Above: Arctic Cod are small forage fish, about the size of capelin, that make up a crucial link in the food chain of the Arctic Ocean. Photos courtesy of Maxime Geoffroy, Marine Institute
Scientists have raised alarms that the population of Arctic cod, a small forage fish related to Atlantic cod, is being severely affected by changing climatic conditions.
A group of 43 scientists, headed by Memorial University of Newfoundland research scientist Maxime Geoffroy, has laid out a series of crucial factors affecting Arctic cod populations. For Geoffroy, the timing of their release reflects the fast-paced changes in the Arctic Ocean, where the rate of warming is two to four times higher than in the rest of the world’s oceans.
“We want to know what this key species — where there’s no other forage species that are endemic to the Arctic — how they will react to these really quick and fast changes,” said Geoffroy.
Arctic cod, being the only forage species native to the Arctic Ocean, serve as a crucial link in the chain that connects energy sources like algae and zooplankton to larger species such as fish, seabirds and marine mammals.
“All the energy is funnelled through them. That’s really their role is to transfer this energy and support the whole food chain over there,” said Geoffroy.
While scientists are alarmed about the situation facing this important forage fish, they want to make it clear that the multitude of factors in the Arctic aren’t causing them to go extinct, but rather limiting their range. Conversely, in the high Arctic regions near the North Pole, warming temperatures might actually have a positive effect on Arctic cod populations.
“The problem is in the other regions, which are called the gateways to the Arctic. The Atlantic region of the Arctic, so Baffin Bay and on the Northeast Atlantic we have the Barents Sea and the Norwegian Sea towards the Arctic. The Pacific side also — the Bering Strait and Chukchi Sea. These areas are really the ones that are changing the fastest, and there we can really see a reduction, especially if the warming continues as it does,” said Geoffroy.
“That doesn’t mean the void wouldn’t be filled by other species. We have capelin that are extending. That’s already something we’re see happening. But we’re losing this endemic, specialist diversity of the Arctic.”
According to Geoffroy, the leaner, less-fatty capelin may not fill the same ecological niche for certain species in replacing the fattier Arctic cod. Arctic species that normally feed of Arctic cod turning to capelin could also have a negative effect on capelin stocks, further endangering a separate forage fish.
“If we think about Baffin Bay, we have turbot — Greenland halibut — which is one of the biggest fisheries in the Canadian Arctic, along with Northern shrimp,” said Geoffroy. “These fish, the Greenland halibut, are feeding on Arctic cod. So, will they be able to compensate feeding on capelin and other species? Maybe, but we know capelin are leaner. They are slightly smaller in general, so Greenland turbot would need to feed more to have the same amount of energy.”
Geoffory and his colleagues have identified eight key factors affecting this northerly forage fish — ocean warming, sea ice decline, increased freshwater input, ocean acidification, changing prey field, increased competition, new predators and pollution.
The warming of the oceans is especially treacherous for young Arctic cod, who have a temperature preference of around zero degrees Celsius.
“Larvae would be okay until around two Celsius, but if you pass that threshold, you start adding more mortality,” said Geoffroy. “And, of course, if you have more mortality in the early life stages of your larvae, you start stock is decreasing.”
Adding to the detriment of Arctic cod larvae, which goes hand-in-hand with ocean warming, is the loss of sea ice.
Sea ice in the Arctic serves as a breeding ground for Arctic cod, who mostly lay their eggs underneath the ice pans during the winter. With sea ice having declined 12.6 per cent per decade over the last 40 years, according to the Massachusetts Institute of Technology, Arctic cod are left with less and less space to spawn.
“The eggs are buoyant. So, they spawn at depth, but the eggs start floating to the surface. They are protected from UV radiation and predation at the surface; at the ice-water interface. So, they’re going to lose this protection and they’re going to drift more, also. They can get pushed more by current and storms outside their optimum growth range and habitat,” said Geoffroy.
With melting ice, comes increased amounts of freshwater. According to Geoffroy, this melting of polar ice is a double-edged sword for the Arctic cod population. While runoff from ice does help to cool down the warming Arctic waters, too much fresh water can dilute the salinity of the ocean to levels that make development unsustainable for saltwater species.
“Especially in regions near glaciers, where you would have almost zero-salinity freshwater released, you can have higher mortality,” said Geoffroy. “And even if the larvae and the eggs survive, they are meant to feed on marine plankton, and that cannot survive in complete freshwater. This is one factor where we identified more uncertainties where we have to do a bit more research.”
Increased levels of carbon dioxide are both the main cause of the exceedingly high ocean temperatures in the Arctic, and for another concern noted by scientists when it comes to the north’s food chain — acidification.
As carbon dioxide is absorbed by the ocean, it forms carbonic acid. As this acid breaks down, it forms bicarbonate ions and hydrogen ions. This growing concentration of hydrogen ions and subsequent reduction of carbonate ions raise the acidity of water. For Geoffroy and his team, the effects of acidification have yet to be conclusively determined.
“It’s difficult to see exactly how much more acidic it will become in the future. There have been some tests done at different levels of acidity in tanks. What we see is more mortality in the young stages. It would be detrimental for the formation of the eggs, mainly, and to some extent the larvae as well,” said Geoffroy.
As the Arctic Ocean warms, both predators and prey from southerly waters move north to waters that are now more favourable to them. Geoffroy claims that leaner, less fatty zooplankton that provide less energy to forage fish are moving in and replacing endemic species like the fatty, nutrient-dense copepods like Calanus Hyperboreus.
“The example I like to give is it’s like popcorn. You might have a lot of popcorn, but it will never be as rich as eating a big steak, for instance,” said Geoffroy.
New predators moving into certain regions of the Arctic include Atlantic cod in the Barents Sea, which are preying on Arctic cod. In Atlantic Canada, redfish have also been observed moving into once-inaccessible regions. The trend continues in Canada’s Pacific regions, where saffron cod, another small forage species, are moving in on Arctic cod’s habitat.
“You still have some Arctic cod, but they have more competitors and more predators that they have to share space with,” said Geoffroy. “The relative abundance and importance is decreased and top predators depending on them will decrease.”
The final piece of the puzzle making up the array of threats to Arctic cod is pollution, particularly from oil exposure. Increased drilling in areas like the Barents Sea as well as the increased prevalence of shipping in the Arctic regions have seen increased oil leakage in these Arctic gateways Geoffroy and his group are concerned about.
“You have more deformities and mortality for the larvae and the eggs. It also has an impact on the spawning individuals, so the quality of the eggs and sperm is lower when they are exposed to contaminants like oil,” said Geoffroy.
The challenges facing Arctic cod are multi-faceted and complex. Geoffroy is aware that the solutions are not simplistic and will require a concerted effort by both governments and the private sector to address the effects climate change is having on this vital forage fish. To him and his colleagues, their main goal is to educate the public about the issues facing Arctic cod.
“In general, people protect what they know and what they like, and they like what they know,” said Geoffroy.
“Our role is to bring information about pieces of the puzzle of the Arctic that people are less aware of. We all hear about the polar bears and the whales, but the reality is that there are other parts of the puzzle of the Arctic that we need to take into account. We’re not just impacting certain species; we’re impacting a whole habitat — a whole ecosystem.”
