Why global warming puts bib on the menu

Nearly two-thirds of fish species in the North Sea have moved further north in search of colder waters because global warming is driving sea temperatures higher.

Nearly two-thirds of fish species in the North Sea have moved further north in search of colder waters because global warming is driving sea temperatures higher.

Scientists have compiled the first unequivocal evidence linking a major northward shift of North Sea fish species with rising ocean temperatures.

The researchers believe the movement is more dramatic than the simple migration of individual fish and represents a fundamental change in the distribution of marine species.

A study that covers 25 years of data has found the range of nearly two-thirds of North Sea species - including commercially important fish such as cod and haddock - have shifted either further north or to colder depths.

As cold-water fish have gone north, exotic warmer-water species such as the bib, scaldfish and lesser weever have extended their range by moving into the North Sea from the south, said Alison Perry, a marine biologist at the University of East Anglia in Norwich.

If trends continue, then Atlantic cod will no longer be able to live in the warm waters of the North Sea by 2080 and its habitat will be totally occupied by the southerly bib, Dr Perry said.

"This is not just a case of individual fish choosing to move into colder waters. It points towards an entire population of fish becoming less viable in response to warming," Dr Perry said.

"It's not just about fish migrating, it's about seeing a whole range of long-term responses to rising sea temperatures over the past 25 years," she said.

Between 1962 and 2001, the average temperature of the North Sea increased by 0.6C. During that period, the world experienced the warmest years on record, which many climatologists have linked to man-made pollution.

The scientists, who included experts from the Centre for Environment, Fisheries and Aquaculture Sciences in Lowestoft, studied the distribution of more than 36 fish species and how each has changed over the past quarter of a century.

They found, for instance, that the centres of population for 15 of 36 fish species had shifted. Atlantic cod for instance has moved north by 73 miles and haddock's southern boundary had shifted north by 65 miles.

Other species such as the monkfish have also migrated north. The witch has shifted its southerly boundary further north by 104 miles and the population centre of the snake blenny has moved north 250 miles.

The study, published in the journal Science, found that, on average, the rate of movement north was about 1.4 miles per year, four times faster than the northwards movement of land species affected by climate change such as butterflies, birds and alpine plants.

Dr Perry said: "We were very struck by the extent and scale of these shifts.

"This could have significant impacts on an ecosystem which is already under heavy pressure [from overfishing]," she added.

The scientists also found the species that moved the most tended to be those with a faster life cycle and smaller size, indicating the migration was a response to higher temperatures, which can affect growth and reproduction. If the differences in the rates of movement between the species are due to shifts in the growth of fish populations then those fish with a faster population turnover would be expected to respond most strongly to climate change, the scientists said.

Climatologists predict the average surface temperatures of the North Sea will continue to rise by between 0.5C and 1C by 2020; by between 1C and 2.5C by 2050; and up to 4C by 2080. "We'd expect to see the changes in fish distribution continue with more southerly species in the North Sea and some of the northern ones retracting," Dr Perry said. The North Sea population of blue whiting and redfish will be eradicated completely by 2050 under such scenarios.

"We need to be more precautionary in terms of putting pressure on existing fishing stocks," she said.