11-year study proves otter numbers are healthy

The Environment Agency Science Department carried out a series of studies into the deaths of almost 1000 otters between 1992 and 2003, which looked at the overall health of otters, factors affecting their survival, the levels of chemicals in otters and the impacts these have on otter populations.

Otter populations have begun to expand again following the decline in numbers across England and Wales between the 1950s and 1980s. The results reinforce the view that one factor behind this recovery may be decreasing levels of organochlorine chemicals (OCs), such as the insecticides dieldrin and aldrin, in the environment.

Conservation Officer Lyn Jenkins, who managed the study into otter health in southern and south west England, said dieldrin was progressively withdrawn from use from 1962 and banned by 1989.

“This was a very potent insecticide, used extensively by farmers, as it remained active for a long time after they applied it. But we now know it can take up to 25 years for 95% of dieldrin in soil to disappear,” Lyn Jenkins said.

“This persistence, and the fact that it passes from animal to animal through the food chain, was the reason it was eventually banned. It had a devastating impact on animals. By interfering with vitamin A levels, it caused reproductive abnormalities and other conditions. Previous research has strongly linked its use to declines in predators such as peregrine falcon, kestrel, sparrowhawk and heron.

“Otter numbers dropped significantly during the late 1950s when dieldrin and aldrin first came into use, especially in the south and east, and it seems otters in this country are only just recovering from the effects.”

Lyn Jenkins said although post mortems revealed a decline in levels of dieldrin in otters over time, this was the first time a link had been made between OC levels in otters and the effects of these chemicals on their physiology.

“In the south west and southern England, almost 30% of otters found between 1996 and 1999 had retinal dysplasia, a distorted retina in the eye linked to low vitamin A metabolism during development, which can make otters almost blind. Concentrations of dieldrin were more than three times higher in otters with this abnormality than in otters with normal eyes,” she said.

“There were numerous stories of ‘blind’ otters being encountered in the 1950s and 1960s, which fits very closely with the widespread use of OCs in the countryside. And we know from the early work in this study that otters with high OC loadings showed this eye disorder.”

The study also found that:

• road traffic accidents are the significant cause of otter mortality, and a major concern is the number of breeding females killed
• most of the otters were in excellent condition, though a small number (particularly orphaned or abandoned young) were suffering from infections, ill health or starvation
• many of the adults had serious bite wounds, almost certainly due to fighting other otters, and some young otters were killed by dogs
• in a small number of otters, some suffered from infections, including lung infections and several tooth abscesses

Lyn Jenkins said there were no population estimates for otters in the UK, but it was unlikely the number of otters killed over the past 15 years was having a significant impact on the population as a whole.

“All we know is that otters lived on all rivers in the UK in the 1920s, and now otter populations have begun to grow and expand again following the decline in numbers between the 1950s and 1980s. It’s also promising that a otter was found on the River Thames last year – not far from Tower Bridge – and it was the first wild otter to be seen in central London for more than a 100 years.

“But we rely on volunteers to help us collate this information. Without the volunteers who report dead otters to us, we wouldn’t be able to undertake this work to increase our knowledge about otter populations in England and Wales,” Lyn Jenkins said.

The series of studies, which are available on the Environment Agency’s website, looked at the results of post mortems carried out on 341 otters found dead in south west and southern England between 1996 and 2003, and on 609 dead otters found in Wales and the rest of England between 1992 and 2003. The post mortems were carried out by Vic Simpson from The Wildlife Veterinary Investigation Centre in Cornwall and Dr Liz Chadwick from Cardiff University.

If anyone finds a dead otter it should be reported to the Environment Agency on 08708 506506.

Otter fact file:
Otters are members of the Mustelid family which includes badgers, polecats, martens, weasels, stoats and mink. There are 13 known species of otter in the world but the only one found in the UK is the European (or Eurasian) otter (Lutra lutra).

Length: Average 1.2m (dogs), 1m (bitches)
Weight: Average 10.3kg (dogs), 7.4kg (bitches)
Colour: Medium to dark brown above and lighter underneath.
Diet: Mainly fish, but also birds, small mammals, amphibians, crustaceans and molluscs.
Breeding: Only once every 2 years as the cubs remain dependent on their mother for a year.
Offspring: 1-4 cubs.
Lifespan: Average 4 years due to so many threats, although they can live 8-12 years.
Habitat: Mainly rivers, but also canals, marshes, small streams, ditches, ponds and lakes. They also inhabit estuaries and coastal areas.
Home range: Depends on food supply but can be from 1km along a rich coast to 40km along some rivers.
Resting sites: Called holts, often tree roots but also drains, caves and holes in rockfalls. They will also rest above ground in vegetation, sometimes called couches.
Social structure: As otters are very territorial they tend to live alone, except during mating and for a period, dependent on the mother, after the cubs are born.
Physical features: Short legs, webbed feet and claws, long stream-lined body, small ears and a broad muzzle, sensitive whiskers around snout to help detect prey, 2 layers of fur – a thick waterproof outer one and a warm inner one.
Special abilities: The European otter has an acute sense of sight, smell and hearing. The eyes are placed high on the head so that it can see when the rest of the body is below water.
Did you know…? Otters can swim at speeds of 12km/hr underwater and can travel for up to 400m before surfacing for air. When it dives, an otter closes its nostrils and ears. Otters are the only truly semi-aquatic members of the Mustelid family.