Study sheds light on ecological ties of whales and krill
The Antarctic Peninsula is home to many predators that chase Antarctic krill (Euphausia superb), a species found in the Southern Ocean that resembles shrimp-like invertebrates and lives in swarms. But information about the ecological ties of whales and krill and reports of 'super-aggregations' of krill have been lacking in the Western Antarctic Peninsula (WAP) ... until now. A Franco-American research team has discovered an extremely high density of humpback whales (Megaptera novaeangliae) feeding on a super-aggregation of Antarctic krill. Presented in the journal PLoS ONE, it is the largest aggregation of krill reported in over two decades, and the highest density of humpback whales ever documented. Scientists from the Laboratoire d'Océanographie Physique et Biogéochimique (LOPB) of the Centre national de la recherche scientifique (CNRS) and the Université de la Méditerranée in France, and Duke University Marine Laboratory in the United States say their findings shed new light on how the region is being affected by rapid climate change. The team tracked the super-aggregation of whales and krill during a six-week expedition to Wilhelmina Bay and surrounding waters in May 2009. 'Such an incredibly dense aggregation of whales and krill has never been seen before in this area at this time of year,' says Professor Douglas Nowacek of Duke University and lead author of the study. Past studies put the spotlight on whale foraging habitats located in waters farther offshore in austral summer. The scientists observed 306 humpback whales (around 5.1 whales per square kilometre) in Wilhelmina Bay. According to them, it was the highest density ever recorded. Krill biomass was calculated at around 2 million tonnes. They also noted that small, floating fragments of brash ice covered only a tenth of the bay. A year after the tracking process, the team recorded similar numbers. They believe that advancing winter sea ice used to cover most of the peninsula's bays and fjords by May, effectively safeguarding krill and compelling humpback whales to hunt for food elsewhere. However, the rapid climate change impacting the area in the last 50 years has minimised the extent of the ice cover and has postponed its annual arrival, Professor Nowacek says. 'The lack of sea ice is good news for the whales in the short term, providing them with all-you-can-eat feasts as the krill migrate vertically toward the bay's surface each night,' points out Ari Friedlaender from Duke, one of the co-authors of the study. 'But it is bad news in the long term for both species, and for everything else in the Southern Ocean that depends on krill.' Krill migrate in austral autumn from open ocean waters, moving to bays and fjords that are rich in phytoplankton. Here, the swarms find shelter under the cover of ice and juveniles feed and grow. 'Changes in the physical structure of the marine ecosystem around the Antarctic Peninsula may have profound effects not only on the abundance of krill and baleen whales, but also on the ecological interactions among all krill predators and their prey,' the authors write. 'Our observations indicate that humpback whales and their prey co-occur in super-aggregations during late autumn in the bays and fjords along the WAP. Efforts to monitor the distribution, abundance and dynamics of these whales should account for these large aggregations.' In terms of what the future holds, the researchers will look into determining how changes in sea ice cover affect the feeding ecology of humpback whales and other predators in the short term and the dynamics of krill populations in the long term. Researchers are also concerned about the growing pressure from commercial krill harvests. 'Failure to account for the effects of climate change on these dynamics will undermine our ability to understand changes in the standing biomass of Antarctic krill and also to predict the recovery of whale populations from a century of mismanagement and overexploitation,' the authors conclude. Researchers from the University of Hawaii Joint Institute for Marine and Atmospheric Research and the University of Massachusetts in the United States contributed to this study.For more information, please visit: PLoS ONE:http://www.plosone.org/home.actionDuke University:http://www.duke.edu/Laboratoire d'Océanographie Physique et Biogéochimique (LOPB):http://www.com.univ-mrs.fr/LOB/
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