An unexpected decline in the otter population in Welsh rivers has led to warnings against complacency over conservation efforts.

The first Welsh survey in more than a decade found a 22 per cent drop in signs of the otter, with the worst affected regions being the Teifi, Loughor, and Conwy catchments.

After its rare success story and “remarkable comeback” in the 1970s, the latest survey has been described as a “wake-up call” over the river pollution.

After being wiped out in parts of the UK in the 1950s and 60s, the species had been recovering, and was considered a rare success story.

Natural Resources Wales, Cardiff University and a host of volunteers have carried out the Welsh National Survey for Otters for the first time since 2010.

Using the same methods as previous surveys to ensure results were comparable, a total of 1,073 sites were visited, with signs of otters found at 756 sites.

It showed a substantive decline in their populations for the first time since the 1970s, from around 90 per cent occupancy in 2010 to 70 per cent in 2015 to 2018.

Reasons for the decline are unclear and further work is planned by NRW and Cardiff to investigate likely reasons

Changes in habitat quality, availability of prey and pollution would all be considered.

Liz Halliwell, team leader for terrestrial ecosystems and species at NRW said: “Monitoring otter population status is important with respect to conservation of this much-loved mammal.

“As well as this, as top predator of our freshwaters, the otter can be an important biological indicator of the health of our rivers and wetlands.

“In Wales as in much of the UK, the otter is a largely nocturnal animal and is rarely observed in the wild.

“But it is possible to detect its presence by searching for its distinctive droppings – spraints – and footprints.

“Otter populations across Britain have been gradually recovering from significant declines in the 1970s.

“The clear message from this report is that we cannot be complacent about the ongoing recovery of the otter in the UK.

“To understand the reasons for the decline, we are working with otter and freshwater habitat experts to review the situation.

“We also have an extensive river restoration programme in development which will bring benefits to many riparian species including otters.”