Cleaner fish and the risks of transmitting infection to farmed salmon

This is the key message in a risk assessment conducted by the Norwegian Scientific Committee for Food and Environment (VKM) on behalf by the Norwegian Food Safety Authority.

Risks of infection and disease

Cleaner fish comprises one of several methods against fighting salmon lice infestations.

Current regulations require removal of all fish, including cleaner fish, from the aquaculture farm following completion of each production cycle of salmon. The Norwegian Food Safety Authority considers changing the legislation, so that cleaner fish may be reused over multiple production cycles.

VKM has assessed the risks of transmitting infection and disease should cleaner fish be reused. In addition, VKM assessed whether relocation of cleaner fish may result in the spread of pancreas disease, from areas endemic for this disease to areas free from it.

Largest risks from wild-caught cleaner fish

Both wild-caught and farmed cleaner fish are used to fight salmon lice.

"Knowledge about diseases and infections in wild-caught cleaner fish is generally poor. Use of such fish may result in exposing farmed salmon to new pathogenic agents. Therefore, wild-caught cleaner fish represent a larger risk of transmitting infection than farmed cleaner fish," says Espen Rimstad,spokesperson for this opinion.

The risks can be reduced by stocking and inspecting these fish prior to releasing them into the net pens.

Amoebic gill disease

VKM concludes that amoebic gill disease is currently the single known disease, for which there are justified suspiscions of disease transmission from cleaner fish to farmed salmon.

"Farmed salmon are vaccinated against agents like furunculosis and classical vibrosis. We consider the risks of transmitting bacterial disease to be low, although this cannot be dismissed," says Rimstad.

Possibilities to reduce risk

Lumpfish and wrasse fish are most common for use as cleaner fish. Lumpfish is very seldom reused. The probability of transmitting infection due to reuse of cleaner fish therefore concerns almost only wrasses.

"Risks could possibly be reduced by avoiding reuse of cleaner fish that have been in contact with sick salmon. Measures such as frequent and complete fallowing of the facility, (both for cleaner fish and salmon), quarantine stocking and fish health inspections, and reusing only a small fraction of the cleaner fish, may alleviate the risks," explains Rimstad.

Data gaps

Knowledge on transmission of pathogens from cleaner fish to farmed salmon is in general limited. In particular information on the disease status in wild-caught cleaner fish is scarce.

Factors such as lack of basic knowledge on disease development, and absence of specific diagnostic tools for cleaner fish infections, prevent a more complete assessment of risks from being made.

The Panel on Animal Health and Welfare is responsible for this opinion.