Every year, the Vermont Fish & Wildlife Department conducts a fish census in Lake Memphremagog, which straddles northern Vermont and the Canadian province of Quebec. In 2013, the team was startled by some of the brown bullhead catfish they were finding in the lake. The fish were marred by unsettling dark streaks.

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“It looks like black tar on the skin — not very pretty,” says Peter Emerson, a fish biologist with the department who has participated in the census since 2009. “You can tell it doesn’t help the fish cause they’re inside the mouth, they’re on the fins.” Sometimes the dark marks are on the lips or the gills or even covering the fish’s eyes.

Just over a quarter of the catfish had the black blotches. Anglers had noticed them the year prior, but before that there had been nothing. “They arrived very suddenly,” Emerson says.

In a paper published on Wednesday in Nature, Emerson and his colleagues report that these inky blemishes are a kind of transmissible skin cancer — one that appears to spread from catfish to catfish.

“It is the infectious agent itself,” says Julie Dragon, a data scientist at the University of Vermont and a co-author on the study. “Those cells can somehow escape the host, maybe live in the environment — the aquatic environment — and then infect a new host.”

Beyond this lake, Dragon believes these catfish may hold clues about how tumors spread throughout our own bodies. “We still don’t really understand this event where a cell leaves the tumor, moves through the bloodstream, gets into a new organ,” she says. In the catfish, “it might have some of the same processes going on that allow these cells to invade a new host, avoid the immune system and then establish themselves.”

A fishy mystery

Shortly after the sinister lesions first surfaced, the Vermont Fish & Wildlife Department worked with researchers to determine that they were melanomas.