Dr. Felix Mark discusses ocean’s acidification problem and its effect on fish

Cod could be back off the menu after recent recovery as climate change warms oceans

Under current emission levels, cod hatching could drop by 60 per cent in key fishing grounds around Iceland and Norway

Josh Gabbatiss
Science Correspondent
Wednesday 28 November 2018 20:00

The future of cod fisheries looks uncertain as warming oceans kill young fish and drive remaining populations into ever smaller areas, according to a new study.

Up to 60 per cent fewer are expected to hatch in key fishing grounds around Iceland and Norway if greenhouse gas emissions continue at current levels, the team of researchers from The Alfred Wegener Institute in Bremerhaven, Germany, suggested.

As these cold-loving fish are pushed further north, they will also be hit by oceans turning acidic as excess CO2 from the atmosphere dissolves in the water, they wrote in their findings published in the journal Science Advances.

After a warning from the United Nations (UN) that world leaders need to amplify their climate commitments, the scientists behind the latest research say only the strictest global warming targets will keep cod populations stable.

Until recently, stocks of North Sea cod were in danger of being wiped out after numbers plummeted by more than 80 per cent in four decades.

Since then, efforts to tackle overfishing have led to a gradual recovery.

The fish was labelled as sustainable by the Marine Stewardship Council last year for the first time in 20 years.

However, climate change is now presenting another major challenge to global fish stocks, and hundreds of species are expected to be forced northwards in search of cooler waters.

Cod in particular need to spawn at temperatures approaching freezing in order for their eggs to develop properly.

Combining their expertise in fish development with models of climate conditions, the scientists behind this latest study predicted survival rates for hatching cod.

“They show that, for the business-as-usual scenario, conditions for the young Atlantic cod will especially deteriorate in the North Atlantic near the end of this century,” said lead author Flemming Dahlke. “In the regions around Iceland and Norway, up to 60 per cent fewer cod larvae will hatch from their eggs.”

The fish are especially sensitive during these early stages, and even small changes in temperature can cause eggs to die or produce deformations in the larvae.

Polar cod are particularly vulnerable to temperature changes. They already live in the extreme north and will therefore have nowhere to migrate as oceans warm. Meanwhile their Atlantic cousins may no longer be able to spawn north of the Arctic Circle by 2100.

Despite their gloomy predictions, the scientists found that if global warming was limited to 1.5C above preindustrial levels, the more ambitious target set by the Paris climate agreement, most of the harmful effects to cod could be avoided.

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