Are you seeing red spots on fish? Those spots could be grub worms. Here’s how they arrived in the pond and how to manage their population. 

Fish grubs are immature forms of parasitic worms that invade fish flesh. They appear as round, bead-like structures embedded in the flesh. Grubs are contained in spherical or oval cysts. If the cyst is opened, the immature worm will straighten and flatten into a form that resembles an adult. Short, flat adult fish grubs live in another animal which uses fish for food.

In an article, Fish Grubs in Freshwater Ponds and Lakes, Texas A&M Extension Fish Disease Specialist Sterling Johnson states cysts and grubs of some worms have characteristic colors. Some are commonly called yellow grubs, white grubs, and black grubs. In Texas ponds and lakes, they live as parasites in almost every kind of fish. Most common are yellow, white, black, and eye grubs.

Yellow grubs are large, infect bass, bream, catfish, and many other species. They invade muscles or edible flesh. Size and color make it easily visible. Although this parasite is not harmful to humans eating the fish, flesh may lose visual appeal. The life cycle is very interesting because it involves not only fish, but also a snail and bird. It begins in water with hatching and release of miracidium from a microscopic egg. Miracidium use cilia (hair-like structures) to swim a few hours, then die. If contacting a certain snail before succumbing, it enters the snail, sheds cilia, and forms a cyst. It then multiplies into numerous individuals after entering the snail as a single individual. They leave the snail, swim with their tail, contact a fish, and penetrate the flesh. Distribution is random and may be seen in fins, gills, and mouth. When skinning or filleting fish, they are visible in the flesh. Affected areas can be trimmed away.

According to Specialist Johnson, if an infected fish is consumed by a fish-eating bird, cysts are digested by enzymes. Freed grubs migrate to the trachea, mouth cavity, upper esophagus, attach themselves, and become sexually mature adults. When the water bird thrusts its beak into the water, eggs laid by adult worms are released into the water. When eggs hatch, the cycle is completed. Development time for the parasite in the egg is a few hours; in the snail, approximately 5-months; and in the bird, a few days. Grubs may live 3-years in fish.

White grubs occur in numerous fish, but mainly among sunfish and minnows. The strain seen in ponds affects sunfishes almost exclusively. Although common, this grub does not attract as much attention as the yellow type because it’s smaller and appears more in organs than flesh. Black grubs represent several species that affect a variety of fish. One common type affects bass and bluegill. It appears peppered as cysts are lodged just under the skin. Eye grubs do not encyst like other grubs. They eventually become situated in the lens chamber of the eye.

I See Red Spots On My Fish

Redear sunfish

As for prevention and control, chemicals that reduce snail populations also can be harmful to fish. Most bird hosts are migratory and protected by federal law. The most common management method is stocking redear sunfish. In the South, they’re called shell crackers. Run your little finger down their throat and you’ll feel hard pads used to crack the snail shell. By feeding on snails, redear eliminate a primary host for parasite development. Standard redear stocking rate is approximately 250 per surface acre.

If you suspect grubs have infested your fish, text a photo. We’ll identify and offer a management plan.