fish louse Argulus sturgeon parasite

The fish louse (Argulus) is another common parasite affecting fish. It belongs to the class Branchiurans, which are ectoparasites of fish (any external parasitic organism). they live mainly in freshwater habitats and have flattened bodies comprised of five limb-bearing segments; the head has well-developed carapace lobes.
This flattened body means that they have a low profile when attached to the host fish but the adult Argulus can be seen without magnification, measuring up to 10mm in diameter.
Branchiurans are all fish parasites, they have a tubular sucking mouth which has rasping mandibles found at the tip of the mouth tube. In Argulus there is also a retractable poison emitting stylet located just in front of the mouth.This is used to inject a secretion into the host. The secretion may contain digestive enzymes to begin to break up host tissues before ingestion.
Argulus attaches itself to the skin or fins, inserts it's tubular mouth into the flesh and feeds on the fishes blood and external tissues. This, coupled with the use of other hooks, barbs and suckers with which Argulus attaches itself causes considerable damage to the fish. In addition to physical damage, affected fish are subjected to severe stress, which often leads to secondary parasite infestations such as white-spot and Costia,reddened lesions develop at the feeding site and may become infected with bacteria or fungus. This type of combined attack on stressed and often weakened fish can result in high numbers of fatalities.

Life Cycle

The female Argulus will leave the fish to lay eggs on plants and other submerged objects. For egg production to take place the water temperature needs to be above 16 degrees C. Eggs take 3-4 weeks to hatch during summer temperatures. They can also over-winter and hatch in the spring when temperatures become more favourable.
Dependant on temperature the cycle takes between 30-100 days.

Symptoms

flashing and rubbing - to dislodge parasite
lethargy - if seriously infected
excess mucus secreted by skin, fins and gills

Treatment

Organophosphates were the most effective treatment against Argulusbut they are now banned in the UK for use in fish treatments.

There are no other treatments currently available that are likely to be totally effective.
 A salt bath may be effective for individual fish but unfortunately there is no simple solution to erradicating these parasites from a seriously infested pond.
As always with all parasites and disease, prevention is better than cure. Healthy fish will be able to fight off most diseases and parasites. Ensure that water quality is at it's best, there is no overcrowding and use the best feed possible in order to prevent stress which will weaken the fishes natural immune system.

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