I’ve always wondered how completely separate, unconnected river systems contain identical species of fish. How can a river in the south of England contain identical species of freshwater fish to a river system in Wales, Scotland or Ireland? Or even river systems in continental Europe or further afield? The fish couldn’t have left the freshwater system by travelling through what would be fatal seawater to populate other freshwater systems. They obviously couldn’t fly or walk from system to system. The same identical fish species couldn’t have evolved independently in each system. Are there any fish species in the British Isles that are entirely exclusive to one river system and found nowhere else?
David Eckersley, Holmes Chapel, Cheshire
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