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Atlantic fish found unexpectedly in the Central Arctic Ocean

As global warming takes effect in the Arctic region, it is predicted that the Central Arctic Ocean will be accessible to all vessels (not only to icebreakers) that seek to exploit the newly available resources. Since most of the area consists of high seas – international waters outside national jurisdictions – possible future human activities here are debated at national and international political levels.

To avoid an unregulated free-for-all in the central Arctic, major international stakeholders, namely Canada, China, Greenland (Kingdom of Denmark), Iceland, Japan, Norway, Russia, South-Korea, the USA and the European Union, got together and negotiated the Agreement to Prevent Unregulated High Seas Fisheries in the Central Arctic Ocean (CAO), which came into force on 25 June, 2021. 

Pauline Snoeijs Leijonmalm is the coordinator of the European Fisheries Inventory in the Central Arctic Ocean (EFICA) Consortium, and a professor in Marine Ecology at Stockholm University.

“This agreement prevents any commercial fishing for at least 16 years to come, and puts ‘science first,’ warranting scientific assessments of the status and distribution of possible fish stocks in the Central Arctic Ocean and the ecosystem supporting them – a wise political decision and a good start towards full protection,” said Snoeijs Leijonmalm.

The ten partners have also agreed to launch a large Joint Scientific Research and Monitoring Program to collect data on fish and ecosystems in the CAO. In fact, the EU has already started this research work by financing the EFICA Consortium’s ecosystem research on the MOSAiC expedition (2019-2020), and the Synoptic Arctic Survey expedition with the Swedish icebreaker Oden (2021). 

The first field data collected by a research vessel in the context of the agreement is presented today in the journal Science Advances. The scientists taking part in the international MOSAiC expedition, with research icebreaker Polarstern, were from Stockholm University, the Alfred Wegener Institute and the EFICA Consortium. 

“Usually, exploitation of newly accessible natural resources tends to precede scientific research and management measures, and internationally shared fish stocks in high seas are especially prone to overexploitation,” said Snoeijs Leijonmalm. It is therefore a valuable exercise to collect data on species present in the CAO at this early stage, in order to be able to monitor any changes that may occur in the future. 

The researchers were surprised to find unusual fish and squid species in the deep waters during their field work. Small fish occur at very low abundances in the 200-600 m deep Atlantic water layer of the Amundsen Basin, according to the data collected during the expedition. The unique hydroacoustic dataset showed a “deep scattering layer” (DSL) consisting of zooplankton and small fish along a 3,170-kilometer survey track, but the abundances of fish were very low.

Therefore, the researchers were surprised to catch four larger fish at a depth of 350 to 400 meters. Three of these fish were Atlantic cod, a predatory species that is not meant to occur so far north. Atlantic cod are coastal fish and are in an uncharacteristic habitat in the deep Amundsen Basin, more than 500 kilometers from the nearest coastline. 

Further analysis of the cod showed that they had originated from Norwegian spawning grounds, and had lived in Arctic water (at temperatures of between -1 and 2oC) for up to six years. The fish preferred the Atlantic water layer, a slightly warmer water mass (0-2oC) that reaches far into the Arctic basin between the surface and deeper water layers, which are both below 0oC.

“So, even if the Atlantic cod does not have its own central Arctic stock, this research shows that it can survive. A small number of individuals seem to find enough food to stay healthy for a longer time,” explained Snoeijs Leijonmalm.

Apart from capturing the cod in this unusual location, the researchers also identified Atlantic armhook squid and Atlantic lanternfish from the footage of a deep-sea camera deployed underneath the sea ice. These species also showed an unexpected distribution, much further north than previously known. 

The findings of the MOSAiC expedition help scientists to understand more about the functioning of the central Arctic ecosystem. The large, predatory fish and squid represent a new trophic level in the pelagic food web in this ecosystem. These organisms, along with the smaller fishes in the DSL could be potential food for Arctic mammals, since seals and walruses can dive down as deep as the Atlantic water layer.  

“The availability of small and even some larger fish in the Atlantic water layer could explain why seals, walrus and polar bear can be found even at the North Pole. Both fish and mammals are very few, but they are there,” said biologist Dr. Hauke Flores of the Alfred Wegener Institute.

The new study also found that daily vertical migrations of the zooplankton in the DSL do not occur during the polar night, half a year of continuous darkness (DSL at 100-250 m), or during the polar day, half a year of continuous light (DSL at 300-500 m). This implies that the carbon flux from shallower to deeper water through daily vertical migration of the DSL is limited in the Central Arctic Ocean, when compared with all other oceans.

“During the short, productive season of the polar day the DSL will remain in the deeper part of the Atlantic water layer 24 hours per day, even when the sea ice disappears, because this process is regulated by the availability of light,” explained Snoeijs Leijonmalm.

Based on their scientific results, the authors of the new paper conclude that – at least in the Eurasian Basin of the Arctic Ocean – there are no harvestable fish stocks today, nor will there be in the near future.

“This was expected because the Central Arctic Ocean has very low nutrient concentrations and very low biological productivity. Even if more Atlantic fish and their prey would be advected with the water inflow from the Atlantic Ocean, the capacity of the Central Arctic Ocean ecosystem to support larger fish stocks is without doubt rather limited,” said Snoeijs Leijonmalm.

In conclusion, Pauline Snoeijs Leijonmalm stresses that it is of great importance that this fragile but fully functional ecosystem receives robust international protection in future, similar to the protection afforded ecosystems in Antarctica.

By Alison Bosman, Staff Writer

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