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Bering Sea ice retreat threatens ecosystem and fisheries – Eye on the Arctic

The number of recorded vessels making the voyage through the Bering Strait between Alaska and Russia has nearly doubled over a four-year period, with 130 in 2009 and 250 in 2012. (Loren Holmes / Alaska Dispatch)
The Bering Strait
(Loren Holmes/Alaska Dispatch)
The trend of the last 40 years shows that the ice is less and less extensive on the Bering Sea, which contributes to reducing the presence of many species of fish, molluscs and crustaceans specific to this area, several of which are prized by the fishing industry, researchers report.

The interactions between the currents, the ice, the reliefs of the seabed and the weather make this sea, located between Alaska and eastern Russia, a very rich ecosystem. Salmon, halibut, cod, but also crustaceans such as snow crab are present in abundance there and feed a fishing industry on which local populations depend, particularly in Alaska. However, the balance is changing et the situation is not likely to improve with global warming.

Scientists at the Naval Postgraduate School in California analyzed data on ice extent each year in the Bering Sea.

They compared them to the data collected concerning the formation, each year, of a layer of dense and cold water at depth in this northern part of the Pacific, what they call the “cold basin” or ” cold pool », in English, and which is crucial to the survival of many species.

All this allowed them to build a computer model to predict the interactions between surface ice and deep cold water each year, noting a strong correlation between the presence of the two.

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The importance of surface ice

Jaclyn Kinney and her team explain in a press release that the ice that forms each winter on the ocean produces a more concentrated water in salt under the surface which gradually migrates to the ocean floor as the season progresses. This creates a pool of cold, dense water at depth that remains for several weeks after the sea ice melts in the spring.

When sunny days return, the melting of surface ice creates a difference in temperature and salinity with respect to the dense, cold layer at depth, allowing phytoplankton to proliferate rapidly in the surface layers. Much of the phytoplankton later sinks to the seabed where it feeds many marine species, contributing to ocean biodiversity.

However, if little or no ice forms on the surface in winter, the cold and dense layer at the bottom does not then have the same extent. In addition, phytoplankton proliferate much later in the spring, the researchers explain. This favors the development of zooplankton and surface (pelagic) fish species, to the detriment of those living at depth. Indeed, during these years when the ice is absent, much less phytoplankton and nutrients reach deep waters in the warm season, they note.

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This puts deep-sea species such as snow crab at risk, the researchers note, which depend on the current balance between surface ice and cold, dense waters deep below.

This illustration shows the extent of the ice in the month of April each year in the Bering Sea. After a winter in 2012 when ice extent peaked, the second half of the 2010s saw ice retreat, including a trough in 2018. (NOAA)

The researchers found that in the summer of 2018, the “cold pool” was only 31% of its 1980-2018 average. This year, the ice extent had dipped below average. This phenomenon repeated itself in 2019 and 2020, which also experienced unusually high temperatures, resulting in less sea ice. Then, in 2021, the crab population crashed, likely due to a reduction of their preferred habitat, say the scientists.

They predict that this subarctic species, like many others, will migrate north into the sea. of the Chukchis (north of the Bering Strait), with global warming.

Species from the Bering Sea will be replaced by other species native to regions further south in the Pacific, they argue.

According to the researchers, whose work appeared in the magazine PLOS ONEit is important to be able to predict future trends in the Bering Sea, both for the consequences on biodiversity and for the future of the fishing industry.

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