THE CHANCES OF survival for the offspring of important fish species will dramatically worsen if the Paris Agreement target of limiting the global rise in temperature by 1.5 degrees Celsius is not achieved.

Under conditions of further warming of the ocean, Atlantic cod and its arctic relative polar cod would be forced to look for new habitats in the far north, researchers have warned. 

This could lead to their populations dwindling, something that “could be disastrous, as the polar cod is the most important food source for Arctic seals and seabirds”.

In addition, the world’s most productive area for catching Atlantic cod, located to the north of Norway, could be lost. However, the results of the study also show that a stringent climate policy could prevent the worst consequences for both animals and humans.

The research, conducted by experts at the Alfred Wegener Institute in Germany, notes that there are some types of fish that prefer extremely cool water – and can only spawn in cold water. The Atlantic cod, a very popular food, is one of them.

Even better adapted to the cold is the polar cod, which spends the winter in the Arctic in large swarms below the sea ice.

The polar cod spawns at water temperatures between zero and 1.5 degrees Celsius because the fertilised eggs can best develop at this temperature. In contrast, the Atlantic cod spawns at three to seven degrees which, from a human standpoint, is still extremely cold.

Researchers are convinced this dependency on cold water could prove fateful for both species. As a result of climate change, the waters of the North Atlantic and Arctic will warm considerably unless human beings find a way to massively reduce emissions of the greenhouse gas carbon dioxide.

Carbon dioxide 

In addition, there is the problem of acidification: the more carbon dioxide finds its way into the atmosphere, the more carbon dioxide dissolves in the ocean. Carbon dioxide bonds with water to form carbonic acid, which acidifies the ocean as it decays.

“That means Atlantic cod and polar cod will be doubly stressed in the future: their habitat will simultaneously grow warmer and more acidic,”  marine ecologist Flemming Dahlke explained. 

He and project director Dr Daniela Storch are the first researchers worldwide top have used experiments to investigate how a simultaneous acidification and warming would affect the eggs of both species.

In this context, the two AWI experts have concentrated on the embryos’ development up to the point where they hatch as larvae, only a few millimetres long. During this stage, they are especially sensitive to changing environmental conditions.

In both species, even a small rise in temperature can cause the eggs to die or produce deformations in the larvae. The experiments show that the situation becomes even worse when the water is acidic: the number of embryos that don’t survive increases by 20 to 30% at a pH level of 7.7, even at optimal temperatures.

Fishing industry 

The findings indicate potential problems for the fishing industry, since the coasts of Iceland and Norway are currently home to the world’s largest populations of Atlantic cod.

Every year, around 800,000 tonnes of cod worth €2 billion are harvested there. If these populations dwindle, as the AWI experts’ findings indicate, the losses could be enormous.

Dahlke stressed that, though the experiments yielded very clear findings, predicting the development of fish populations is extremely difficult.

“For instance, whether or not the embryos and larvae survive also depends on the ocean currents and available food.”

The Atlantic cod now spawn near Lofoten, an archipelago to the northwest of Norway. The current takes the eggs floating in the water, and later the larvae, farther north, where ideal living conditions await them.

“If the Atlantic cod populations and their spawning grounds shift to the northeast in the future, the fish will most likely spawn in completely different systems of currents,” Dahlke explained. “If that happens, we can’t yet begin to gauge the effects.”