The otter casualty collection is part of a national programme

The Wildlife Veterinary Investigation Centre say traffic is a major threat to the otter population.

Spokesman Vic Simpson said there was a "dramatic decline" in numbers of otter road deaths last year, which are used to determine the total population.

Post-mortem tests are held to determine problems with an otter's health and can indicate the state of rivers.

New threats

Cornwall and Devon are recognised as the main strongholds for otters in England, with the majority of casualties collected from the region.

In the last decade, the numbers from Somerset and Dorset have increased.

Mr Simpson said: "At this time of the year we are normally inundated with otter corpses from the South West.

There has been a dramatic decline in the number of casualties collected Wildlife Veterinary Investigation Centre

"The high number of road kills we have, indicates there is a healthy, expanding population.

"However, this year there has been a dramatic decline in the number of casualties collected and we don't know why."

He said that as otter numbers increased, they spread out, looking for new territories which meant crossing more roads.

Lyn Jenkins, from the Environment Agency, said: "We monitor the level of man-made chemicals that accumulate in otters and this is an important indicator of the health of our rivers."

The Wildlife Veterinary Investigation Centre was set up in Chacewater in 2001, in partnership with the Cornwall Wildlife Trust.