Many fish species in the North Sea are steadily moving northwards to escape warming waters, researchers report.
Cod is one species that is moving north
Commercially important fish such as cod, whiting and anglerfish have shifted significantly north, while some other species moved to colder depths.
Scientists warn in Science magazine that some fish may disappear from the North Sea by 2050.
They say commercial fisheries will have to take account of global warming as well as dwindling fish stocks.
"Some of these species are already depleted and this is yet another challenge that they face," Allison Perry, of the University of East Anglia, told the BBC News website.
Predators and prey
Dr Perry and her team have studied data on 36 species of fish going back to the 1970s. Of those species, 21 have moved northwards, some by hundreds of kilometres.
Since the 70s, the average winter temperature at the bottom of the North Sea has risen by around one degree Celsius, and the researchers believe that rise, which they say is attributable to global warming, is forcing populations to shift.
In some cases, this relocation might be a mere inconvenience for fishermen, but in others it could prove very dangerous for the fish.
The main problem is that marine species are shifting at different rates. So some predatory fish might be migrating north, while their prey stay put; and likewise other species might be unable to move even though their habitat is becoming more and more unsuitable for them.
"For example the coley is a large-bodied fish and it can live up to 30 years, and it is currently considered to be over-fished," Dr Perry said. "It is not one that is shifting and it could be in danger, depending on how much its habitat is changing."
The scientists say further research is needed, but as a precaution, greater protection should be put in place for stocks already threatened by over fishing.
"This research adds more weight to what scientists are advising in terms of the need to reduce fishing pressure," Dr Perry said.