Researchers have identified a transmissible melanoma in brown catfish in Lake Memphremagog, on the Vermont–Quebec border. It is the first documented contagious cancer in fish and the first in a freshwater ecosystem, showing that cancer cells themselves can sometimes spread between animals rather than developing independently. The discovery also matters because the lake supplies drinking water to more than 175,000 people, although the article does not indicate a human health risk.

Black lesions were first noticed in 2012, with melanomas found in 23–37% of examined fish—an unexpectedly high rate. Genome sequencing showed that tumours from different catfish were genetically closer to one another than to their hosts and shared hundreds of thousands of variants, strongly indicating a single clonal cancer lineage. Researchers found no virus or bacteria specifically linked to the tumours, and transmission may occur through close contact during reproduction or through cancer cells surviving in water or sediment.