Black Spot (Black Grub)


A common disease in earthen bottom ponds and lakes, this disease is caused by a parasite (larval trematode) that burrows into the skin of a fish causing the formation of a cyst approximately one millimeter in diameter. This parasite has a complex life cycle that requires fish eating birds or mammals, snails, and fish at different stages in order to survive.

In general, even heavy infestations of these parasites do relatively little damage to the fish. There is some evidence that heavily infested juvenile fish may experience excessive blood loss, physiological stress, and even death. Also, fish with heavy infestations on the eyes may be blinded.

While not recommended by Fisheries Division, it is possible to reduce the incidence of the parasite by removing or reducing the intermediate hosts such as snails. This will disrupt the life cycle of this parasite. Copper Sulfate (CuSO4) applied to a pond at a rate of 1.0 part per million will reduce snail populations. This should be done with caution, as copper sulfate will also unnecessarily harm other mollusks. Additionally, the application of copper sulfate may need a permit from the Department of Environmental Quality if the waterbody is connected to waters of the state.

Control of this parasite is really not necessary as these parasites are incapable of infesting humans and the fish are safe to eat. It may be more aesthetically pleasing to skin a heavily infested fish prior to eating. In any case, cooking kills the parasites.