Otter populations underwent a dramatic decline between the 1950s and 1970s, shrinking to a few strongholds in south-west England and west Wales.
But collated results from post-mortems of more than 1,000 otters found dead, mainly from road accidents, since 1988 indicate that populations are healthy and regaining ground.1Otters are an important indicator of river health. As a top predator they are particularly vulnerable to persistent pollutants like the pesticide DDT which bioaccumulate up the food chain.
Most of the otters tested had traces of DDT derivatives, the pesticide dieldrin and polychlorinated biphenyls (PCBs). But contamination patterns were complex, varying by age, sex and region.
Figures from across England and Wales suggest concentrations of organochlorine pesticides are now considerably lower than in the 1960s and 1970s but may have stabilised in some areas. However, there was little evidence that PCB levels had declined, and animals in the north had the highest levels.
A more detailed study of animals from the south and south-west, regions dominated by agriculture, showed a clear and continuing decline in pesticide concentrations. The authors believe the otter’s recovery in these areas is the result of organochlorine pesticide bans introduced in the 1980s.
Laboratory studies have shown organochlorines interfere with vitamin A metabolism. Almost half of the 100 otters tested for vitamin A had abnormally low levels. Low vitamin A can in turn affect eye development. Distorted retinas were found in about 30% of otters from the south and south-west.
Links were also found between organochlorine concentrations and sexual development in young males.
A host of possible explanations have been put forward for the decline in otter populations in the past, including chemical contamination, changes in land use and river modification. Previous studies on the Clyde suggested PCBs from marine sewage sludge dumping and industrial discharges were high enough to disrupt reproduction (ENDS Report 214, pp 8-9 ).