A national endangered species charity has called on wildlife enthusiasts to help monitor a declining animal population as part of an annual survey.

The People's Trust for Endangered Species (PTES) has launched the survey to monitor water vole numbers as part of the annual National Water Vole Monitoring Programme (NWVMP).

Water voles were once a common sight on UK riverbanks and waterways, but have faced the most severe decline of any wild mammal in the UK over the last century, due to threats from the American Mink, habitat loss, agricultural infestation and river pollution.

The survey, which was launched in 2015, will take place from Sunday, April 15 until Friday, June 15.

Emily Thomas, key species monitoring and data officer at PTES, said: "Volunteers are crucial to helping us collect robust data about the state of our water voles across the UK.

"We use the data gathered to monitor population trends year on year which in turn help guide our conservation efforts and inform us where action is needed most."

Since 2015, the survey has seen more than two hundred volunteers take part, with volunteers collecting data from 222 sites across England, Scotland and Wales last year, of which 82 had water vole signs present (48 per cent of sites surveyed).

The distribution of positive sites is encouraging, however there are gaps in survey areas where PTES needs more help, including the South West and Dorset.

Volunteers are invited to survey one of the 900 pre-selected sites across the UK, record all sightings and signs of water voles along a 500 meter length of riverbank once during the course of the two-month period.

Sites that are being surveyed can be registered with the NWVMP.

Though no prior experience is needed, volunteers will need to learn how to identify water vole field signs.

This information is provided by PTES in the form of a survey pack, which includes clear instructions on how to carry out your survey and a field signs ID guide.

To find out more, or to take part in PTES's 2018 National Water Vole Monitoring Programme, visit ptes.org/watervoles