Yellow, black or white cysts or "grubs" are often found in fish and frogs. The sunfish family, which includes bluegill, largemouth bass, and green sunfish are all susceptible to infestation by this worm. These fish are sometimes known as perch, brim or goggle eye. Channel catfish are also susceptible to grub infestations that can render this commercial species unmarketable.

Grubs are most often found in or at the base of fins and just below the skin between muscle segments. However, they can be found in almost any part of the fishes body. Yellow grubs range in size from as small as a pin head to as large as a pea. Black grubs are small and resemble black pepper sprinkled on fins or flesh of fish. Grubs are not harmful to humans. Eating fish containing grubs will not cause sickness or parasitic infection. However, most people find it objectionable.

Life Cycle Of The Yellow Grub
The yellow grub, Clinostomum complanatum, (formerly known as C. marginatum), is a parasitic fluke that requires snails and fish eating birds to complete it's complex life cycle.

The life cycle of black and white grubs is essentially the same. Grubs begin life as eggs released from an adult fluke living attached by a sucker, to the throat of a fish eating bird, commonly, a great blue heron. Eggs hatch into a free swimming stage called a mericidium.

Mericidium will die if they do not find a Heliosoma spp. snail (first intermediate host) within a few hours. When contact is made with the snail, the mericidium burrows into the digestive gland or gonads. Mericidium undergo developmental changes to produce sporocysts.

Sporocysts have no digestive system, a characteristic that differentiates them from the redia, the next developmental stage. Germinal cells in the sporocyst produce many redia that continue to migrate about digestive gland. The redia produces the cercarial stage of the grub.

Cercaria differ structurally from adults only by size, addition of a tail and other changes necessary to reach the fish (second intermediate host). Free - swimming cercaria leave the snail through the mantle cavity. On contacting a host fish the cercaria burrow under it's skin and gradually form a thick walled cyst or "grub" that is readily visible when the fish is examined. The grub is the metacercarial stage of the adult fluke. Fish respond to some metacercaria by surrounding it with pigment. This gives the black grub it's distinctive color.

The encysted grub is freed when a fish eating bird captures and swallows an infested fish. A combination of temperature, digestive juices and carbon dioxide in the stomach of the bird activate the grub and digest the cyst wall. On release from the cyst, the grub migrates up the esophagus and attaches itself to the throat or mouth cavity.

Grub Control In Fish
Complete grub eradication is rarely possible in ponds. Fish eating birds and snails can not be kept away from the pond at all times. However, any steps taken to disrupt the life cycle of the grub will reduce numbers found in fish.

Reduce Snail Population
Snails feed on aquatic vegetation. Removing aquatic plants from ponds will reduce the amount of food available to them. Snails also prefer hard bottomed, clean ponds. They are often found attached to rocks or other hard surfaces. In ponds with soft muddy bottoms, snails are most often found on the surface of leaves and other vegetation. Gravel, rocks and vegetation should be removed where possible to reduce or eliminate optimum snail habitat.

Red eared sunfish or "shell crackers" can be stocked. These fish are known to eat snails however, they will not eradicate them. Removal of aquatic vegetation will make the snails more vulnerable to red ear sunfish predation.

Mow vegetation along pond banks. Snails deposit egg masses on vegetation near pond banks. Mowing allows sun and wind to dry vegetation and kill these eggs.

Draining and drying a pond will greatly reduce snail populations, however, this is not always desirable or feasible for many pond owners.

Discourage Fish Eating Birds
Deepen shorelines to 24 inches or more. Wading birds have difficulty fishing in water this deep. Deep water also discourages aquatic vegetation growth.

Use of a variety of bird scare devices can help frighten birds away from ponds. Vary times and techniques when scare devices are used to prevent birds from becoming habituated to the methods employed.

Most fish eating birds are Federally protected by law . Shooting them is not a legal option.

References
An Illustrated Laboratory Manual Of Parasitology. Fifth Ed. Raymond M. Cable