Parasites

All animals, including fish, may have parasites that live on and in their bodies such as fish lice. Parasites may cause disease. For example, northern pike may get black spot disease from the parasite Uvulifer ambloplitis that causes the entire skin of the fish to be covered by small black spots.

A higher number of parasites observed on the outside or inside of fish than what is normal for the population could indicate that the fish is experiencing stress. Stress can occur during spawning, from natural changes in the fish’s normal environment, or from changes in the environment occurring from human activities.