In 1974, a German fisherman released thousands of baby catfish into a Spanish river; 50 years later they are threatening native wildlife as an 'ecological time bomb'

• 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.