Koi Ponds and Leeches from the Garden. Do Fish Eat Leeches?
QUESTION ... Eight years ago, while gardening, I discovered two very large leeches, identified as horse leeches, in different parts of the garden. Recently I found another very large leech about 1ft from the raised wall of the pond. My pond has never had plants in it and I have never seen a leech in it up to now. Do leeches travel across land? What do they eat and do Koi eat them?
Rod Bowley, Leicester
Answer: The strongly marked adult Hirudi medicinalis or Medicinal Leech is the largest leech to be found in this country coming in at 90mm, whereas the Horse Leech (Haemopsis sanguisuga) makes only 60mm and is seemingly black or dark olive in colour. The Medicinal Leech needs blood to survive and will thrive on frogs and fish in its juvenile stages, but once adult, needs the blood of mammals. This it finds, I presume, by lying in wait for grazing animals in or near its aquatic habitat. Unfortunately this is the tag that the Horse Leech has landed itself. This beastie is quite content to merely swallow worms.
As long as they are still moist there is no reason why they cannot roam the countryside willy-nilly looking for a source of nourishment or even a new habitat if the old one is drying out. They are certainly difficult to keep contained in a jar they can get out of anything. So a raised pond will certainly prove much of an obstacle. Further proof of this is that grazing animals have been found with leeches of this type in the mucous membranes of their nostrils.
In parts of the USA they breed or collect leeches for bait for fishing with. Catfish and salmon love them and so undoubtedly Koi would certainly be in line in the dinner queue, if they dont get sucked on first.
In Dorrigo, Australia they have The World Leech Racing Championships. Competing leeches are stood in the center of a trampoline and are encouraged to wriggle to the edge. Apparently, one leech owner claimed that he drank coffee all night then let his leech suck his caffeinated blood, but despite that the leech still came in last.