic predators, as effective grazer on phyto- and zooplankton organisms as well as on the microbial sea ice biota .
In the SO krill has a circumpolar distribution with a total biomass between 100 and 500 million tons. However, the largest krill stock (58-71%) is located in the Southwest (SW) Atlantic sector of the SO between the western Antarctic Peninsula and the Scotia Arc. To put the high biomass of krill into perspective, annually, less than 100 million tonnes of all species of fish and shellfish are currently harvested from the world’s oceans. In the last 30 years, the krill stock seem to declined in the SW Atlantic sector, which is associated with the seasonal sea ice dynamic. It has been demonstrated that duration of sea ice coverage and extent during winter have an negative effect on larval development during winter and reproduction of adults in the forthcoming spring in terms of food supply.
The krill life cycle
The animals grow to a maximum length of 65mm, have a life span of five to seven years and their age at sexual maturity is after two years . Krill have evolved a successful and complex life cycle adapted to exploit the highly seasonal environment of the Southern Ocean. Seasonal growth and reproduction cycles of krill are synchronized with seasonal cycles of food supply, sea ice cover and day length.
The reproductive period of krill is restricted to a 1.5-3 month season during the Antarctic summer, alternating with a long period of gonadal rest. After spawning, eggs sink to depths of 800-1000m and hatch. The nauplius larvae commence their re-ascent to the surface while developing via the metanauplius stage to the calyptopis larvae (“developmental ascent”). The calyptopis I stage is the first feeding stage of krill. Krill ontogenesis proceeds via two more calyptopis stages, followed by six furcilia stages, which develop during summer, autumn and winter and recruit to the juvenile population in the following spring. As krill mature and become adult they begin to aggregate into huge schools or swarms, with many thousands of krill turning the water red or orange. The high abundance of krill and this swarming behaviour make krill attractive to commercial harvesting. The Antarctic krill fishery has been the largest fishery in the Southern Ocean, in terms of tonnage caught, since the late 1970s.