They are released into the water by adult mussels and, when a fish passes close enough to disturb them, the glochidia attach themselves to the skin or gills of the fish by means of their barbed valves. The larvae (glochidia) of freshwater mussels are parasitic on fish. They leave characteristic Y-shaped scars, which may be susceptible to infection by other parasites. They attach themselves to the skin with their sucker and feed on the blood of the host. Leeches are also parasitic on freshwater fish. There is no means of control available, and several waters in south-western Victoria now contain fish populations which are heavily infested with this nematode. Fillets taken from infected fish may have several worms in them, but if the flesh is cooked properly the worms are easily removed and the flesh is quite safe to eat.Įustrongyloides is spreading steadily through Victorian waters, lakes particularly, and this is due to the natural cycle of fish-eating birds consuming infected fish and transferring either live worms or worm eggs to other waters. Heavily-infected fish may be severely emaciated and have numerous lumps under the skin, each cavity containing a thin, coiled, red worm about 20-30 mm long. The adult trematode is generally found infecting fish-eating birds.Įustrongyloides is a nematode worm, the larval form of which is usually found in the muscle and internal organs of trout, redfin (English perch) and some native fish. The organism causing "black spot" has been previously identified as Neascus, but there are several species of trematodes which have larval stages which cause black spots these species have yet to be identified. "Black spot" infections occur in several species of freshwater fish, but galaxiids appear to be particularly susceptible to infection, with some fish carrying hundreds of cysts on the body and fins. The cyst then becomes surrounded by pigment cells, giving it the characteristic dark colour. When the parasite infects the fish it forms a cyst within the host tissue. Several larval trematodes infecting fish causing what is commonly known as "black spot" because of the characteristic, small (about 2 mm in diameter) dark brown or black spots which develop in the muscle and on the body, fins, gills and eyes of infected fish. Galaxiids (minnows), particularly the Common Galaxias, are often infected by Ligula. Heavily-infected fish are weak and slow-moving, making them an easy target for predators. The contained worms are white, segmented and may be up to 200 mm long, often longer than the infected fish. One such tapeworm is Ligula whose life cycle consists of several stages which are parasitic in turn on crustaceans, fish and birds.įish infected with Ligula exhibit grossly distended abdomens. Several cestodes (tapeworms) and trematodes (flukes) are parasites on freshwater fish. Ichthyophthirius infections are known commonly as "white spot" because the parasite causes the formation of small, white cysts on the skin and fins of infected fish. Protozoans cannot usually be identified except under a microscope, but disease symptoms, such as a grey cloudiness of the skins and fins, laboured respiration, emaciation and unusual swimming behaviour, may give some clue to the causative agent. Protozoan (single-celled animal) parasites, such as Costia, Chilodonella, Triochodena and Ichthyophthirius, may infect fish, especially under aquarium or intensive culture conditions. The fungus is often identified as Saprolegnia, but several other species of fungus may also infect fish. Usually fungus infects only fish which have suffered some wound or damage or are weakened or stressed from some other disease. Fungal Infectionsįungal infections are characterised by white growths, like cotton wool, on the body and fins of infected fish. In this situation fish will often lose condition, making them more susceptible to predation, or may even die from the effects of the parasites.įish which are injured in some way, such as after an attack by a predator, may carry a wound which is then infected by a parasite. However, in times of stress, resistance of fish is often lowered and some parasites may greatly increase in abundance and affect the health of the fish. These are sometimes obvious but more often are difficult to detect other than by specialist techniques and usually appear to have little effect on the host fish. Most fish in the wild carry some parasites. Further details of their life histories, effects on fish and methods of control are given in the references. Some of the more obvious or commonly encountered parasites of freshwater fish are briefly described below.
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