COLLEGE STATION — Black spots or worm-like cysts on fish skin may not be appetizing, but with proper cleaning and cooking techniques, these catch are safe to eat, according to Dr. Jim Davis, Texas Agricultural Extension Service fish specialist.
“Fish do have parasites, just like all other animals,” Davis said. “A parasite does a fish some harm, but most do not kill the fish unless they are present in very large numbers.”
A parasite is any animal that lives on or in another animal or plant and obtains its food and shelter from that host, Davis explained. Though there are many parasites found on or just under the skin of fish, the two of most often seen by Texas anglers are yellow grub or black grub.
He said people can safely consume fish infected with either of these grubs, provided that the flesh is thoroughly cooked.
“If you are concerned about the looks of the fish flesh,” Davis added, “you can cut out the parasites if there are not too many.”
The yellow grub, what leads some people to call a fish “wormy,” infects most freshwater fish but is usually found in ponds with high populations or crowded conditions caused by falling water levels.
“When an angler finds the worms, they are usually in a cyst just beneath the skin,” Davis said. “From the outside, it appears as a bulge near the fins or tail. When cut out of the fish, they are yellowish or white and about one-fourth-inch long.”
Davis said the yellow grub’s life cycle is complicated. The adults life in the mouths or throats of fish-eating birds such as herons. The grub eggs, which are shed into water while birds feed, hatch and swim around to find a snail. After boring into the snail, the larvae lives there until it is eaten by a fish. Once in the fish, it matures and remains encysted until it is eaten by a bird to begin the cycle again.
Black grubs actually are neither grubs nor black, Davis noted. These tiny animals are actually flukes that are white but look like a black spot on fish.
“What an angler really is seeing is the pigment deposited around the grub,” Davis said. The black grub’s life cycle is similar to the yellow grub except the parasite passes through a bird’s intestines after an infected fish is eaten. The adult lays eggs which pass into the water with the bird droppings. The black grub larvae continues its life in a snail just as the yellow grub does.
“It is possible to rid a pond of these parasites, but it is not very easy or practical,” Davis said. “If you prevent all fish-eating birds from entering the pond or rid the pond of all snails, the problem would go away.”
But, he said, birds move from pond to pond and carry the parasites with them. Likewise, it would be difficult to get rid of all snails.
“What this means is that people will sometimes catch a fish with one or more of these parasites,” Davis said. “The worms are not generally a problem to the fish, and after the fish is cleaned and fully cooked, it is completely safe to eat.”
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