Sea lice mitigation events graph

Sea lice occur naturally in the marine environment and their abundance is influenced by seasonal and annual variations in wild salmon, ocean salinity, temperature, and other environmental factors. Our conditions of licence for marine finfish aquaculture require monitoring and reporting of sea lice throughout the year. There are additional monitoring requirements for Atlantic salmon facilities in the month of February, leading up to the juvenile wild salmon out-migration (March 1 to June 30) when wild salmon could be more vulnerable to sea lice and may be found closer to facilities. Atlantic salmon facilities must be below a threshold of three motile salmon lice (L. salmonis) per fish at the first counting event of the out-migration window, and attempt to stay under this threshold through the rest of this time period. If they exceed the threshold, they must take action to reduce levels. Due to species differences and susceptibility to sea lice, Chinook salmon facilities (currently the only species of Pacific salmon farmed in BC) do not usually require mitigation as sea lice levels typically remain low.

Motile refers to a developmental stage of the sea louse. They initially start as free floating eggs and larvae. If they find a host, they physically attach to the fish until they grow to the motile stage, where they can move independently on the fish surface. Therefore motile sea lice are those pre-adult to adult lice observable on fish.

Atlantic salmon aquaculture companies use an integrated pest management approach to manage sea lice at marine facilities. This means that numerous treatment methods are developed, including in-feed medication, mechanical removal, and medicinal and non-medicinal bath treatments. 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, and these lice can continue to grow to motile stages, requiring repeat treatments. Additionally, unlike in-feed treatments, bath and mechanical removal treatments do not provide residual protection from new infections and require a lot of handling, which may lead to increased stress and mortality.

In some cases, licence holders may choose to harvest fish in a timely manner to reduce sea lice numbers rather than use other mitigation measures. Conditions of licence require licence holders to submit notification of sea lice mitigation events and to prevent captured sea lice from re-entering the marine environment for some types of treatments.

The figure below shows an annual break-down of sea lice mitigation events used at marine finfish aquaculture facilities in BC. These events are categorized according to the type of mitigation:

In-feed treatment

An anti-louse treatment of emamectin benzoate, or “SLICE©”, is added to the feed and kills all attached and motile stages of lice on farmed Atlantic salmon. 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. Aquaculture operators must monitor the effectiveness of in-feed treatments in order to protect against the development of resistance. Any reduced efficacy must be reported to us.

Mechanical removal

A mechanical removal treatment, the Hydrolicer©, uses a spray of water to remove motile stages of sea lice from farmed Atlantic salmon. The fish are caught using seine nets and pumped onto a treatment vessel where they are individually sprayed, lice are removed and collected, and the fish are returned to the net pen.

Medicinal bath

A medicinal bath treatment of hydrogen peroxide, or “Paramove 50©”, can be used to remove motile stages of sea lice from cultured Atlantic salmon. These treatments are most commonly delivered by pumping the cultured fish into large well boats where the concentration of medication and water quality can be closely monitored. After a short treatment, the fish are returned to the marine net pens. Hydrogen peroxide dissipates quickly after being discharged back to the marine environment, but use of a well boat gives farmers the ability to discharge used bath water 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.

Non-medicinal bath

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

Harvest

Harvesting is an approved approach to reducing 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. During the juvenile salmon out-migration period (March through June), licence holders must harvest all fish, or bring the average sea lice abundance below the threshold within 42 days of exceedance.

Graph: sea lice mitigation treatments at marine finfish aquaculture sites in BC, 2011 to 2023

Detailed information on specific sea lice mitigation events can be found in DFO’s Sea lice mitigation events at British Columbia marine finfish aquaculture sites report.