Mother Nature can be very complicated in her day-to-day ecosystem operations. While most life cycles are fairly simple, others can be extremely entangled and perplexing. Such is the case with a couple of common fish parasites, as we will learn. This fact will be interesting to anglers and birders alike.
Anglers have undoubtedly caught fish with visible parasites from inland lakes, but may not recognize them. Two of the most common parasites anglers encounter are black spot and yellow grub. Black spot, which is very common in Michigan lakes, looks like grains of pepper on the skin or fins. Individual fish appear as if someone has taken the pepper grinder to them and seasoned them prematurely. Yellow grub appears as a white to yellow worm, about 1/4-inch long in the flesh and does not have any external signs.
Black spot and yellow grub are the adult stage of parasites (flatworms) that infest birds, but have to spend part of their larval life cycle in snails and fish. The bird host for black spot is the kingfisher or loon, while the Great Blue heron is the bird host for yellow grub.
The complex life cycle is as follows: Mature microscopic eggs hatch in the water in a state known as a miracidium, which we will call LV1 (larval stage 1). The LV1 is a free swimming individual that will die in a few hours unless it comes into contact will a snail. The LV1 enters the snail's body and forms a sporocyst-LV2, which goes through several larval stages, LV3, LV4, and LV5. The LV5 stage (cercariae), which leaves the snail host, is called a daughter cell and is free-swimming. This stage, like LV1, has hours to find a fish or it will perish. Those that find a fish burrow through the skin and form cysts or metacercariae-LV6. The infected fish is now eaten by a fish-eating bird, where the LV6 walls are digested by enzymes. The freed grubs now migrate to the throat or mouth area of the bird where they attach and become sexually mature adults that produce eggs. When the bird thrusts is mouth into the water, eggs are released and hatch into the LV1 stage, completing the life cycle. Got that? Quiz at the end ...
Black spot commonly infects rock bass, bluegill, sunfish, bass, pike, perch and minnows, but can occur in brook trout. Black spot occurs in fish from northern Canada to the Amazon. Fish almost completely covered with pepper spots (cysts) have been observed, yet the parasites do not appear to impact growth or longevity. However, young fish may experience mortalities. Some older, larger fish with black spot will have 1/8-inch long or less white worms in the flesh, just under the skin, which are observed when the fish is filleted.
Yellow grub has been reported from almost every species of freshwater fish in North America and so apparently no fish is immune. My experience is, it is most common in yellow perch. Yellow grub normally does not kill fish, but may under the right conditions.
Fish parasites are not harmful to humans, with the exception of broad fish tapeworms. The parasites are killed by proper cooking and the flavor of the fish is not impacted. While neither of these parasites is capable of infesting humans, they are unsightly and unappealing. Some anglers take the time to dig the worms from the flesh, prior to cooking, thus making them more aesthetically pleasing to the consumer. Others just consider them additional protein. Bon appetit!
The incidence of these parasites appears to be increasing and can be attributed to the increase in Great Blue heron, kingfisher and loon populations, a sign of a healthy environment.