In 1974, a German fisherman released thousands of baby catfish into a river; 50 years later they are threatening native wildlife

German biologist and recreational fisherman Roland Lorkowsky released thousands of juvenile Wels catfish into Spain's Ebro River in 1974. His goal was simple and positive: to create a world-class sport fishery by introducing one of Europe's biggest freshwater fish. But today, the Wels catfish has become one of the dominant predators in many Spanish waterways. While they swim as the ultimate trophy catch for the sport fishermen, for conservationists these swimmers pose an ecological threat.Europe's freshwater kingThe Wels catfish is the largest freshwater fish native to Europe. It can grow to more than 8 feet (2.5 meters) in length, with exceptional individuals reportedly exceeding 9 feet and weighing more than 440 pounds (200 kilograms). Its enormous size, long lifespan and ability to thrive in a wide range of habitats have made it one of the continent’s most successful freshwater predators.The species is also a fantastic feeder, consuming almost anything it can swallow. Its diet includes fish, crayfish, frogs, water birds and even small mammals that venture too close to the water’s edge. Adult fish are estimated to consume around 5 per cent of their body weight every day. In its n...