Cod are doomed to disappear from the North Sea because of climate change and not just as a result of over-fishing, researchers have discovered.

In the past 40 years the average temperature of the North Sea has increased by 1C with catastrophic effects on its delicate eco-systems.

Species of plankton, on which cod larvae feed, have moved away in search of cooler waters. The decline in cod stocks has led to an explosion in the populations of crabs and jellyfish, on which the adult fish feed. The shortage of predators at the top of the food chain has had a knock-on effect on flat fish, such as plaice and sole, whose offspring are eaten by crabs.

The cumulative consequences of warming for the North Sea have been spelt out in detail in the study published yesterday in the Proceedings of the Royal Society Biological Sciences journal.

Richard Kirby, a Royal Society Research Fellow at the University of Plymouth, and Grégory Beaugrand, from the French Centre National de la Recherche Scientifique, warn that stricter quotas or a ban on fishing would not be enough to save the North Sea’s cod. They add, however, that quotas are important to protect those cod that are left for as long as possible.

The researchers studied the distribution of surface-dwelling copepod plankton on which young cod feed. Copepod numbers have declined by more than 60 per cent as the sea has warmed over the past four decades.

Dr Kirkby said: “The plankton that young cod usually eat during March, April and May prefer cold water and so they have become much less frequent as the North Sea has warmed.

“These copepods have moved north by about 1,200km (750 miles), or 30km per year, and the plankton replacing them come later in the year, which is no good for the young cod. The cod will not simply move north to follow the plankton, however, because the water there is too deep.”

Dr Kirby said: “As top predators such as cod are declining, this appears to have had a cascading effect on the whole ecosystem.

“The increase in temperature has affected the whole food chain from the plankton to fish and jellyfish, including animals that live on the sea bed such as crabs, sea urchins, and bivalves such as mussels and scallops.

“Plaice and sole also appear to be declining in abundance. This reveals how changes in fisheries may be related through indirect links in the food web.”

Dr Kirby added: “If the increase in global temperatures projected by the [Intergovernmental Panel on Climate Change] continues, cod will inevitably disappear as a commercial species in the North Sea whatever the reduction in fishing. New areas such as the Barents Sea may become a habitat for cod.

“It is therefore particularly essential to limit fishing mortality to enable the survival of fish such as cod.”

A separate report by Natural England, the Government’s conservation body, suggests that three quarters of people are prepared to pay more for fish that is caught in ways that minimise damage to the environment.

Research has shown that only a quarter of 20 British fish stocks are being harvested sustainably, and that almost a third of the North Sea catch is discarded — thrown back because it is an unwanted species, under size or over the quotas set.

Fishing methods can also affect wildlife in the seas, the report said.

However, some practices in the English fishing industry are environmentally friendly, including handline fishing, which reduces the amount of fish caught by mistake, adapting gear to reduce its impact, and closing fisheries periodically.

Fish caught sustainably should command a premium price to ensure that fishermen get better returns for taking steps to limit damage to the environment, Natural England said yesterday.

It also called for an urgent overhaul of the European Common Fisheries Policy, which governs fishing in European waters and has been criticised for poor management and failure to prevent dwindling fish stocks.

The report called for the policy, which is due to be reformed by 2012, to put environmental sustainability of the seas at its heart, to hand the management of fisheries down from an EU level to regional and local areas and to ensure that the size of the EU fleet matches the available fish stocks.

Helen Phillips, Natural England’s chief executive, said: “We need a radical change of approach to avoid a permanent collapse of marine life around our shores and the end of livelihoods that, for decades, have depended on it.

“We can avoid the bleak future that England’s fishing industry currently faces but we have to accept that far-reaching changes, from policy through to purchase, are now needed.”

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