Scientists at Cardiff university have told how post-mortem examinations on almost 1,000 otters have helped them study their diet and environment.
The field centre in is Newbridge-on-Wye , near Llandrindod Wells
Carcasses, usually spotted by the public, have been collected over the course of the 13-year otter project.
Mainly roadkill from across Wales and England, they are examined in Cardiff and at a field centre in Powys.
One finding is a drop in lead levels in otters since the early 1990s which reflects the ban on lead in car fuel.
Project manager Dr Liz Chadwick, said post-mortem examinations were a key way to learn about otters as they are so elusive.
They had received 962 dead otters since the project began, most of which are initially reported by a member of the public, she said.
The carcasses are sent in frozen by organisations such as the Environment Agency, the Countryside Council for Wales or the Wildlife Trust.
The alternative to post-mortem examinations, is looking for otter "spraint" (faeces) along the river bank, explained Dr Chadwick.
"They are very difficult species to study, they are quite elusive. Very few people see them in the wild alive."
She added: "If you are surveying them by looking at poo on rocks, it doesn't tell you a great deal about otters that way.
"With carcasses, it's unfortunate they have been killed by a car but you can get a whole range of extra information.
"Their stomach content tells you what they've been feeding on, we test the liver for pesticides and other pollutants."
Examining dead otters is a useful way of monitoring river pollution levels because pollutants accumulate up the food chain and they are at the top.
The team have recently discovered otters also eat worms and slugs
It appears that the otter population is on the increase, said Dr Chadwick - one sign of this was the higher number of carcasses being sent in.
She said: "We do have to treat that with caution because there are more cars on the road now and more people are aware of the project. But it ties in with survey data."
Wales had always remained a "stronghold" for otters, said Dr Chadwick, explaining: "There are so many rivers, good quality rivers with good fish stocks that are not too polluted."
However, they are now receiving dead otters from more areas than before, including middle England where they had largely disappeared.
"It looks like they are spreading back into areas where they had become extinct," said Dr Chadwick.
If several otters have been killed on the same road, the team can take steps to get ledges built underneath the highway to provide them with a safer route.
Another element of their work involves analysing bone samples to monitor lead levels.
Since the early 1990s, they have noticed a significant drop in lead levels found in otters which reflects the ban of lead in car fuel.
In recent months, the team have also started using DNA profiling to get a more accurate picture of the otters' diet.
Traditional methods had relied on finding hard remains, such as bone, in otter faeces.
Dr Chadwick explained: "We can investigate some of the prey items you would not find using other ways.
"We've found good evidence they are feeding on slugs and worms, which is new information and quite exciting as it's not been found before because they don't have bones."