Scientists in Lake Memphremagog, which straddles the Vermont-Quebec border, have identified a contagious form of skin cancer spreading among brown bullhead catfish, the first documented case of a transmissible cancer in fish. The discovery is significant because it suggests such cancers, in which living tumour cells pass directly between individuals rather than arising independently in each host, may be far more common in nature than previously thought, and could offer researchers a useful natural model for studying how tumours spread and metastasise in other species, including humans.

The melanoma, which appears as dark streaks or blotches on the fish's skin, fins, lips, gills and eyes, was first spotted in the lake in 2013, when just over a quarter of sampled catfish showed symptoms. Researchers, including Peter Emerson of the Vermont Fish & Wildlife Department, Julie Dragon of the University of Vermont, and Elizabeth Murchison of the University of Cambridge, believe the disease spreads when the fish gather closely to spawn and accidentally scratch one another with their sharp spinal fins, allowing cancerous cells to pass into new hosts. More than a decade on, about 30% of the lake's catfish now carry the disease.