ONE of Scotland’s rarest seabirds is making a dramatic comeback in its colonies bordering the North Sea because of climate change.
Numbers of lesser black-backed gulls at some coastal colonies on the east coast have doubled, and scientists believe there is a direct link between the boom in chicks and an explosion in the number of swimming crabs that form a major part of their diet.
Unlike the scavenging gulls which make life an increasing misery for the residents of coastal towns and villages, the lesser black-backed gulls get most of their food at sea. Now they are feasting on a proliferation of native crabs and newer species in more northerly oceans due to a 1C rise in sea temperatures, according to marine scientists.
The research has been carried out by an international team and has been published in the latest edition of the Royal Society journal Biology Letters.
Dr Richard Kirby, from the Marine Institute at Plymouth University, explained that the increase in gull numbers was part of a “cascade” of effects on the food chain in the North Sea since the temperature of the waters began rising more than three decades ago.
His team had uncovered evidence of a substantial rise in the numbers of swimming crabs already native to the North Sea and had also identified the arrival of a new warm water species – the Henslow’s swimming crab – in the area. The crab, once only found in the Bay of Biscay and the English Channel, has spread as far north as the Moray Firth.
“The North Sea has warmed by a degree since the 1980s and, as a result, the numbers of crabs have increased,” Kirby said. “And the species of crab that has increased the most are swimming crabs. While species that already occurred in the North Sea have increased in abundance, a warm water species – the Henslow’s swimming crab – has also invaded the North Sea.
“We first noticed the crab three or four years ago and now it can be found throughout the North Sea as far north as the waters off the north-east of Scotland.”
The lesser black-backed gull is slightly smaller than a herring gull, with a dark grey to black back and wings, a yellow bill and yellow legs. More than half the UK population is found at fewer than 10 sites.
There are an estimated 112,000 breeding pairs in the UK, but the scientists believe there is a direct link between the rise in crab numbers and an increase in populations of the lesser black backed gulls in several countries surrounding the North Sea.
In Scotland, colonies in the Moray Firth and Scottish Borders areas have doubled.
Kirby said: “The lesser black-backed gulls aren’t particularly numerous – it’s on the amber list of UK birds as it is considered a rare species – but the UK has about 40 per cent of the global population.
“But the lesser black-backed gulls eat swimming crabs, which spend quite a portion of their adult life at the surface. And when they are they are on the surface they are a food source for the lesser black-backed gulls, which bring back the crabs to feed to their chicks.”
“We have shown that an increase in the larvae of swimming crabs in plankton is followed by an increase in adult crabs the following year. And this in turn is followed three to four years later by an increase in the number of breeding pairs of lesser black-backed gulls.”
The scientists also believe that the decline in cod numbers in the North Sea may be a factor. Cod also have a fondness for crabs and the decline in predation may also have helped the crustacean populations to boom.
The researchers have also raised the possibility that the increased crab diet may be affecting the ecology of the coastal areas the gulls have colonised by improving the fertility of the land.
Although welcoming the rise in gull numbers, James Reynolds, a spokesman for RSPB Scotland, urged caution. He said: “There may appear to be some winners in our changing climate, like swimming crabs, but the bigger picture for the marine environment is certainly one for concern.
“The biomass of zooplankton – the tiny creatures which fuel our seas – in the North Sea has declined by over 70 per cent since the 1960s, and the knock-on effects as the food chain breaks down have been disastrous for many of Scotland’s seabirds.
“Early indications from this year’s breeding show that kittiwakes and Arctic terns remain in real danger, with declines in some colonies and across the country.”