Winter habitat selection by Antarctic krill will increase krill-predator-fishery interactions during ice free years

Climate change will affect populations and fisheries in the Southern Ocean as area typically covered by seasonal sea-ice become ice free in some winters (Stammerjohn et al. 2008). For Antarctic krill (Euphausia superba), a key forage species (Laws 1977; Smetacek and Nicol 2005) and a target of a commercial fishery (Nicol et al. 2010; Watters et al. 2013), recent declines in seasonal sea-ice extent and duration has negatively impacted their populations (Loeb et al. 1997; Loeb et al. 2009; Saba et al. 2014; Atkinson et al. 2004) and is likely to increase krill-predator-fishery interactions during autumn and winter (Nicol 2006; Flores et al. 2012a; Nicol et al. 2011). Research cruises conducted around the Antarctic Peninsula in winters with contrasting ice conditions provide the first acoustic estimates of krill biomass, habitat use, and association with top predators to examine these likely interactions. Krill were virtually absent in offshore waters of the Drake Passage during all three winters, compared to summer. In Bransfield Strait, median krill abundance was an order of magnitude higher (8 krill m ^-2) compared to summer (0.25 krill  m ^-2 ) regardless of ice concentration. Krill biomass was an order of magnitude higher (~5 500 000 tons in 2014) than summer average biomass (520 000 tons) in Bransfield Strait.  This concentration of krill represents 79% of the mean summer biomass (19 yrs; 6.9 million tons) in the larger (124 000 km² ) study area. Ice obligate, krill dependent predators (e.g. crabeater seal (Labodon carcinophagus)) were concentrated in Bransfield Strait regardless of sea-ice extent. Winter biomass estimates show krill are overwintering in coastal basin environments independent of ice, or primary production and in areas that are becoming more frequently ice free (Stammerjohn et al. 2008; Hill et al. 2013; Flores et al. 2012) increasing their availability to autumn and winter krill fisheries. In the near term, climate change induced variability will increase the risks of negative fishery-krill-predator interactions during low ice years when ice obligate predators are habitat limited and open waters are available to fishing vessels. Changes to conservation measures may be necessary to limit this interaction and ensure that risks to krill-dependent predators are minimized in accordance with Article II of the Convention for the Conservation of Antarctic Marine Living Resources.