Science of the Seasons: Leeches eke a living on fish

By Dr. David Wartinbee, for the Redoubt Reporter

Photo courtesy of Dr. David Wartinbee

Over the past couple of months, I have enjoyed being out ice fishing on a number of local lakes. Though I’ve caught some trout, my catch in some cases — from Egumen, Duckling, Nest and Snag lakes — were often infested with worm ectoparasites.

The slender worms were easily dislodged from the fish as I removed the hook or as they lay on the ice beside the hole. They were between .5 and 1 inch in length and seemed to have a slight greenish banding pattern on their body. Even without the aid of a microscope, I could identify a small sucker at the posterior end, so I was pretty sure I was seeing a leech of some kind. With a little magnification, I could see that there was a small sucker disk at the mouth end to accompany the larger posterior disk.

After consulting a variety of literature sources, I believe that I have been seeing a fairly common fish leech in the genus Piscicola. The species may be geometra but I have not had time to send specimens to colleagues who specialize in these aquatic worms. They are related to some of the other, larger leeches that are found worldwide and are sometimes parasitic on human swimmers.

Like their larger, aggressive relatives, these small leeches are able swimmers and will actively search out a fish for their meal. Often, they attach their posterior end on submerged vegetation and reach out and attach to a fish as it swims past.

This group of leeches does not have jaws so it is unable to chew through the host skin. However, it has a proboscis that can be forced through the skin of a passing fish so it can feed on the bodily fluids. Once attached to a fish, it may feed for a few days and then drop off and rest in the bottom detritus. It may not need to feed again for more than a month, although young fish leeches often repeat this process three or four times over a year as they mature into an adult.

Some of the fish I caught seemed to have a large number of the worms on them while others had only a few. The worms supposedly prefer being on the gills or at the base of pectoral and pelvic fins. However, I saw them attached to a variety of other places on the trout. While these leeches would not be able to pierce the scales of a fish, there are many other areas where bodily fluids are accessible. When the fish is irritated by their presence, it attempts to scrape off the leeches by rubbing against objects or on the lake bottom.

These particular fish leeches are well known among fish fanciers and fish breeders. Within a small fishpond or tank, their impact can be costly, since younger fish or small fish can be killed outright by heavy infestations. Adult rainbow trout are generally able to prosper in spite of modest leech infections. However, larger adult fish are known to die because of these leeches. There is an additional issue here since the holes left by a satiated leech could be an entry site for more-problematic bacteria or fish viruses.

In an interesting twist, a popular lure used in our streams and rivers is a leech mimic. While an “egg-sucking leech” fly pattern is based on a larger, nonfish parasite leech, others, like the green leech, are patterned after these fish parasites. Fish will actively feed on swimming leeches and they don’t discriminate on those that are their own parasites and those that are not.

Piscicola leeches are hermaphroditic, which means they have both male and female reproductive organs. Their reproductive activity starts when the daylight-to-darkness ratio reaches 12 hours (the vernal equinox on March 21 to 22). During mating, adults swap spermatophores and then produce a small number of eggs. The eggs are individually wrapped in a cocoon that is left on the lake or stream bottom to mature and hatch. They apparently produce only 50 and 90 cocoons that then hatch a few weeks later.

The adult leech probably dies at this time and the young soon hatch to begin the cycle again. Some laymen fishing literature from England indicates that trout are healthier in the late spring than during the winter because they have shed their leech parasites. It could be that the adult worms are dead in the early spring and the very young and very small leeches are not big enough to be seen or to have a major impact on adult fish.

These worms do not use humans as a food source and they are strictly found on the outside of the body. They have no impact on the edibility of the fish and will be removed during normal cleaning or preparation. Because they are relatively small, many fishermen will never even see these parasites. However, if you look for these parasites and find some, remember that they are just a part of the normal world our local fish are able to survive.

David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Cook Inlet watershed.


1 Comment

Filed under ecology, science of the seasons

One response to “Science of the Seasons: Leeches eke a living on fish

  1. justin

    Thanks. I recently fished campfire lake off of swan lake road and caught two rainbow trout with about 50 of these leaches a piece on them.

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