Otter populations are the healthiest they've been in nearly a century and the animals are continuing to expand across England and Wales, a new study says.
Scientists from the Environment Agency carried out post mortems on almost 1000 otters between 1992 and 2003.
Results show that the banning of pesticides like dieldrin and aldrin have played a major role in their recovery.
Conservation Officer Lyn Jenkins, who managed the study, said: "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.
"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."
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.
The study also found that road accidents are the biggest killer of otters. While the animals can live for up to 12 years, the average lifespan is four.
"Otters lived on all rivers in the UK in the 1920s," Jenkins added. "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.
"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."
Anyone who finds a dead otter it should be reported to the Environment Agency on 08708 506506.