This is the common "grub" found in our freshwater fish as a yellow worm up to 1/4 inch (6.4 mm) long just under the skin, or in the flesh. Yellow grub has been reported from so many kinds of freshwater fish in North America that apparently no fish is immune to it. The grub is the larval stage which must be eaten by fish eating birds, such as herons and bitterns, to develop. The grub matures in the throat of the bird, and eggs wash into the water from the bird's mouth when feeding. The eggs hatch and the first larval stage (miracidia) swim by means of fine hair like cilia until they find a snail of the genus Helisoma. Unless they find this snail they die within several hours. In the snail they go through several developmental stages during which they multiply a thousand fold, finally leaving the snail as free swimming cercariae. Unless the cercariae find a fish within a few hours, they die. When they find a fish, they burrow through the skin and encyst, where they develop into metacercariae, which are the yellow grubs. There they remain until eaten by the bird host, thus completing the life cycle.
Yellow grubs beneath the skin of the tail of a yellow perch
The grubs may live for several years in the fish, thus in many lakes rather heavy infestations accumulate and the fish are classed by fishermen as unfit for food. It is possible that yellow grub may kill fish under some circumstances, but normaly a fish is not noticeably affected by the parasite.
A perch with filet removed to show the yellow grubs in the flesh. Note that some of the worms were encysted during filleting, and have assumed an elongated form more commonly associated with a "worm"
Normal cooking of the fish destroys the grub and the flavor of the fish is not altered.