Overwinter habitat selection by Antarctic krill under varying sea-ice conditions: implications for top predators and fishery management
Climate change will affect Antarctic krill Euphausia superba, krill-dependent predators, and fisheries in the Southern Ocean as areas typically covered by sea ice become ice-free in some winters. Research cruises conducted around the South Shetland Islands of the Antarctic Peninsula during winters with contrasting ice conditions provide the first acoustic estimates of krill biomass, habitat use, and association with top predators to examine potential interactions with the krill fishery. Krill abundance was very low in offshore waters during all winters. In Bransfield Strait, median krill abundance was an order of magnitude higher (8 krill m-2) compared to summer (0.25 krill m-2), and this pattern was observed in all winters regardless of ice cover. Acoustic estimates of krill biomass were also an order of magnitude higher (~5500000 metric tons [t] in 2014) than a 15 yr summer average (520000 t). Looking at krill-dependent predators, during winter, crabeater seals Lobodon carcinophagus were concentrated in Bransfield Strait where ice provided habitat, while Antarctic fur seals Arctocephalus gazella were more broadly distributed. Krill overwinter in coastal basin environments independent of ice and primary production and in an area that is becoming more frequently ice-free. While long-term projections of climate change have focused on changing krill habitat and productivity declines, more immediate impacts of ongoing climate change include increased risks of negative fishery-krill-predator interactions, alteration of upper trophic level community structure, and changes in the pelagic ecology of this system. Development of management strategies to mitigate the increased risk to krill populations and their dependent predators over management timescales will be necessary to minimize the impacts of long-term climate change.