The study was introduced in Global Change Biology, an international journal.
"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," he added.
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.
He also states that global warming seldom has positive effects on the cod, but this change is the odd point. "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."
The researchers made the discovery by reviewing figures from the International Council for the Exploration of the Sea (ICES).
These figures showed that the ocean temperatures at various times that the average sizes of the fish at various times, that the location of the fish at various times and that when the fish became sexually mature at various times.
"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 stocks in the North Sea can be preserved.