Invasion Of The Body Snatchers

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Published on: Jun. 2, 2002

Last revision: Nov. 12, 2010

Most anglers have encountered fish parasites at least once, so Conservation Department fisheries biologists have to answer a lot of questions about these unique organisms. Although fish parasites may be aesthetically repulsive, these little critters are generally harmless to humans. In fact, the parasites that affect Missouri fishes are not at all infectious to humans. Even if they were, as long as your fish are properly cooked, there's no way for you to acquire an infection.

Fish parasites come in a variety of sizes and shapes. Some species are flat (flatworms) or tubular (roundworms). Some (thorny-headed worms) have distinct structures that allow them to attach to a fish's intestine. Some fish parasites are found within the body (endoparasites), and some attach to the outer body (ectoparasites).

Some fish parasites are very specific about the type of fish they choose as hosts. Others are found in a wide range of fish hosts. Some parasite species are restricted to certain geographical regions, while other species are broadly distributed across the Midwest. These creatures are an important part of the biology of lakes, ponds and streams, and all naturalists should be aware of them.

Parasitic infections are normal among fish populations, and some fish may die as a result. It's just one of the many ways that Mother Nature helps keep population levels in check. That said, severe losses of fish due to parasitic infections are rare.

The actual causes of fish deaths aren't always obvious. A large percentage of mortality is either caused by physical damage to tissues and organs, by the parasites continually "robbing" fish of critical nutrients, or by secondary bacterial and/or fungal infections. Parasites can also function as vectors (carriers) of viral diseases.

That's not the worst of it! Many parasites require several hosts in order to complete their life cycles and become reproductively mature. For example, a parasite may be living inside a fish today, but the parasite must somehow get inside a fish-eating bird to complete it's life cycle. In order to increase the chances of a bird eating a host fish, some parasites actually alter the behavior the fish, causing it to swim awkwardly and nearer the surface, where a bird will more likely grab it.

Most of the parasites a fish harbors will never be seen by an angler. For example, protozoans (single-celled organisms) may live on the gills and fins of a fish, but you

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