The return of otters to a Sheffield river once thought to be Europe's most polluted has been hailed as evidence the city is cleaning up its act.
Eurasian otters are thriving along the River Don, according to the results of a new survey, which experts say shows the once-filthy waterway is today in much better health.
The 'Otterly Amazing' project saw teams of specially trained citizen scientists, supported by ecologists, spend 18 months examining the Don for evidence of the 'king of the river'.
They found droppings, footprints and feeding signs at multiple locations along a 24km stretch of the river, as well as capturing shots of the elusive mammals using cameras triggered by heat and movement.
Sara Blackburn, the project coordinator, said: "We were so incredibly excited to record the first video of an otter back in July 2016, and since then we’ve captured these elusive creatures no less than 22 times at numerous location across the city.
"We've also recorded some really interesting behaviour. What is most exciting is to see these amazing animals amongst a backdrop of industrial buildings - it really does show how the river has improved, even in the most industrial areas, and we hope that everyone is as pleased as us to see the return of otters to the Don."
More than 125 verified signs of otters were spotted along the river, including many in very built-up areas.
DNA analysis showed at least seven otters had passed through the study area during 2017.
Evidence was found repeatedly at numerous locations, suggesting the mammals are resident there and not just passing through.
The otter sits at the top of the river food chain, making it a useful indicator of the extent of wildlife supported by waterways.
The UK's otter population nose-dived in the middle of the 20th century due to the use of harmful pesticides, pollution, persecution and the destruction of habitats.
In Sheffield, the booming steel industry made it almost impossible for any wildlife to survive in the Don as factories spewed waste into the river.
The 'Otterly Amazing' project was part of the Nature Counts survey, which was coordinated by Sheffield & Rotherham Wildlife Trust, using a £100,000 Heritage Lottery Fund grant, and involved a range of other organisations within Sheffield.
The wildlife trust and its partners plan to use the findings to inform further conservation work and surveys in the north-west of Sheffield as part of the Sheffield Lakeland Landscape Partnership project.
They have not revealed the locations of the sightings as they fear this could endanger the animals.
You can learn more about otters and the survey's findings at Weston Park Museum's Nature Counts exhibition or by visiting wildsheffield.com/otterly-amazing.