A common nuisance to swimmers, leeches are flattened worm-like animals.  Most species feed on worms, snails, insect larvae and other small aquatic animals, but, a few species, if given the opportunity, will also feed on human blood.

Leeches are typically found in shallow, protected waters, concealed among aquatic plants or under stones, logs and other debris.  They are attracted to water disturbance around docks and swimming areas.   On hot summer days leeches are most active.  In winter they burrow in mud.

A measure which can be successful in controlling leeches is bait trapping.  A metal can with a lid (a one pound tobacco or coffee can is ideal) drilled with small holes (depending on the size of the nuisance species) and baited with raw meat may trap large numbers of leeches from a heavily infested area.  After feeding, the leeches will have difficulty leaving the can.   Destruction of the contents of the can will help in reducing the size of the leech population.

Leeches are important in the food web of a lake.  As such, many fish species will eat leeches.  A healthy pond fish population will generally keep leeches under control, and redear sunfish are especially good leach eaters.  If you discover you have a leech problem, consider introducing redear sunfish in your pond.

Ducks have also been used to control leech populations, but an over abundance of ducks can create other more serious problems.  In order to keep ducks near your area for leech control they will also have to be fed.  Duck feed is high in phosphorus and travels rapidly through the duck's digestion system.  A readily available form of phosphorus is then added to the lakes.  If the lake is small and the ducks numerous, enough phosphorus can be added to the lake to cause algae to grow.   Ducks are also an intermediate host for a number of parasites that may not be desirable in a swimming area.

Water level manipulation can be used to control the bloodsuckers on small private ponds in areas where temperatures fall below freezing in the winter.  Dropping the level of the pond at least four feet after ice has started to form on the pond will freeze the leeches in the mud.  This measure is effective but it will also kill other forms of aquatic life that burrow in the mud.  

Since leeches prefer the shallow areas of lakes, swimming in deeper waters will reduce the risk of a leech attaching to a swimmer.  A boat or float can be used to get to the deeper water or a person can swim out to the deeper waters.