By Staff
Nearly every fisherperson has experienced it. You get permission to fish a private pond and sally forth with great expectations.
Certainly these seldom bothered fish will be of such immense proportions and quantity no frying pan will be big enough to cook them. Expectations hold and you go home with a nice mess of bluegills, perch or bass but while cleaning you notice tiny black spots all over the fish just under the skin, like it's been pre-peppered for cooking.
Or perhaps you carve off tantalizing filets but in zipping the skin off you see yellow, worm-like things embedded in the flesh. Yuk! What are those?
Whether they be the black spots or the yellow worms they are what fishermen commonly refer to as grubs. They are most prevalent in fish inhabiting ponds and small lakes in the Midwest but they can appear most anywhere. I've even seen them in trout from high mountain lakes in the Pacific Northwest. They're most often found in bluegills, sunfish, perch and bass but can appear in any species of fish.
Technically they are called digenetic trematodes. A third form of this group resides in and around the fishes internal organs but these are much less noticeable because they are small and are discarded with the offal during cleaning. None pose a hazard to humans, though the thought is hardly appetizing. While nearly all fishermen in these parts have encountered these parasites I'll bet you don't know the slightest thing about them.
Their life cycle is most unusual and requires three totally different host creatures, birds, snails and fish. In the case of the black grub, those pinhead-sized black spots in the skin, tail base and fins, the typical host for the adult grub is kingfishers. The mature grub resides and reproduces in the bird's intestine.
Eggs are produced and hatch into one of the larval stages. These are introduced into the water in the bird's feces. The larvae must quickly find a snail for their second stage of development for they don't survive long in open water. Those successful in finding a snail host develop into a second larval stage. They then leave the snail and go free swimming in search of a fish. Again, they must do so quickly as their survival time free swimming is short. They burrow in through the fish's skin where they transform into a third life stage. When the infected fish is eaten by a kingfisher or other bird the grub matures and reproduces, completing the life cycle.
The life cycle of the yellow grub, those eighth to quarter inch long, worm-like grubs found in the fish's meat, is similar though the host bird is more often a great blue heron. The mature grub resides in the bird's throat and eggs are released into the water when the bird thrusts its beak into the water. From there the intermediate hosts of snail and fish is the same. The life cycle of the tiny white grub found in the fish's internal organs is identical to the yellow grub. All these grubs are so often a curse of small ponds because of the prevalence and close proximity of snails and fish. In larger bodies of water the snails and fish are more spread out and it's much more difficult for the parasites to make the transition.
In normal quantities these parasites have little effect on fish but extreme infestations can impair or kill them. They are very difficult to control as they can survive up to four years in a fish. Snails eat decaying vegetation and algae and their numbers can be lessened by clearing vegetation around the pond and controlling aquatic vegetation and algae.
Sunfish eat snails and can be introduced but that enhances the risk of overpopulation. Keeping kingfishers and herons out of the pond is pretty much mission impossible. They become accustomed to noise makers, flashers and other deterrents and nets or screens over most ponds is impractical. One solution is to realize protein is protein and just cook and eat the fish, grubs and all. Personally I choose solution number two, get my fish dinner elsewhere. Carpe diem.