Mink numbers drop as the otter bites back

The American mink, one of the countryside's most ruthless killers, has finally met its match. Across Britain, mink numbers are plummeting - thanks to a resurgence in native otter populations.

Research by zoologists at the University of Oxford has found that, for the first time in 40 years, otters are displacing American mink along river banks across the country. In rare cases, otters are even killing mink in direct combat. And the decline is dramatic, with mink numbers falling in the Home Counties by as much as 50 per cent. From a nationwide peak of 110,000 mink in the 1980s, mink numbers could now be as low as 40,000.

The new findings represent a remarkable reversal of fortunes for both species. After escaping from fur farms in the late 1950s, mink spread throughout Britain and earned the reputation as one of the country's most reviled species. Widely blamed for decimating Britain's water vole population, mink were able to exploit a collapse in otter numbers in the 1950s and 1960s. The otter was almost wiped out in England by the build-up of toxic organo-phosphate pesticidesin the food chain.

But now, thanks to tougher water purity and pesticide regulations, and Government-funded reintroduction projects, otters are getting the upper hand. Otter populations today are up to about 4,000 and the much larger animal is out-competing mink for food and territory. In one incident, a naturalist in Northumbria witnessed an otter attacking and killing a mink.

One project in the upper Thames area, covering rivers such as the Colne, Thames, Windrush, Churn and Leach, tracked mink numbers after otters were reintroduced there in 1998. Mink numbers initially fell by 75 per cent. They recovered slightly but by 2002, they were still living in only half their previous range. Similar results were found in south-west England.

Laura Bonesi, the zoologist who conducted the research with Oxford university's Wildlife Conservation Research Unit, has also analysed statistics from the last national otter survey. Overseen by the Environment Agency, the survey uncovered a 55 per cent rise in otter numbers since 1994 and a 530 per cent rise since 1979.

In an as yet unpublished study, Ms Bonesi estimates that the rise in otter numbers is largely responsible for the slump in the mink population. "It appears that mink are declining, especially in western areas of the country, and this is strongly correlated with an increase in otters in those areas," she said. "My prediction is that as otters recolonise the UK, mink numbers will fall even further." And as otters have only recaptured a third of their potential territories, the full impact on mink has not yet been felt

In addition to being displaced by otters, mink are also being trapped by naturalists trying to save the water vole from extinction - particularly in counties such as Somerset, Dorset and Wiltshire.

Ms Bonesi said this transformation was a telling example of the way man damaged the environment. "We introduced pollutants which made the otter population go down, and then introduced mink because we wanted fur. When we do these things, we don't realise how far-reaching the effects are. There is a chain reaction."

TALES OF THE RIVERBANK

Otter

Scientific name: Lutra lutra

Length: Up to 1.2m

Weight: Up to 10kg

Physique: Muscular and powerful with short legs and clawed feet. Two layers of fur. Acute sense of sight, smell and hearing.

Food: Mainly fish, but birds, small mammals, frogs and shellfish

Lifespan: Four years on average, but up to 12 years

Numbers: Roughly 4,000. Now recolonising areas across Britain after catastrophic decline in 1950s and 1960s.

American Mink

Scientific name: Mustela vison

Length: 30-40cm

Weight: Up to 1.5kg

Physique: Agile, quick swimmers and climbers. Slim, with poor eyesight but acute sense of smell. Ruthless but opportunistic predators. Aggressive and highly territorial.

Food: Small fish, water voles, bird eggs, rabbits, shellfish, frogs, rats and small birds

Lifespan: Up to 10 years

Numbers: About 40,000. Found across Britain, after first breeding in Devon in 1957.