Fish Leech (Piscicola geometra)

Photo © Ondrej Zicha, www.biolib.cz, ondrej.zicha@gmail.com

by Karen Paul

Piscicola geometra is the most common fish leech found in garden ponds. It differs from other true parasites, such as Argulus and Lernaea, in that it only needs to attach to the fish for feeding. After feeding the leech leaves it's host and hides among stones or plants while it digests it's meal. The fish leech can survive for long periods without feeding and is often introduced to the pond on plants.

The adult fish leech is clearly visible without magnification, measuring up to 25mm in length. Leeches are strong swimmers and can actually be seen swimming across the pond towards their victims.

The fish leech can affect any part of the fish, firmly attaching itself with the sucking disc and feeding by piercing the skin and sucking blood with its specialised mouthparts. Wounds left by feeding leeches may become infected with secondary bacterial infections or fungus. There is also a risk of spreading viral diseases, the fish leech is known to be a factor in the spread of SVC (Spring Viremia of Carp).

Life Cycle

Reproduction occurs after several meals. Leeches are oviparous, laying their eggs in dark brown oval cocoons attached to debris on the bottom of the pond, plants, planting baskets, etc.

The life cycle of Piscicola geometra takes up to 30 days. The life cycle of other species may take anything from 24 days to several months depending on water temperature and avaiability of hosts.

Symptoms

Attached fish leeches can be easily seen without magnification even from a distance. They measure up to 25mm in length and appear to be pale brown, or brown and white striped, in colour.

Leeches may cause the fish to flash or rub against the sides and bottom of the pond to relieve the irritation. The fish may also be lethargic, stop feeding and sit on the bottom with their fins clamped. The skin may have a milky appearance due to an increase in mucus secretion.

photo © Ondrej Zicha, www.biolib.cz, ondrej.zicha@gmail.com

Treatment

The most effective chemical treatments against fish leeches were organophosphates, Dimilin, Masoten and Malathion, but they are now banned in the UK for use in fish treatments.

The attached leeches can be carefully removed with tweezers, the fish may need to be anaesthetised for this. Once removed the wound should be treated with an antiseptic spray or gel to reduce the risk of secondary bacterial infections or fungus. Alternatively, Malachite Green can be applied with a cotton bud. Because leeches will still be present in the pond, the fish are likely to be re-infected when returned to the pond.

The only way to eradicate leeches is:
1. Move the fish from the pond to an aerated holding/quarantine tank, taking care to remove all attached leeches. Salt may be added to aid recovery and reduce the risk of secondary infections.
2. Leave the pond empty of fish for about a month to break the life cycle, the newly hatched leeches will soon die with no hosts to feed on. Alternatively, drain the pond and leave it dry for about a week. Drying will destroy the eggs as well as the adult leeches.
This is fairly easy to do with a small pond but not practical for a large pond or lake.

Photo © Ondrej Zicha, www.biolib.cz, ondrej.zicha@gmail.com

I have read that numbers can be reduced by hanging a small piece of bloody meat in the pond on a piece of string to attract the leeches. The feeding leeches can then be removed and destroyed. Repeat with a fresh piece of meat until no more leeches are found. This will not completely eradicate the leeches but may help in a large pond or when it is otherwise impractical to move the fish out.

As with all parasites and disease, prevention is better than cure. Poor water quality, overcrowding and poor nutrition will cause stress and weaken the immune system. Healthy fish will be able to fight off most diseases and parasites.

Before using any chemical treatments it is advisable to test the water quality. Some symptoms, such as flashing, may be caused by high nitrite levels. Adding chemicals in these circumstances will do more harm than good.

Midge Larvae

Don't confuse fish leeches with midge larvae, which are often found on the sides of the pond and in the filter. Midge larvae are only about 10mm long and usually, but not always, darker in colour. They are harmless and an excellent free source of natural protein. Don't reach for the chemicals, feed them to the fish.

Back to top