OUTDOOR CORNER: FISH PARASITES NOT HARFUL TO HUMANS

Have you ever enjoyed a relaxing fishing trip, only to notice that some of your fish have little yellow and black bumps? What did you do? Did you release the fish? Or, did you take them home and fillet them as usual?

If you normally return these "infested" fish to the water, you may be pleased to learn that there is really no problem at all. These bumps, descriptively called "yellow grub" and "black spot", are fish parasites that cannot infect humans. These parasites will be killed by cooking and cannot be tasted, but some anglers prefer to remove them.

So how do fish become infested? Both, black spot and yellow grubs are the juvenile stages of parasitic worms called flukes. Adult flukes live in the digestive tract of fish-eating birds, like herons or kingfishers. Eggs are produced, which pass from the bird and fall into the water.

The tiny, free-swimming young that hatch can survive only if they happen to find a snail. If a snail is found, the parasite bores in and undergoes more growth and development. Later, a second form emerges from the snail. These must locate a fish to survive, and, if they do, they penetrate and change into the grubs that fishermen see. If an infested fish is eaten by a bird, the grubs develop into adult flukes, and the cycle is repeated.

It is likely that every fish you catch is host to at least a few parasites. Most are very small and located in the internal organs, so you won't eat them and probably won't even see them. No parasite is good for fish, but most fish live their entire lives without suffering any serious effects from parasites. Spawning failure in fish has been caused by heavy parasite infestation of the reproductive organs, and fish have been killed by excessive parasitism of the liver or kidneys. However, such occurrences are rare.

How do we control parasites? That question poses another question: is control really necessary? When you remember that the parasites can't infect humans and don't normally hurt the fish, the answer is usually "no". Besides, the complicated parasite life cycles make treatment difficult and often impractical. In lakes with severe grub infestations, fishery managers have stocked redear sunfish, which eat snails, to reduce the number of hosts for larval parasites. This tactic sometimes lessens the problem.

The best way to deal with fish parasites is not to worry about them. If you have any questions, call the Missouri Department of Conservation in Cape Girardeau at (573) 290-5730 and ask for the publication on fish parasites.

Brad Pobst is a fisheries management biologist with the Missouri Department of Conservation.