‘Small changes can make a big difference’: how getting farmers, fishers and beavers together could save a Cornish river

Cornwall council organises boat trips on the River Fal as part of its strategy to reduce runoff. Photograph: Luke Gartside/The Guardian

On a rocky beach in Cornwall, a group of wetsuit-clad freedivers and snorkellers are wading into the shallows. Below St Mawes Castle, built by Henry VIII to defend Falmouth harbour, they disappear in a swish of florescent pink and green fins.

They are volunteers, trained by the Cornwall Wildlife Trust to use underwater cameras and floating GPS devices to detect changes in marine habitats over time.