Workers decided to have the fish tested when they noticed an increase in deaths at Klamath Falls, Rock Creek and Elk River hatcheries, officials said. A fish hatchery is where fish are bred and raised in a controlled and safe setting before being released into natural waters, the U.S. Fish and Wildlife Service said. Officials try to uncover mysterious illness in fish At first, the trout were tested for bacteria and viruses, but nothing turned up, officials said. They were also given antibiotics, but the fish “did not respond” to the treatment.