The recent explosive increase in size of the North Sea cod is baffling marine scientists.
A new study now claims to have found the cause: global warming.
The study has just been published in the international journal Global Change Biology.
”When the seawater gets warmer, all life processes in the cod speed up,” says Peter Grønkjær, an associate professor of marine ecology at the Department of Biological Sciences at Aarhus university, who headed the study.
“This causes the cod to digest their food faster and makes them convert it into extra muscle tissue.”
Researchers had anticipated that the North Sea cod would get smaller.
Overfishing has started a process in which the cod spend their energy on reproducing themselves rather than on growing bigger.
”But over the past 30 years the increased ocean temperatures have compensated for the anticipated decrease in cod size,” says Grønkjær.
Global warming rarely causes positive associations, and this new finding is merely a tiny bright spot in a very dark sea:
“On the small point that deals with the growth of the cod, this is good news,” he says.
“But this doesn’t mean that the warming of the North Sea is generally good for the cod. The higher temperatures alter the ecosystem, which causes the composition of the crustaceans that the cod feed on to change too. And that makes it harder for the fry to survive.”
In fact, there is a risk that the cod may disappear from Danish waters altogether.
In the 1980s, the average North Sea cod reached sexual maturity after four years, measuring 70 cm.
In 2000, it was only 2.5 years old and 50 cm long.
Today, the cod are back to their 1980 size. This is due to the increase in ocean temperatures.
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In the North Sea, they will look to the North where the sea is colder. And in the Baltic Sea they may become extinct because they cannot breed in the low salt levels of the northern Baltic, where the temperature is otherwise ideal for cod.
The researchers made the surprising discovery by reviewing figures from the International Council for the Exploration of the Sea (ICES).
These figures showed:
”By linking all this data together, we can conclude that the weight change is caused by changes in the temperature,” says Grønkjær.
The new findings will be used for developing strategies for how cod in the North Sea can be preserved.
Read this story in Danish at videnskab.dk
Cod tend to shrink considerably when a stock has been exposed to overfishing.
The ‘80s and ‘90s saw a great deal of overfishing of North Sea cod. Some 60 percent of all 2-4 year-old cod were taken annually in this period.
This meant that more or less the only cod that had an opportunity to spawn were those which – due to their genetic makeup – spawned early on in life. The consequence is that the gene for early spawning was passed on in the cod stocks. This is why today’s cod spawn several years earlier than they did 30 years ago.
“When cod spawn earlier, they spend their energy on producing eggs and sperm rather than on growing. In the 1980s the cod reached sexual maturity when it had a length of 70 cm. Today this occurs when it’s 50 cm,” says Grønkjær.
“However, thanks to climate change, today cod continue to grow, so that they reach the same sizes as they did in the ‘80s.”