Current trends

As Britain's water temperatures increase, southern fish species such as red mullet, anchovies and sardines are extending their range north. But does that spell the end of the coldwater species? Zoe Corbyn reports

It is 8am at the Plymouth fish market in Devon, and Doug Herdson is on his weekly round, dodging forklift trucks to peer into the crates that make up the day's landing. He spies a crate of red mullet. "They've got bigger," he says. "They've got more common, and they are here not just in summer but all year round."

Herdson manages the UK fish recording scheme at the National Marine Aquarium at Plymouth. For nearly a decade it has collected fishermen's and anglers' observations of unusual visitors to our waters. And as temperatures warm with climate change - average annual sea surface temperatures around Europe are the warmest they have been since the measurements began in the 1860s - the data is beginning to show not just the occasional unusual find, but pronounced trends.

"Particularly the triggerfish are getting much further north," Herdson says, citing recordings in Scotland. "And off the south-west, the sea bream that used to be rare are now getting common. At least two breeding areas [for Couch's sea bream] are now known in Devon and Cornwall."

Herdson calls them "range extension" species. "They used to be off north Africa, and the coast of Spain and Portugal, but just as things are changing, they are spreading," he says.

It was in the mid-1990s that the increases became most noticeable, but Herdson is yet to see corresponding declines of cold water species or retreats northward. Hence the fish market visits. "That's the difficult thing - everyone spots something new, but few people notice when things disappear," he says.

Dave Cuthbert, a net fisherman for nearly 30 years and secretary of the Plymouth Fishermen's Association, agrees. "There's a lot more triggerfish turning up in crabbing pots," he says. "And there's an enormous increase in gilthead sea bream." Both he and Herdson stress that the area has always attracted bizarre fish.

The database may not be the most accurate tool, as trends are masked both by increased reporting and reductions as people stop noticing species they see regularly. But scientists are increasingly documenting changes in the UK's marine life. Water temperatures in the region have risen 0.2C to 0.6C per decade over the last 30 years - faster than in other parts of the world.

"We have the equivalent of a shift [in the ecosystem] of 100 miles towards the north," says Alan Southward, an ecologist at the Marine Biological Association (MBA) in Plymouth. He says he has seen dramatic changes in the relative abundance of warm water and cold water species of plankton, barnacles and fish. Some species have colonised new territory as a result.

Biggest shift

Martin Edwards, from the Sir Alister Hardy Foundation for Ocean Science, says warm water plankton have shifted 1,000km north over the last 40 years. It is the biggest recorded shift - terrestrial or marine.

Keith Brander, from the Copenhagen-based International Council for the Exploration of the Sea, says there is a lot of evidence from all kinds of species of fish. "From Portugal up to northern Norway, it is pretty consistent and pretty fast," he says. Red mullet and bass have extended their ranges northward to western Norway, while dory have moved up the west coast of Ireland.

Mike Heath, a scientist at the Fisheries Research Services (FRS) in Aberdeen, says sardines and anchovies have moved into the southern North Sea. "There were none there 30 years ago," he says.

A paper published in the journal Science in 2005 showed that the populations of nearly two-thirds of North Sea fish species had now shifted their distributions north and the range margins of half have moved one way or another, with cod retreating north, although debate rages about whether the study adequately accounted for over-fishing and the existence of multiple populations.

Yet river fish have not changed their ranges. "It is so much more difficult for the fish to move into new areas," says John Armstrong, group leader of freshwater ecology at the FRS freshwater laboratory in Pitlochry, though he adds that fewer salmon seem to be returning every year to rivers for spawning, which may be linked to them straying further north in the ocean.

As to how a fish move, while it is easy to picture schools swimming to warmer waters, this does not necessarily seem to be the case. More likely, explains Heath, they are transported earlier: "Some species will be carried as eggs and larvae northward with the water currents - and if the temperate and plankton conditions are suddenly more appropriate for them, more of them will survive."

But to an extent, we have been here before. European waters were also warm in the 1950s, giving valuable insight into the ecosystem's responses. "Between the wars there was a warming of the sea and increase in southern species of fish and plankton," explains Southward, which he attributes to more solar radiation from "sunspot activity". "This phase reached a peak in 1959 and there was marked cooling after that."

Since the 1980s, the current warming has, however, been exponential - and likely to be a combination of both natural decadal influences and a human induced trend. "Without the latter, things would much cooler," says Southward.

As for the future, the gradual decline in the typical colder water fish such as cod is expected, as well as increases in the more southern species: red mullet, anchovies and sardines.

But according to Heath, it will not necessarily mean a reduction in the overall amount of fish in the system: "It is just that there will be a different type of fish that may be more or less commercially attractive," he says, emphasising that fishing will drive changes too.

But changing stocks can also have knock-on effects, for example for seabirds. Edwards speaks of a current reduction in the abundance of sandeel fish linked to climate change: "[This means] the chick survival success of puffins drops ... There is nothing to replace their food source."

Local changes

Meanwhile, MBA scientists are also observing local changes. Sampling the same piece of ocean bed since 1913, their data show marked increases over the past 20 years in some smaller non-commercial southerly distributed fish - including poor cod, dragonet and lesser spotted dogfish. "The species are comparatively abundant today and also during the warm period in the 1950s," says Martin Genner, a scientist working on the project with a colleague, David Sims.

As warming continues, catches and menus will likely increasingly resemble those hundreds of miles to the south. Such large-scale trends are easy to predict, but the details of how this will affect marine ecology are far from clear. The ocean is a big place. "If you pulled the plug on the English Channel it would be a very interesting experiment," says Sims.

Bitter predicament

The oceans are not only becoming warmer, they are also becoming more acidic. The world's seas are reckoned to have absorbed about a third of all carbon dioxide spewed into the atmosphere by human activity since the industrial revolution, and scientists are now starting to notice the effects.

Carbon dioxide dissolves in sea water to form carbon acid and researchers think the overall pH, or measure of acidity of the oceans has changed by 0.1 in that time. The nature of the pH scale makes this a much larger change than it first appears: equivalent to about a 30% increase in hydrogen ions.

What could the effects be of acidifying seas if global emissions of CO2 continue to rise? Scientists cannot be sure, but if it continues they expect it to be bad news for marine life, especially those such as corals and shellfish that rely on alkaline conditions for the calcium they build into their shells and skeletons. Plankton - tiny plants and animals that live in the ocean - are among the creatures that could be most harmed by the change.

A mass extinction of huge numbers of deep-sea creatures around 55m years ago was caused by ocean acidification after the sudden natural release of massive amounts of carbon.

In 2005, the Royal Society published a study of the possible effects. It said marine ecosystems, especially coral reefs, are likely to be badly affected, with fishing and tourist industries based around reefs losing billions of dollars.
David Adam


· The article above was amended on Thursday July 19 2007. We inadvertently placed Plymouth in Cornwall. It is in Devon. This has now been corrected.