Strangers in the seas
 

Exotic marine species are turning up unexpectedly in the cold waters of the North Atlantic. This may be exciting as a spectacle, says Steve Connor, but it suggests that something sinister is going on

5 August 2002

First there was the flying gurnard, followed a couple of years later by the blue marlin. Two years after that came the first appearance of the sharp-nosed shark and a year after a fine specimen of the big-eye tunny. In more recent years we've seen the short-snouted seahorse, saddled seabream and sailfin dory – to name but a few of the strange fish seen off the coastline of Britain.

Flying gurnards, which have greatly elongated pectoral fins that enable them to move quickly through the water, were previously unknown to British waters before 1980, when the first one was caught in the nets of Cornish fishermen. The same goes for the sharp-nosed shark, the first recorded specimen being caught just south of the Lizard in 1984. During the Eighties, a new species of tropical or semi-tropical fish tended to be recorded once every couple of years but during the Nineties the new arrivals became even more frequent. Towards the end of the decade first-recorded arrivals of new fish species were nearing two every year – culminating in the appearance of both the big-eye thresher shark and the barracuda in 2001.

The past 20 years has seen an ever-more diverse array of exotic fish from warmer, southern waters swimming north into British coastal waters. And it's not just fish. Tropical plankton have been seen moving north, as well as warm-water sea snails and other invertebrates normally seen much further south. Earlier this year an unusual lobster was caught off the Isles of Scilly. The 5in-long (12cm) slipper lobster, Scyllarus arctus, is normally a native of the Mediterranean and only a dozen have been sighted in Britain in 250 years of record keeping. But this specimen, brought up off St Mary's in the Scillies, was the fifth specimen to be caught since 1999.

(It is not just Britain, of course, that is seeing this trend. President Bush was recently photographed landing a huge striped bass that his daughter Jenna had caught on a fishing holiday off the coast of New England. Striped bass is a warmer-water species, which only a generation ago would never have been seen so far north along America's Atlantic coast.)

The question arises as to what is going on? Are these strange foreigners blown off course by storms and strong currents? Are they the result of a statistical blip caused by better record keeping? Or is their appearance an indication of something more sinister – namely a genuine change in the temperature of the seas caused by global warming? Some scientists believe it is the latter and think they now have more than a smoking gun to show it.

The first systematic study of marine records dating back 40 years points to a strong link between the northward migration of fish and rising sea temperatures. A team of marine biologists has for the first time linked the arrival of tropical and semi-tropical fish off the coast of Cornwall – the southern-most tip of the British mainland – to increases in the average temperature of the North Atlantic Ocean.

Studying records dating back to 1960, the scientists found that more exotic species of fish are being caught or washed ashore now than ever before and that the sightings can be directly linked to a corresponding rise in sea temperatures. The link is a "significant correlation" and could explain why Cornwall in particular has seen so many exotic species of marine wildlife coming from warmer regions of the world, says Tony Stebbing, a recently retired biologist from the Plymouth Marine Laboratory, who was funded by the Natural Environment Research Council.

"As the world warms, the only way for wildlife species to live in the temperature they prefer is to move their ranges slowly poleward," says Stebbing, the lead author of the study published in the Journal of the Marine Biological Association. This is especially true for fish, a cold-blooded animal that has to rely on the outside heat of its watery environment to control its own body temperature.

"Fish are good indicators of temperature change because they are unable to regulate their temperature independently of the surrounding water. They, therefore, swim to keep themselves in waters of their preferred temperature range. Not only are changes in fish distribution likely to reflect temperature increases, but the arrival of new fish species are well monitored by fishermen, as well as scientists," Stebbing says.

Climate modelling and temperature records suggest that species would have to move north at a rate of between 50km and 80km (31-50miles) per decade in order to maintain a constant average temperature of their surrounding environment. The Cornish peninsula would be the first bit of Britain to detect this. As Stebbing says: "It is as though Cornwall is at the bow of ship which is slowly steaming south at a speed of between 50km and 80km a decade."

Trawling through the archives of the Cornwall Biological Records Unit, the researchers – Stella Turk, Alwynne Wheeler and Bob Clarke – attempted to document the first sightings of warm-water fish since 1960. The team decided to concentrate on appearances that were made within 12 miles of the coastline, as recorded by the Environmental Records in Cornwall and the Isles of Scilly Automated (Erica) database. The records revealed that, in fact, it wasn't until about 1980 when the first new fish appeared – it was the famous flying gurnard. Over the past 20 years, 17 more were recorded, including the Barracuda caught six miles off the Lizard.

At the same time, the team looked at temperature records of the North Atlantic compiled by American scientists to try to see if there was any correlation. What was intriguing was that it took until the end of the Seventies for average sea temperatures to rise consistently above the average for the period, and that the rise in temperature over the past 20 years or so has been accelerating, just like the accelerating appearance of new species of fish off Britain.

Oceans in general have warmed by an average of 0.06C over the past 40 years but the surface water, where many fish live, has risen faster, by up to 0.31C. The North Atlantic in particular is warming faster than any other ocean with a temperature increase of 0.5C over the past 20 years – a rise that is increasing at an accelerating rate.

It is now understood that something like 90 per cent of the heat generated by global warming is locked up in the oceans. The increase in temperature this creates is not always equally distributed. Surface waters are warmer, presumably because this is where the exchange of heat with the atmosphere takes place.

However, deeper water is not immune to even bigger temperature rises. Stebbing points out that the shelf-edge current, which flows from Spain to the Shetlands at a depth of between 200 metres and 600 metres (656ft-2,000ft), has warmed by 2C between 1972 and 1992. Because this current runs north at a speed of about 35 miles a day, it is the most likely route for some tropical and semi-tropical species to find their way to Britain. Fishy visitors from foreign shores may be interesting and welcome spectacles but there is a serious side to this migration polewards. As the new immigrants move in, some of our native species are moving north.

Cod, which is at the southerly most reach of its range in Cornwall, is particularly vulnerable to rising sea temperatures. In the past, as cod have dwindled, it has been difficult to tease out the reason why. Over fishing has evidently played a major part, but these latest findings suggest a more complicated picture, with rising sea temperatures exacerbating an already difficult habitat problem for the nation's favourite fish dish.

"Whatever the mechanism of the northward shift in the distribution of fish in European waters, the clear trends suggest they are doing so in a way that makes future changes predictable," Stebbing says. "They may help to forecast changes in the future distribution of commercial species due to climate change. It seems likely that in time our fishing fleets will have to go further north to make commercial catches, while fish farming in our native waters will be able to grow more sub-tropical species."

What began with the flying gurnard may end with the disappearance of the cod and the many other native fish of the British Isles, which can no longer take the heat of a warmer world.