Sea lice mitigation events at B.C. salmon farms

Sea lice occur naturally in the marine environment. Their abundance is influenced by seasonal and annual variations in wild salmon populations, ocean salinity, temperature, and other environmental factors. Conditions of Licence for marine salmon aquaculture require monitoring and reporting of sea lice throughout the year.

From July to January, salmon farmers must monitor and report sea lice abundance on a regular basis. If the management threshold is exceeded, licence holders must increase monitoring and reporting and may undertake voluntary sea lice treatments. In February, additional monitoring is required and licence holders must ensure that the abundance of sea lice on the farm is below the management threshold at the beginning of the out-migration window from March to June, when migrating wild juvenile salmon could be more vulnerable to sea lice. During the out-migration window, sea lice management requirements are elevated and if the abundance of sea lice at a farm exceeds the management threshold, licence holders must take action to ensure that the sea lice abundance is below the threshold within 28 days. Until July 2024, licence holders had 42 days. The current threshold at which action is required is 2.4 motile sea lice per fish. In 2025, the threshold was 2.8 and before 2025, it was 3 motile sea lice per fish.

Chinook salmon are typically less susceptible to sea lice than Atlantic salmon. Therefore, farms growing Chinook salmon are only required to monitor sea lice opportunistically and rarely require intervention to keep sea lice levels low.

Mitigation approaches

Companies growing Atlantic salmon use an integrated pest management approach to manage sea lice. This means using multiple treatment methods, often together, to reduce or prevent sea lice infections on farms. In B.C., in-feed medication, mechanical removal, and medicinal or non-medicinal bath treatments are utilized. This approach decreases the reliance on one treatment type, reduces the likelihood of resistance developing, and allows the use of the most appropriate tool for different situations. While mechanical and bath treatments are very effective, attached stages of sea lice are not always removed. These lice can continue to grow to motile stages, requiring repeat treatments. Mechanical removal and bath treatments require fish to be handled, which may lead to stress and mortality. Conversely, in-feed treatments do not require handling and provide longer lasting protection from lice re-infestation; therefore, companies often utilize a combination of approaches. In some cases, licence holders may choose to harvest fish in a timely manner to reduce sea lice.

Conditions of Licence require licence holders to notify us when treatments occur, and all captured sea lice must be prevented from re-entering the marine environment for all types of treatments where collection is currently possible.

In-feed treatment

When added to feed, emamectin benzoate, or “SLICE©”, an anti-louse treatment, kills all attached and motile stages of lice. This type of treatment provides residual protection, which can last for several weeks. In-feed treatments can be used to treat active infections as well as to prevent future infections. Licence holders must monitor the effectiveness of in-feed treatments in order to protect against the development of resistance. Any reduced efficacy must be reported.

Mechanical removal

The “Hydrolicer©”, is a type of mechanical removal treatment, which uses a spray of water to gently remove motile stages of sea lice from farmed Atlantic salmon. On board the treatment vessel, fish are individually sprayed and lice are removed and retained. The fish are then returned to the marine net-pens. Due to the simplicity and effectiveness of this method, it has become one of the most commonly used sea lice mitigation tools in B.C.

Medicinal bath

A medicinal bath with hydrogen peroxide, or “Paramove 50©”, can be used to remove motile stages of sea lice from farmed Atlantic salmon. These treatments are most commonly delivered in large well boats where the concentration of medication and water quality can be closely monitored. Lice are removed and retained, then the fish are returned to the marine net-pens. Hydrogen peroxide dissipates quickly after being discharged into the marine environment, away from ecologically sensitive habitats. The Aquaculture Activities Regulations require all licence holders to submit notifications for planned pesticide deposits at least 72 hours prior to usage and notifications of any observed wild fish mortalities following a pesticide deposit.

Non-medicinal bath

A non-medicinal bath treatment of fresh water can be used to remove motile stages of sea lice from farmed Atlantic salmon. Sea lice live in saltwater environments and drop off the salmon with prolonged exposure to fresh water. These treatments are most commonly delivered in large well boats filled with fresh water, where the water quality and fish health can be closely monitored. Lice are removed and retained, then the fish are returned to the marine net-pens.

Harvest

Harvesting may be used to reduce the absolute number of sea lice at a facility. In some cases, where a facility is near harvest or other treatment options are not viable, licence holders may choose to harvest fish rather than employ other mitigative measures.

Reports

The following figure shows an annual breakdown of sea lice mitigation events at B.C. salmon farms. These events are categorized according to the type of treatment used as described above. In cases where an event utilizes multiple treatment types, the primary type is reported on the graph. Detailed sea lice mitigation reports are available.

Graph: sea lice mitigation events at salmon farms in B.C., 2011 to 2025

Related links

• Average number of lice per fish on B.C. salmon farms
• DFO sea lice audits of BC marine finfish aquaculture sites
• Industry sea lice counts at BC marine finfish aquaculture sites
• Sea lice mitigation events at BC marine finfish aquaculture sites
• DFO marine finfish aquaculture audit activities in British Columbia