Diseases and parasites in wild fish

Abnormalities, lesions, or parasites may sometimes be observed in aquatic wildlife, in all types of environments. These natural phenomena are not necessarily related to water or habitat quality. They depend notably on the presence of all living organisms in the ecosystem, such as crustaceans, birds, and predators, which enable their development.

Most fish diseases are harmless to humans. In addition, parasite transmission between fish and humans is rare. However, certain parasites can cause health problems. To avoid them, it is recommended to take certain precautions when consuming wild fish.

When sick fish or abnormal fish deaths are observed in the natural environment, an assessment of the magnitude of the situation is carried out and, if possible, the cause of the mortality is identified. Most abnormal mortalities are attributable to specific environmental causes, such as high water temperature, lack of oxygen in the water, or the presence of cyanobacteria.  

A thorough investigation of reported cases may prove necessary in the following situations: 

• The mortality is unexplained;
• A new pathogenic organism is suspected;
• The number of affected fish is high;
• The affected species are vulnerable, such as species at risk and sport fish species.

In Québec, several diseases and parasites (PDF 10.98 Mb) (in French only) affecting various fish species have been documented, as well as diseases that could be introduced: 

• Infectious salmon anaemia
• Anguillicola crassus
• Parasitic copepods in fish
• Diphyllobothrium
• Heterosporis sp
• Lymphocystosis
• Black spot disease in fish
• Fish lice
• Saprolegniosis in fish
• Walleye dermal sarcoma
• Viral hemorrhagic septicemia
• Blotchy bass syndrome
• Red vent syndrome
• Whirling disease

Various surveillance operations are carried out to:

• assess the health status of Québec’s wild fish populations;
• detect the presence of diseases or parasites that may affect wild fish or human activities, such as fishing and aquaculture;
• monitor the progression of existing diseases and respond quickly if a new pathogenic organism appears. 

Surveillance reports

Each year, the Gouvernement du Québec publishes a report on reported cases.  

These reports present the results of the analyses carried out, the main causes of observed mortality, the affected species, and the areas concerned. 

• Wild Fish Health Surveillance 2025 (in French only)
• Surveillance reports from previous years (in French only)
• Wildlife Disease Surveillance 2015-2020 (in French only)

Everyone’s participation is essential to slow the introduction and spread of aquatic diseases and parasites.  

Here are some good practices to adopt:  

• Thoroughly clean, drain, and dry your boat (french only), equipment, and footwear before visiting another waterbody.
• Do not throw organic waste, such as fish remains, into waterbodies or on shorelines. Instead, place them in the trash.
• Respect the regulations concerning the use of live and dead bait.
• Do not move live fish from one waterbody to another without authorization.
• Report any abnormal mortality or suspicion of an officially reportable disease to the competent authorities. 

It is important to report sick or dead fish to quickly detect concerning situations, protect wild fish populations, and prevent the spread of diseases.

Certain parasites or pathogenic organisms are commonly present in Québec and do not pose a significant risk to fish populations. It is therefore not necessary to report an isolated fish carrying known parasites, especially if its behavior appears normal.  

Abnormal mortality

You must report a situation if you observe several dead fish without an obvious cause (abnormal mortality).  

These observations must be reported to the direction régionale de la gestion de la faune in your region (French only). 

Officially reportable diseases

Certain fish diseases are officially reportable in Canada and must be reported without delay to the Canadian Food Inspection Agency (CFIA) as soon as a suspected case is observed.

Among the diseases present or under surveillance in Québec’s wild fish, the following are notable: 

• Infectious salmon anaemia
• Viral hemorrhagic septicemia
• Whirling disease