Trout living in rivers polluted by metal from old mines across the British Isles are genetically “isolated” from other trout, new research shows. The work appears in Diversity and Distributions.

Researchers analyzed  at 71 sites in Britain and Ireland, where many rivers contain metal washed out from disused mines.

While trout in metal-polluted rivers appear healthy, they are genetically distinct—and a lack of diversity in these populations makes them vulnerable to future threats. By comparing the DNA of trout in rivers with and without metal pollution, the researchers found that metal-tolerant trout populations split from the wider species during periods of peak mining activity.

The study, by the universities of Exeter and Cardiff, was carried out in four regions: west Wales, northeast England, southwest England and southeast Ireland.