Overwinter habitat selection by Antarctic krill under varying sea-ice conditions: implications for top predators and fishery management

Abstract

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 ), and this pattern was observed in all winters regardless of ice cover. Acoustic estimates of krill biomass were also an order of magnitude higher (~5 500 000 metric tons [t] in 2014) than a 15 yr summer average (520 000 t). Looking at krilldependent 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 icefree. 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.

The winter pack-ice zone provides a sheltered but food-poor habitat for larval Antarctic krill

“…This implies that larvae have access to this high algal biomass within the sea ice. However, recent observations 9,10 indicate that the linkage between sea ice and krill recruitment success is not as direct as has been suggested. The timing of ice-edge advance and annual ice-season duration is highly variable, and does not necessarily show a clear link to krill recruitment in the following year ( Supplementary Figs.…”

“…According to bioenergetics models, part of the variability is due to interannual variation in reproductive output 11 as well as autumn blooms that may govern the possible overwinter survival rate of larvae 11,12 . In the Bransfield Strait, three krill winter surveys have shown that krill abundance is an order of magnitude higher than in summer, regardless of concurrent sea-ice conditions 10 . Furthermore, although algal concentrations within sea ice can be…”

“…According to bioenergetics models, part of the variability is due to interannual variation in reproductive output 11 as well as autumn blooms that may govern the possible overwinter survival rate of larvae 11,12 . In the Bransfield Strait, three krill winter surveys have shown that krill abundance is an order of magnitude higher than in summer, regardless of concurrent sea-ice conditions 10 A dominant Antarctic ecological paradigm suggests that winter sea ice is generally the main feeding ground for krill larvae. Observations from our winter cruise to the southwest Atlantic sector of the Southern Ocean contradict this view and present the first evidence that the pack-ice zone is a food-poor habitat for larval development.…”

Spatio‐temporal trends of the Krill fisheries in the Western Antarctic Peninsula and Southern Scotia Arc

“…Those risks are even higher when changes in krill populations due climate change are considered (Klein, Hill, Hinke, Phillips & Watters, ). The changes taking place in the Antarctic Peninsula from climate change are already having consequences for top predators populations (Reiss et al., ; Trivelpiece et al., ), and therefore, those links need to be better studied.…”

Accounting for seasonal and composition‐related variability in acoustic material properties in estimating copepod and krill target strength

“…Scientific echosounders are commonly used to survey populations of commercially important species such as fish and euphausiids (Simmonds and MacLennan ; Fielding et al ; Siegel and Watkins ; ICES ). Many species of euphausiids are found in swarms suitable for detection with the common fisheries echosounder frequencies, and hence are quantitatively monitored using acoustic techniques routinely in the Antarctic (Reiss et al ), sporadically in the North Atlantic (Jech et al ), in the Bering Sea (Ressler et al ), off the California coast (Santora et al ), and in the Barents Sea (Ressler et al ). Euphausiids (also referred to as “krill”) are important ecosystem members throughout the world's oceans and are well studied in many regions (e.g., Antarctic krill, Bering Sea, and California Current) (Nicol ; Santora et al , ; Ressler et al ).…”