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Idaho Fish and Game

Lake Cascade's Clinging Crustaceans

They're back... and on Cascade's trout. Small, whitish copepods less than one-quarter inch in length are once again being found on fish taken from Lake Cascade. The worm-like organisms are the reproductive stage of Lernaea cyprinacea, a crustacean related to freshwater shrimp. "Many Idaho waters contain these copepods," Fish and Game fisheries manager Don Anderson explained. "Lake Cascade simply has a higher concentration, making them more noticeable to anglers." The parasitic stage eventually drops away from the fish, growing into the free-swimming "shrimp-like" adult. At this life stage, the copepods find the tables turned; they are a favorite fish food. The parasites are rarely lethal to their host fish. "They must reach concentrations of dozens on the gills of a half-pound fish to cause any damage," Anderson noted. "And we usually see fewer than ten copepods per fish." Anglers may also notice a reddish tint around the site where the copepods are attached, the result of skin irritation. The crustaceans are as common as "fleas on a dog" according to Anderson and pose no threat to humans. Because only their small mouthparts actually penetrate a fish's skin, simply skinning and/or filleting the fish will effectively remove the parasites from the edible portion of the fish. Adapted to cold water, the copepods die quickly if cooked; human body temperatures are also lethal, providing peace of mind to those who accidentally ingest the organism. There are no plans to attempt eradication of the copepods from Lake Cascade. "In fact, we wouldn't want to," Anderson said. "The non-reproductive stage is a critically important food item for all of Lake Cascade's fish, particularly young trout and perch."