Trout living in rivers polluted by metal from old mines across the British Isles are genetically isolated, according to new research conducted by the universities of Exeter and Cardiff. The study analysed brown trout at 71 sites in Britain and Ireland, where many rivers contain metal washed out from disused mines. Although trout in metal-polluted rivers appear healthy, they are genetically distinct from other trout populations, which makes them vulnerable to future threats. Metal-tolerant trout populations split from the wider species during periods of peak mining activity, leading to reduced genetic diversity in these populations.
The study focused on four regions: west Wales, northeast England, southwest England, and southeast Ireland. Professor Jamie Stevens from the University of Exeter explained that trout inhabiting metal-impacted waters carry a high burden of metals, which can lead to suffocation. Trout that can tolerate these conditions have emerged in polluted rivers, becoming genetically isolated from other trout populations. This lack of gene exchange with other populations leaves the metal-tolerant fish vulnerable to future changes and threats, as their genetic diversity has been significantly reduced.
Researchers found that common metal pollutants in former mining areas include lead and copper, which have already wiped out many fish and other species in affected rivers. The emergence of metal-resistant trout, with high genetic differentiation from neighbouring populations and reduced diversity, was observed in polluted rivers in Cornwall, Wales, and northeast England. Dr. Daniel Osmond of the Westcountry Rivers Trust, who conducted the research as part of his PhD, noted that the selection pressure caused by the ability to withstand metal pollution has driven the evolution of these metal-tolerant trout populations.
The study highlighted that rivers polluted with metal and affected by physical barriers, such as dams and weirs, showed the highest levels of genetic isolation among trout populations. Metal pollution levels fluctuate, often rising during periods of high rainfall, causing further stress on fish living in these environments. The polluted landscapes in areas such as West Cornwall, which are filled with old mines, can lead to mine water wash-out that flows into rivers during rainy periods. This has contributed to the decline of freshwater migratory fish populations, including trout, in many areas.
While trout are not currently considered a threatened species, the research suggests that they may be more vulnerable than previously believed due to genetic isolation and reduced diversity in populations living in metal-polluted rivers. The researchers recommend strategies such as reducing mine water wash-out and removing physical barriers in rivers to help mitigate genetic isolation and increase genetic diversity among trout populations. The study used simulations of populations through time to model different historical evolutionary scenarios and predict the impacts on the genetic diversity of trout populations in metal-polluted rivers.
Overall, the research sheds light on the genetic consequences of metal pollution on trout populations across the British Isles and emphasizes the importance of maintaining genetic diversity to ensure the resilience of these fish in the face of environmental challenges. Funders of the study included the Natural Environment Research Council, the Game & Wildlife Conservation Trust, and the EU Interreg SAMARCH project, highlighting the significance of this research for conservation efforts in polluted river systems.
