Climate change impacts on Antarctic krill behaviour and population dynamics

Krill habitats in the Southern Ocean are impacted by changing climate conditions, reduced sea ice and rising temperatures. These changes, in turn, affect krill occurrence, physiology and behaviour, which could have ecosystem impacts. In this Review, we examine climate change impacts on Antarctic krill and the potential implications for the Southern Ocean ecosystem. Since the 1970s, there have been apparent reductions in adult population density and the occurrence of very dense swarms in the northern Southwest Atlantic. These changes were associated with latitudinal and longitudinal rearrangement of population distribution - including a poleward contraction in the Southwest Atlantic - and were likely driven by ocean warming, sea-ice reductions and changes in the quality of larval habitats. As swarms are targeted by fishers and predators, this contraction could increase fishery-predator interactions, potentially exacerbating risk to already declining penguin populations and recovering whale populations. These risks require urgent mitigation measures to be developed. A circumpolar monitoring network using emerging technologies is needed to augment existing surveys and better record the shifts in krill distribution. Krill are food sources for megafauna, are drivers of carbon export and are being impacted by sea-ice declines and changing climate conditions. This Review examines changes in krill populations, habitats and behaviour in the Southern Ocean, and discusses their potential drivers and implications for fishery management in the future. The massive abundance and swarming behaviour of Antarctic krill makes them the primary prey for numerous Antarctic megafauna and important mediators of biogeochemical cycling in the Southern Ocean, particularly in processes that enhance carbon sequestration.Analyses of an existing dataset collectively support southward habitat contraction in the Southwest Atlantic sector, likely due to ocean warming and hence changing sea-ice dynamics, including the reduction of sea ice.Changes in habitat conditions affect krill physiology, which in turn could be accompanied by changes in their behaviour, such as changes in distribution and swarm size and frequency.Reduction in krill biomass is potentially associated with a reduction in the number of swarms. Such changes will have implications for predator-prey relationships and interactions with the fishery.An overall reduction in krill biomass and abundance could decrease the contribution of krill to the biological carbon pump in the region, with fewer faecal pellets, carcasses and moults sinking to the deep ocean, thereby reducing this important route of carbon sequestration.Future studies must aim for deeper understanding of the energy budget in krill as it pertains to their swarming behaviour, and should deepen our understanding of climate change impacts on the pelagic lifestyle of krill.