What are those parasites on my trout?

Fish with these parasites are still good to eat.

Parasitic copepods on a lake trout. Photo: Ontario Fisheries Resource Centre

Parasitic copepods on a lake trout. Photo: Ontario Fisheries Resource Centre

Many recreational fishers who have been fishing in the Keweenaw Bay and Munising Bay areas of Lakes Superior have reported external parasites on lake trout and splake (cross between a lake trout and brook trout) they have caught. Some have even expressed concern if these fish are safe to eat.

These parasites are in a group known as parasitic copepods. This parasitic species found on the body of lake trout and splake is known as Salmincola siscowet with other species of Salmincola occurring on other salmonid fishes. The parasite resembles a small white or yellow grub-like organism attached to the gills, fins, or general body surface.

The males of most parasitic copepods are rarely seen and consequently identification is based on the females. The adult female produces two clusters of 60 to 300 eggs twice during a 9- to 13-week life span. These two egg cases can be seen protruding off the end of the trunk of the parasite. In two to three weeks the eggs hatch and the young must find a host lake trout or splake within two days or die. On the fish they mate, the males die and the females develop into adults.

The parasite feeds on the epithelium or skin tissue and blood of the fish and at times the fish may become susceptible to secondary infections, especially fungi. Although those fish which are heavily infested may not be aesthetically pleasing to the eyes, if the fish is skinned after filleting most of the areas where the parasite was imbedded will not be noticeable. If some areas of muscle penetration by the parasite are evident in can be trimmed out. These parasites are not harmful to humans.

Heavy infestations of the parasite Salmincola siscowet are not common in the wild such as that of the Keweenaw Bay and Munising Bay areas of Lake Superior. Lake trout and splake present in these bays may not range or travel as much as those outside the confines of these bays and thus may be more prone to reinfestation by the parasites. This parasite can be found anywhere in Lake Superior and northern Lake Huron.

Michigan Sea Grant helps to foster economic growth and protect Michigan’s coastal, Great Lakes resources through education, research and outreach. A collaborative effort of the University of Michigan and Michigan State University and its MSU Extension, Michigan Sea Grant is part of the NOAA-National Sea Grant network of 33 university-based programs.

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