Lesions

A lesion is usually a naturally occurring sore on the surface of a fish’s body that is not bleeding; caused by diseases from bacteria, viruses or parasites.  However, a higher number of lesions observed on fish than is normal for the population could indicate that the fish is experiencing stress.  Stress can occur from spawning, from natural changes in the fish’s normal environment, or from changes in the environment occurring related to human activities.

Lymphosarcoma is an example of a lesion caused by a virus, affecting northern pike in the spring and early summer. The virus causes red sores on the body and makes the scales ‘mushy’. While Lymphosarcoma is not common, it can be subject to localized outbreaks and is probably spread by direct contact between fish during spawning. This disease does not affect humans but can kill fish.