Possible Reason for Otters' Decline
08 July 2004
Otters began to decline in numbers soon after organochlorine pesticides like DDT were introduced to control agricultural pests. These long lasting compounds were later found to have chronic toxic effects on species at the top of the food chain, such as birds of prey. So scientists have long suspected that otters were affected in the same way - but to date there has been no evidence that these pollutants cause any pathological damage to otters at the levels found in the environment.
However, in The Veterinary Record (July 10, pp 52-56), Dr David Williams of the University of Cambridge describes malformations in the eyes of free-ranging otters that correlate closely with levels of one particular organochlorine pesticide, dieldrin.
Dieldrin and related compounds were gradually withdrawn from the market in the US and Europe during the 1980s in the face of mounting evidence of their long-term environmental effects. Its use was finally banned completely in Britain in 1989 but, like other organochlorine pesticides, it is still used in many developing countries.
Most of the 88 otters in the study were animals that were found dead in Cornwall and Devon. They were collected between 1990 and 2000 and, in the majority of cases, the animals had been killed in road traffic accidents.
Williams carried out post mortem examinations of their eyes and found that a third of them had abnormalities of the retina similar to those that will occur in an animal that receives insufficient amounts of vitamin A during development. Dieldrin is known to affect vitamin A metabolism by blocking its transport around the body and increasing the rate at which it is excreted. Sure enough, analyses of liver samples from the affected animals showed that they had abnormally low levels of vitamin A and their dieldrin concentrations were more than three times higher than in healthy animals.
"In the uterus and in the days immediately after they are born, the cubs are growing like crazy and need a great deal of vitamin A for their development. Vitamin A is a fat-soluble vitamin and the cubs will get most of that in their mother's colostrum. The problem is that dieldrin is also fat soluble and so it too gets concentrated in the milk," said Vic Simpson, of the Wildlife Veterinary Investigation Centre in Truro and co-author of the paper.
The fact that the otters in the study had survived to adulthood implies that their eye abnormalities were not severe enough to prevent them from catching fish. But Simpson believes that the extent of the damage caused to individual animals by dieldrin has declined since the ban. The compound has a 'half life' of between 5 and 7 years depending on environmental conditions and so it degrades much faster than compounds like DDT which takes over 50 years to break down to half its original concentration. The reduction in environmental contamination with the compound has coincided with a resurgence in the fortunes of British otters with their numbers increasing significantly over much of their former range.
Notes for Editors:
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Vic Simpson can be contacted on 01872 560623.
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For further information please contact Chrissie Nicholls or Helena Cotton in the BVA Press Office on 020 7636 6541 or email chrissien@bva.co.uk.