Scientists have identified a rare and strange transmissible cancer affecting brown bullhead catfish in lakes across Canada and the U.S. This novel melanoma-type cancer, which behaves more like a parasite than a traditional tumor, marks the fourth known contagious tumor among animals. The contagion has been confirmed in fish from Lake Memphremagog and other freshwater systems, raising concerns about its spread and impact on aquatic ecosystems. Researchers are investigating this unprecedented case of a contagious cancer in fish populations.