Already struggling to survive in warming Arctic seas, the polar cod are now at risk from rising oil pollution.
LONDON, 2 June, 2021 – They are small – on average around 25cm long. But polar cod (Boreogadus saida) are a vital part of the Arctic food chain, a major ingredient in the diet of seals, narwhals and a wide variety of seabirds.
The Arctic is warming faster than any other area on the planet, and a study published in 2020 found that declines in winter sea ice cover in the Barents Sea region of the Arctic, plus warmer sea temperatures, were causing declines in polar cod reproduction rates.
The latest research indicates that the polar cod is now under threat not only from warming Arctic seas, but because of oil pollution as well, as the region’s rapidly diminishing ice cover allows more shipping traffic and commercial activity.
Morgan Lizabeth Bender is a researcher in the department of Arctic and Marine Biology at the University of Tromsø (UiT) in northern Norway. Her research has found that when the polar cod is exposed to a combination of warmer waters and only very slight levels of oil pollution, its development is interrupted, with abnormalities common.
“Polar cod is a somewhat difficult species that hasn’t been researched that much,” Dr Bender told the Science Norway website. “The fish are a difficult species to find and to take care of in the lab. However, this species has a very important ecological role.”
“Increased water temperature can increase the harmful effects of oil exposure”
The fish, monitored during the breeding process, were sorted into aquariums – some at a current Arctic water temperature of 0.5°C, others at a warmer 2.8°C to mimic an Arctic affected by climate change.
The aquariums contained either pure water or water contaminated by minuscule amounts of crude oil. “The pollution level would be the equivalent of about five drops of oil in an Olympic-size swimming pool,” says Dr Bender.
Though the study found that polar cod eggs in the warmer water hatched much faster than those in the colder water, at first there was little difference between survival rates in the various aquariums.
But then something strange started happening to the fry – the young fish – that were exposed to oil.
“When they first hatched, there wasn’t much difference,” says Dr Bender. “But as their jaw, face and eyes started to develop, we saw very clearly that they weren’t forming properly.”
Lower survival rates
The research found that the fry were very sensitive to even the slightest amount of oil pollution: death rates were highest among fry exposed to both warmer water and oil.
When the fry became large enough to start feeding, only 8% survived in the contaminated warmer water and 23% in the contaminated cold water.
Marine scientists say that polar cod numbers have shown a downward trend since 2010, despite the fact that they are not a fished species.
Sonnich Meier, of the Norwegian Institute of Marine Research, has been examining the impact of both global warming and oil pollution on Arctic fish species for a number of years.
“Polar cod is one of the fish species that is hardest hit by climate change in the Arctic,” he says. “The study shows that increased water temperature can increase the harmful effects of oil exposure.” – Climate News Network