Scientists said Monday that they have for the first time documented that an Asian carp species has reproduced within the Great Lakes watershed, an ominous development in the struggle to slam the door on the invaders, which could threaten native fish. Four grass carp captured last year in the Sandusky River, a tributary of Lake Erie, had spent their entire lives there and were not introduced through means like stocking, researchers with the United States Geological Survey and Bowling Green State University said. Grass carp are among four species imported from Asia decades ago to control unwanted plants in settings like sewage treatment lagoons. They escaped and have spread into the Mississippi and other waters across the heartland. Of greatest concern in the Great Lakes region are bighead and silver carp. But because all Asian carp species require similar conditions to reproduce, the Sandusky discovery suggests it is likely that any of them could spawn there and in other Great Lakes tributaries, said Duane Chapman, a fisheries biologist. “It’s bad news,” he said. “It would have been a lot easier to control these fish if they’d been limited in the number of places where they could spawn.”