CLIMATE change, rather than overfishing, is the pricipal cause of falling cod stocks in the North Sea, and conservation measures imposed by the European Union will do nothing to reverse the decline, a new study has revealed.

Changes in the North Atlantic current, caused by global warming, have disrupted supplies of plankton, which are essential for the survival of newly hatched cod.

According to the study by the Natural Environment Research Council, the amount of vital plankton in the North Sea has plummeted by 50%.

Researchers investigating marine productivity say the decline in cod stocks is now irreversible.

“No amount of fishery regulation is going to bring the cod stocks back. The fate of cod is commercial extinction,” said Dr Martin Angel, a government adviser who led the study.

Academics at the Southampton Oceanography Centre, the marine laboratory in Aberdeen and the University of Strathclyde analysed hundreds of plankton samples collected from the North Sea. The plankton, which used to be carried to cod spawning grounds by the North Atlantic current, is now drifting further north.

“Probably the main reason for the lack of success of cod to recover after overfishing is that the ecosystem has changed and the Calanus finmarchicus is no longer in the right place at the right time,” said Angel. “Normally this plankton is carried to cod spawning grounds, but the current has changed and they are now drifting to the southeast of Greenland.

“At this critical period, when cod require to feed on high plankton densities, the plankton they favour is no longer there.”

While Calanus finmarchicus used to account for about 80% of plankton in the North Sea, it now accounts for just 40%. A new species of plankton that thrives in the warmer waters is becoming more common, however it spawns at the wrong time of year for the cod larvae.

Angel believes the changes in the North Atlantic current will have far-reaching implications for the environment.

“The engine of the Atlantic current — the cold, salty water that used to drive it — has been switched off,” he said. “This will have a major impact on global climate because the oceans distribute energy over the surface of the globe. The seas off Britain appear to be getting warmer and the animals are responding to that.”

Dr Richard Dixon, director of WWF Scotland, added: “These findings do look very significant and it may already be too late for some species.

“While we have seen interesting changes with birds moving further north, butterflies changing their distribution and trees flowering at the wrong time of year, this is clear evidence of a fundamental change in the marine ecosystem, which will make a really big difference.

“Half of Scotland’s plants and animals live in the sea and a large part of our diet and economy are based on there being a diversity of life in the oceans.”

Michael Park, chairman of the Scottish White Fish Producer’s Association, said Angel’s findings were supported by the experience of Scottish fishermen.

“At every branch meeting I have attended — which constitute around 70% of the UK catching industry — my members have asked when the water temperature and other environmental issues are going to be taken seriously,” he said.

“There have been big changes. We have a big squid fishery in the northeast, which we never had before, while the pink shrimp has disappeared.

“The North Sea is increasing in temperature by about one degree per decade, which is a sprint in ecological terms. There has been a decrease in plankton since the 1970s, but it has fallen off the edge of the scale recently, and cod, which are dependant on it during the early stages of life, are suffering as a result.

“Meanwhile, the targets we are facing to get the stocks up to recovery are extremely difficult, if not impossible, to achieve.”

Angel’s finding follows recent research by the UK National Oceanography Centre, which found that the Gulf Stream — which carries warm water to Europe from the tropics — has weakened dramatically in recent years.

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