In 2012, anglers and biologists reported strange black tumors on catfish in a Vermont-Quebec lake. Years later, genetic testing revealed the shocking outcome: The tumors are a rare contagious cancer spreading between fish

For years, strange black growths on brown bullhead catfish in Lake Memphremagog puzzled anglers and biologists. The lake, which stretches roughly 30 miles across Vermont and Quebec, is known for its trout, salmon and other fish. But its brown bullheads have developed something far more unusual.

The dark lesions, found on the fish’s fins, gills, lips and even eyes, were identified as melanomas. Now, genetic evidence suggests the cancer may be spreading from one catfish to another, potentially making it the first transmissible cancer documented in a fish species.