Wild Things columnist Eric Brown learns of anti-mink measures introduced at a Bexley nature reserve and recounts his own worrying encounter with the dangerous species.

Nothing could be seen of it except for a pair of large, dark, staring eyes and the top of a black head above water disturbed by its v-shaped bow wave. It moved quickly past my wife and I as we swam in the lake, scrambled from the water and revealed itself as a mink. Then it headed towards a tree where our toddler son was playing with toys. Cue pandemonium. My wife reached our son first and scooped him up away from danger as the startled mink disappeared down a hole in tree roots. Mink, with their razor-sharp teeth and claws, are among the most fearless and devastating small predators around. Larger than a grey squirrel but smaller than an otter, they have voracious appetites and will eat almost anything. One has been photographed killing a grey heron. My sighting occurred in southern Finland around 1980. Within a few years of mink being discovered there the enormous lake had been almost cleared of fish, a local delicacy that sustained generations of people. Next morning I found smoke spiralling up from the tree roots where it vanished and my father-in-law watching over a controlled fire. He later filled in the hole.

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Mink quickly became a pest in the UK too. American mink were imported in large numbers from the 1950s as get-rich-quick-merchants realised there was cash to be made breeding them for the lucrative fur trade. Inevitably some escaped. Other mink were freed by activists who failed to appreciate the consequences. Numbers of water voles, their main prey, have dropped by 97 per cent since the 1970s. Coots, moorhens, ducks, kingfishers etc were also targeted. A gamekeeper complained about mink killing his young pheasants while nesting birds and their eggs were regularly taken. Fur farming became illegal in 2000 but this led to large numbers of mink being freed by unscrupulous individuals suddenly unable to cash in on them. East Anglia suffered particularly badly yet the Waterlife Recovery Trust have just declared the region mink free after years of trapping. Water vole numbers are recovering there as a result.

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Now Belvedere's Crossness Wildlife Reserve is using WRT methods to ensure there is no mink threat to its own water vole population. Mink have become established on the nearby river Darenth. One was spotted at Crossness a few years ago but there is no indication of a current population there. A floating raft mink trap has been deployed with a remote device that sounds telephone alerts when the trap is triggered. Reserve manager Karen Sutton formed an eight-strong squad of volunteers to respond to alerts, check the trap, release any "accidental" inhabitants, re-set the trap and then notify officials. She said: "We are unaware of any mink population at Crossness but want to be sure. We are grateful to the volunteers. They only check during daylight hours as we don't want people stumbling around our pools in the dark.

"Rob Martin, South London Project Officer for Waterlife Recovery Trust, did a superb job supervising the trap installation and volunteer training."