Lymphosarcoma in Hatchery‐Reared Yearling Tiger Muskellunge

Yearling tiger muskellunge (northern pike Esox lucius × muskellunge E. masquinongy) being cultured within the Colorado Division of Wildlife's fish hatchery system were found to have external lesions that were grossly and microscopically consistent with descriptions of esocid lymphosarcoma. This neoplasia has been described as a tumor of adult northern pike and muskellunge; a retroviral etiology has been proposed for it. However, esocid lymphosarcoma has not previously been reported in tiger muskellunge. Owing to concerns about the potentially infectious nature of the condition in a hatchery environment, an experiment was conducted to determine whether the lesion could be transmitted to naive young‐of‐the‐year tiger muskellunge in the laboratory by means of cell‐free filtrates. At 32 weeks postchallenge, grossly visible lesions were observed on the challenged fish. Histological evaluation of these lesions confirmed that they were esocid lymphosarcoma. We believe that this is the first report of the natural occurrence of this disease in tiger muskellunge as well as in any esocid that was not an adult.