The North Sea is undergoing "ecological meltdown" as a result of global warming, according to startling new research. Scientists say that they are witnessing "a collapse in the system", with devastating implications for fisheries and wildlife.

Record sea temperatures are killing off the plankton on which all life in the sea depends, because they underpin the entire marine food chain. Fish stocks and sea bird populations have slumped.

Scientists at the Sir Alistair Hardy Foundation for Ocean Science in Plymouth, which has been monitoring plankton in the North Sea for over 70 years, say that an unprecedented heating of the waters has driven the cold-water species of this microscopic but vital food hundreds of miles to the north. They have been replaced by smaller, warm-water species that are less nutritious.

"A regime shift has taken place and the whole ecology of the North Sea has changed quite dramatically", says Dr Chris Reid, the foundation's director. "We are seeing a collapse in the system as we knew it. Catches of salmon and cod are already down and we are getting smaller fish.

"We are seeing visual evidence of climate change on a large-scale ecosystem. We are likely to see even greater warming, with temperatures becoming more like those off the Atlantic coast of Spain or further south, bringing a complete change of ecology.

"Some of the colder-water fish species that people like to have with chips are at the southern limit of their range, and if the warming trend continues, cod are likely to become extinct in the North Sea in the next few decades."

This year stocks of young cod were at their lowest for 20 years. The numbers of wild salmon have almost halved over the past two decades and this year the numbers returning to British rivers to spawn fell to a record low. Meanwhile, warm-water fish such as red mullet, horse mackerel, pilchards and squid are becoming increasingly common.

Overfishing has played a part in the decline, but scientists have been surprised to see that stocks have not made their expected recovery after severe cuts in fishing quotas. They say that continued warming will effect all forms of marine life, including seabirds and dolphins.

Research by the Royal Society for the Protection of Birds has established that seabird colonies off the Yorkshire coast and the Shetlands this year suffered their worst breeding season since records began, with many simply abandoning nesting sites.

The society puts it down to a record slump in sand eels, which normally breed in their millions, providing the staple diet for many seabirds and large fish. The eels depend on the plankton that are now being pushed out by the warming waters.

The survey concentrated on kittiwakes, but other species that feed on the eels, including puffins and razorbills, are also known to be seriously affected. Dr Euan Dunn of the RSPB said last week: "We know that sand eel populations fluctuate and you do get bad years. But there is a suggestion that we are getting a series of bad years, and that suggests something more sinister is happening."

He too pointed the finger at global warming and added: "Everything points to the conclusion that there are major ecological changes going on in the North Sea."

Plankton

Microscopic creatures found in their billions in every square foot of sea. As the base of the marine food chain, they are vital to young cod, salmon and sand eels. As North Sea temperatures have risen, cold-water plankton have moved hundreds of miles to the north, disrupting ecology. Warmer-water species tend to be smaller and less nutritious.

Crustaceans

Crab and lobster fisheries are thriving in the warmer water around the UK and on warm-water plankton which have taken the place of cold-water species.

Sea birds

An RSPB survey this summer shows east coast colonies of kittiwakes, guillemots, puffins and razorbills had the worst breeding season on record. Nest counts in east Yorkshire and Shetlands show kittiwakes not laying or hatching eggs because of a severe shortage of their favourite food - sand eels. Some colonies have even been abandoned.

Seals

Populations of common seal were hit in the late Eighties by viral infection. Numbers had almost recovered when they were hit by a second outbreak last winter. Both viral outbreaks coincided with influxes of warm Atlantic water into the North Sea, and some scientists believe that two events might be linked.

Salmon

Numbers estimated to have almost halved in 20 years, and this year adults returning to UK rivers fell to a new low. Studies show salmon are highly dependent on plankton on their journey to feeding grounds in the north Atlantic.

Mediterranean fish

As seas have warmed, large numbers of Mediterranean species, such as red mullet, squid and sardine, have moved into UK waters. Red mullet, popular in Spain and France, are now being caught commercially in the North Sea. In the Channel there are emerging sardine fisheries.

Sand eels

Make up between a third and half of the weight of all fish in the North Sea. Caught in huge quantities by Danish factory ships, which turn them into food pellets for pigs and fish. This summer, the Danish fleet caught only 300,000 tonnes out of its 950,000-tonne quota - a record low.

Cod

Stocks of young cod this year at their lowest for 20 years. Waters around the UK are the southern limit of their range. The International Council for the Exploration of the Seas says numbers are lower than previously thought, and has called for a ban on cod fishing in the North Sea and Irish Sea.

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