An anonymous reader quotes a report from CBC News: It's rare that cancerous tumors can spread from one individual to another. But a genetic study suggests that's what's happening with melanoma tumors among catfish in Quebec and the northeastern U.S. The discovery represents the first known transmissible cancer in fish and one of very few transmissible cancers ever found, reported the study published in the journal Nature on Wednesday. Julie Dragon, co-leader of the new study, noted that only three other transmissible cancers have ever been identified so far -- in Tasmanian devils, dogs and shellfish. "These conditions are incredibly rare in nature," she said. "So something has to be happening to allow this to happen." Anglers in Lake Memphremagog, which spans the border between Quebec and Vermont, first reported catching brown bullhead catfish with strange black spots and lumps in 2012. They turned out to be melanoma skin cancer tumors. (Humans can also get this kind of cancer.) Now, about a third of the brown bullheads living in the lake have them. It's not clear how sick the fish become. In some cases, the cancer spreads to other organs, such as the brain or liver. But many fish with lesions seem relatively healthy and some older fish even have them, suggesting they can live with the disease for a time. Because of the sudden appearance of the black lesions in Lake Memphremagog the year after a huge flood caused by post-tropical storm Irene, locals worried they were caused by carcinogens washed into the lake by floodwaters. Tests for carcinogens haven't been conclusive, although a study published in May found higher concentrations of seven metals, including zinc and the carcinogen arsenic, in the skin of fish with the tumors. Researchers suspect the tumors may spread among adult fish during spawning, when crowded fish rub against one another and may be punctured by their spines. "They... swim all over each other and we think it's possible that they poke each other and cells can get into other fish that way," Dragon said, though transmission has not yet been demonstrated.Read more of this story at Slashdot.