Parasitic copepods in fish

Description

Parasitic copepods are small crustaceans that can be found on the skin, gills, fins and mouth of freshwater and saltwater fish.

Wild animals at risk

Copepods feed mainly on the blood and flesh of freshwater and saltwater fish. Fish heavily infested with copepods are rare. In Québec, one copepod in particular affects salmonids, notably Arctic char, brook trout and lake trout.

Signs of the disease

Copepods are often visible to the naked eye and can measure between 0.1 and 300 mm. They tend to attach themselves to the same host. It is usually adult copepods that parasitize fish. Females mate before attaching permanently to the fish. The parasites usually attach to the base of fins or gills, or in the oral cavity, depending on the species and size of the infested fish. They can also be attached to the entire body of the fish. Lesions may appear where copepods have attached themselves, causing damage to scales, skin and gills.

Parasite of rainbow smelt © Kathleen Sévigny, Ministère de l’Environnement, de la Lutte contre les changements climatiques, de la Faune et des Parcs

Life cycle and persistence in the environment

Copepods can swim to infect a fish. Some copepods can also move from one fish to another through direct contact. Survival, development, growth and infection rates depend on salinity level and water temperature. The likelihood of the copepod detaching from the fish or dying increases when conditions are less than optimal. Ideal temperature and salt level vary according to copepod species. Some can survive in salt water on anadromous fish, i.e. freshwater fish that spawn in salt water.

Several factors contribute to the spread of copepods:

The natural movement of water and fish;
Transfer of infected fish (e.g., bait fish, stocking, introduction of a species into another body of water);
Transfer of contaminated water (e.g., ballast water ^{Read the content of the note 1} , fish transport water);
Use of contaminated boats or equipment.

When the egg hatches, a larva usually emerges. The larva moults into the adult stage and attaches itself to a host. Copulation takes place on the host. The females remain attached to their host. They can produce several batches of eggs during their lifetime. The females lay their eggs in the water and the cycle begins again. — Copepod life cycle ©MELCCFP, Ariane Côté

Protection and prevention

To prevent the contamination of water bodies by copepods, it is important not to transport fish from one body of water to another, and not to discard fish waste (e.g., viscera) in the water. Boats and equipment should also be cleaned thoroughly before and after visiting a new body of water.

Risk to wildlife health

In small numbers, copepods are fairly harmless in the wild, unless they attach themselves to vital organs such as gills, or if a secondary infection (virus, bacteria or fungus) develops. Some copepods can cause mortality episodes, especially in an aquaculture context.

Risk to pet health

Copepods pose no risk to pets. They can be found on fish in aquariums and artificial ponds.

Risk to human health

Copepods do not pose a danger to humans. They can be manually removed from the fish before cooking. It is always important that we take the usual precautions prior to consumption of wild fish.

Monitoring and control

It is well known that parasitic copepods are present in Québec, so there is no need to report suspect fish.