Scientists are surprised by the first documented case of transmissible cancer in fish, detected in a US lake. The cancer, a melanoma, has researchers investigating a potential transmission method between the fish. This discovery is particularly concerning as it suggests the cancer cells are being passed from one fish to another, a rare phenomenon previously observed in dogs and Tasmanian devils. Researchers are now focused on confirming how the disease spreads and understanding the implications for the fish population. The finding raises questions about the potential for similar transmissions in other aquatic environments. Further study is crucial to determine the extent of the outbreak and develop potential management strategies.