Fisherman Klaus Hidde wades through shallow ponds. He shakes out his nets. Hundreds of crayfish tumble into his basket. He’s on a mission: get rid of the invaders!

 Mr. Hidde has an unusual fishing spot: Berlin, the biggest city in Germany. Louisiana crayfish come from Mexico and the southern United States. And now they’re taking over Berlin. The crayfish—also known as crawdads and mudbugs—crawl over city bike paths. When it rains, they walk across the street to new rivers and lakes. People have never seen such creatures running wild in Berlin. They call city offices. They say, “There are scorpions in the street!