Arctic Ocean: The jellyfish are coming

Ctenophores

https://aslopubs.onlinelibrary.wiley.com/doi/full/10.1002/lno.12568

As the oceans warm and sea ice dwindles, jellyfish can spread further and further north – including into the Arctic Ocean, as researchers have now discovered. As a result, the fire jellyfish and other species of jellyfish could dramatically change the food webs in the Arctic Ocean and, above all, endanger fish. Because the cnidarians are usually superior competition for cod and co.

Jellyfish are real adaptation artists and can also thrive where more sensitive species suffer from pollutants, fishing or other disturbances. As a result, the cnidarians often benefit when the marine world becomes unbalanced due to warming, over-fertilization or overfishing. As a result, there are increasing mass proliferations of jellyfish in various marine areas. They are so successful that entire food webs in the ocean are shifting – there is a risk that the seas will become “squalid”.

The Arctic Ocean’s habitat is particularly at risk

Such a dominance of jellyfish would be a massive threat, especially to the Arctic Ocean. Because it is particularly badly affected by climate change and the changes associated with it. “Of all the oceans, the Arctic Ocean is warming the fastest,” says lead author Dmitrii Pantiukhin from the Alfred Wegener Institute for Polar and Marine Research (AWI). In addition, the Arctic Ocean is burdened by dwindling sea ice, increasing stratification and changing current patterns. A particularly large number of fish that are valuable for our diet, such as cod, live and spawn in the cold waters of the Arctic. “The Arctic represents around ten percent of global fishing yields,” explains Pantiukhin.

But it is precisely the fish in this sea area that would be at risk from a massive jellyfish invasion: “The cnidarians can often assert themselves against food competitors such as fish. This in turn has consequences for the entire food web,” explains lead author Dmitrii Pantiukhin from the Alfred Wegener Institute for Polar and Marine Research (AWI). “Many jellyfish feed on fish larvae and eggs and thus delay or prevent the recovery of fish populations that have come under pressure, which are also usually heavily managed by humans.”

Eight species of jellyfish as a test case

Pantiukhin and his AWI colleagues have now investigated how great the risk of “squalidization” of the Arctic Ocean is. For their study, the researchers collected data on the temperature tolerance and physiological requirements of eight jellyfish species common in the Northern Hemisphere and fed this into a simulation of the Arctic Ocean and its future development. In addition to the melon jellyfish (Beroe sp.) and the fire jellyfish (Cyanea capillata), which also occur in the North and Baltic Seas, the jellyfish species also included the crown jellyfish (Periphylla periphylla), which lives in the deep sea.

“By coupling it with the MPI Earth system model, we were then able to calculate for these jellyfish species how their distribution will change from the reference period 1950 to 2014 to the second half of the current century 2050 to 2099,” explains senior author Charlotte Havermans. She and her team assumed a climate development with medium to high greenhouse gas emissions and took into account not only the changes on the sea surface, but also those in deeper water layers.

The Arctic Ocean is conquered

The simulations showed that, with one exception, all jellyfish species examined could massively expand their habitat towards the pole. The fire jellyfish has particular potential to spread: it benefits from rising water temperatures and shrinking sea ice and could almost triple its Arctic range by 2100. “These results highlight how dramatically climate change can change Arctic Ocean ecosystems in the future,” says Pantiukhin. “The projected expansion of jellyfish habitat could have massive and cascading impacts throughout the food web.”

The relatively large fire jellyfish in particular is known for being able to displace cod, for example, from their habitats. The jellyfish would also endanger the spawning grounds of this commercially important fish species. “Especially in the regions around Spitsbergen and Novaya Zemlya, the northern expansion of the fire jellyfish could significantly affect the spawning grounds of cod,” report Pantiukhin and his colleagues. But other species of jellyfish, such as the crown jellyfish, are also known to drastically decimate cod offspring. This has already been the case in the Norwegian fjords with mass proliferation of this type of jellyfish.

The researchers therefore already see a need for action: “Management plans in the fisheries sector must also urgently take this dynamic development into account if they want to avoid the collapse of heavily fished stocks in the future and manage them sustainably,” says Havermans.

Source: Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research, specialist article: Limnology & Oceanography, doi: 10.1002/lno.12568

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