Cod and other coldwater fish in the North Sea and North Atlantic could soon be replaced by subtropical marine species such as tuna, sharks and sea horses lured by warmer waters caused by climate change.
One of Britain's leading marine scientists has warned that a minor change in temperature of the seas off the north-west coast of Scotland and the rest of the UK is having a dramatic effect on traditional marine life.
The once-thriving cod stocks, depleted by over-fishing, are being replaced by tuna, red mullet, horse mackerel, pilchard, squid, john dory, sea horses and leatherback turtles.
Martin Angel, a government adviser and chairman of a steering group investigating marine productivity, said close examination of the tiny zooplankton species Calanus finmarchicus, which provides a vital food source for young cod, salmon and other coldwater species, shows it is being driven further north as the seas around Britain warm.
"Calanus finmarchicus used to be extremely abundant in the North Sea," Dr Angel said. "If you dipped a plankton net into the waters off Britain it would make up about 80 per cent of what was caught. But as the seas warm, the whole ecological system is changing; plankton more usually associated with southern waters takes over and the species of fish which feed on it expand their traditional territories."
Dr Angel said the largest change has been in the past 15 years, and that within 10 years tuna could be almost as common in Scottish waters as cod. "These subtropical species have been migrating north at a rate of 50 kilometres a year," said the scientist who wrote the last Oslo and Paris Convention report into the health of the Atlantic and North Sea and who was the UK chairman of the International Year of the Ocean.
"The instance of leatherback turtles finding their way to Britain has increased in the last few years because they feed on jellyfish which thrive in the warmer water. Since the mid-1980s, there has also been a rise in the amount of tuna caught in Scottish waters, especially the smaller skipjack variety. This kind of change is happening with just a minimal warming of sea temperature but it is producing quite enormous environmental changes."
Dr Angel, who is based at the Southampton Oceanography Centre, said that within the next few years sea horses will be a common sight around the Scottish coast as will several species of sharks and john dory, an olive-green spiny fish more usually associated with continental shelf waters from south-east Queensland, south and north-west Western Australia, the western Indian Ocean, eastern Atlantic, Mediterranean and the waters of New Zealand, Japan and Spain.
"The rate by which john dory-like fish are advancing up the coast is incredible," said Dr Angel. "Already they can be found off Wales and within a few years they will undoubtedly be in Scotland." The UK crab and lobster fisheries are thriving as the crustaceans enjoy the warmer waters and other fish species traditionally more popular in Spain and France are being caught in the North Sea.
"A squid fishery is developing off the coast of Aberdeen," a spokeswoman for WWF Scotland said. "There have been reports of squid in Scottish waters for years but now it's actually reaching big enough proportions for fishermen to make money from it. It's not just the temperature of the sea; the chemical balance of the water is also changing and having an effect on the ecological system which is attracting different fish."
The influx of new species is not necessarily good news for the beleaguered fishing industry. Dr Angel said: "It's not just about replacing one species with another as these new species are not as productive as cod. Even if the fishermen could fish as much as they wanted they would never catch as much; there are just not the numbers to sustain that."