Identification and Control Methods for Fish Lice (Argulus sp.)

Signs of Infection Fish lice are disc-shaped parasites, 8 to 12 mm in diameter, and come in various colors, from light green to brown. They can be seen attached to the fish's body and fins.

Fish Affected Fish lice can infest many species of coldwater fish. They are almost impossible to find on tropical species unless the parasites are brought into the aquarium on live food or plants. Even then, fish lice cannot survive for long in warm water.

Details of Infection These parasitic crustaceans have eight feathery legs, which they use to swim in search of a host fish. The small tail is used for steering and contains the reproductive and respiratory organs. On the underside of the body, two powerful suckers allow the fish louse to firmly attach itself to the host's scales. Between the two suckers is a venomous needle, which can inject an anticoagulant into the fish's body and connect with the body tissues via the needle.

There are about 200 species of fish lice worldwide. The most common are Argulus foliaceus, Argulus coregoni, and Argulus japonicus.

Fish lice usually lay eggs in midsummer, on stones or other solid surfaces, and about a month later the larvae hatch and swim freely until they find a host fish. When satiated, the larvae move freely from one fish to another, sometimes going without food for several days. Five or six weeks after hatching, they are able to reproduce. Fish lice are suspected of possibly transmitting diseases, such as dropsy.

Recommended Treatment Fish lice can be removed with tweezers. If necessary, lightly brushing with a paintbrush dipped in strong salt water, or soaking in a salt solution, can help remove them. An alternative method is to add a new type of aquarium anti-crustacean treatment to the water. Trichlorfon is effective, but must be used according to the instructions.