eliminate leaches in small pond ( 3000 ft2 )

Asked September 13, 2018, 10:27 AM EDT

please , sources of information to deal with leaches .

Windham County Vermont

1 Response

Greetings Anonymous,

This question was assigned to me in Ohio even though the system is not supposed to send me out-of-state questions. (I'm assuming Vermont Extension doesn't have staff who feel competent to address your issue.) There are no pesticide applications approved for the public to apply to aquatic leeches in Ohio. I know nothing of Vermont's pesticide regulations, but they probably are at least similar. I thus will offer nothing by way of advice regarding toxicants.

Ah leeches…

Leeches like organic sediment, and lots of sediment that makes good leech habitat will make for more leeches. Ponds with healthy fish populations, especially of panfish, often have limited numbers of leeches simply because fish in general like eating them.

A properly designed aeration system would slow the accumulation of organic muck in promoting the growth of aerobic bacteria and promoting efficient nutrient processing, but I’m not certain it would help much with the problem at hand other than reducing the likelihood that it becomes worse. (A healthy, non-stunted fish population would go farther.) The point of diffuser aeration is to destratify a pond. Ideally, that’s begun in the spring and run all day every day until the pond could be expected to mix naturally in the fall. There are some tradeoffs to destratifying a water column during the hottest days in the hot season (mixing and thus warming the water column to surface temperatures can stress fish when it’s very hot). However, shutting down an aerator in the heat of a day may allow temporary stratification that’s enough to cause hypoxia even in only a few cm along the sediment that can then allow the dissolution of phosphorus and promote algae growth and subsequent problems. Tradeoff.

Trapping leeches is possible (usually by rigging a closed coffee can with holes and baiting it with raw meat). Trapping anecdotes give mixed results. While such a trap can certainly be used to remove whatever leeches it happens to catch, you’re not likely to catch and remove enough to substantially reduce the population.

The most effective method to kill leeches of which I’m aware: late in the fall, as the water is becoming cold and leeches have burrowed into sediment to overwinter, drop the water level to expose mudflats. As winter comes on, mudflats freeze, and most or all of the leech population is eliminated in one fell swoop. To be effective, the pond would need to have enough deep-water refuge remaining for fish and some mechanism to controllably lower water level.

…And of course, leeches are a natural part of aquatic environments and can be useful as a food source for fish populations. Some numbers of them can be expected.

However, how much conflict with human use do these leeches cause on only 3,000 feet^2? Can they be simply tolerated? Most of the management approaches I've discussed above are applied to somewhat bigger sites.