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SUSTAINABILITY LIBRARY 2023 Fish health and fish welfare Sea lice

Sea lice


Salmon and trout farming entails a relatively higher level of host density compared to in the wild. Infection pressure of wild salmon stocks with sea lice from farmed salmon is one of the main challenges in terms of environmentally sustainable aquaculture.

Due to the host density in farming, Lerøy attempts to keep average numbers of mature female sea lice as low as possible throughout production. Controlling sea lice levels entails higher costs and have the highest priority in Lerøy.


Salmon lice live naturally in the sea and will be able to use our fish in aquaculture as host animals. It is important for Lerøy that we monitor the level of lice in the sea and on fish when the fish is under production. Undesirable quantities of lice will increase the risk that possibly escaped fish from our farms, and smolts that are migrating from the river and passes our farms to the sea, may be attacked by lice. If the lice surcharge becomes too high on our fish, it may exceed the limit values in Norwegian regulations and lead to increased costs for the company, fines, downgrading of fish, reduced fish welfare and, in worse case, fish mortality. Lice infestation on wild smolt could have a negative impact on this species.

Our ambitions in this area

The Group´s efforts to control salmon lice follow the principles of an IPM strategy to control salmon lice and keep the numbers at a low level in the long term. The overall goal is to reduce any negative impact on the environment and the need for active interventions.

The overall principles underlying this control strategy includes establishing acceptable levels (see details in Policy), preventive measures (as structural measures relating to use of locality, coordinated operations over larger geographical areas, zone collaboration, fallow periods in between production cycles, smolt quality and smolt weight as measures to reduce period of exposure, use of various types of physical barriers, (e.g. skirts), monitoring, biological control (i.e. cleaner fish) and active interventions.

We work to minimize levels of adult female lice per fish as close to zero as possible.

How we work in the area

We use targeted measures to reduce the number of lice by reducing exposure in the sea. To do so, we produce larger smolt using RAS technology so we can have shorter production periods in the sea. Biological delousing using our own produced cleaner fish is also a target area for Lerøy.

Since 2010, Lerøy Seafood Group ASA (LSG) has developed technology for semi-closed facilities. This technology takes water from depths of 20-30 metres and transports this via a laminar flow in a tube. Water from these depths helps reduce infection exposure for the fish in the facility as the majority of lice larvae are found in the upper water layers, down to depths of 10 metres.

Requirements from the authorities provide a definition, at any given time, of the upper maximum limit for fully grown female lice. Lerøy has also established their own limit values to provide guidelines for when to implement preventive and active measures to control sea lice levels.

The average values for lice in the facilities shall be reduced by more than 50% during the period from April to June when compared with the rest of the year. This is when wild salmon migrate as smolt from the rivers to the oceans.

How we measure our impact

Sea lice counts and registration are carried out at a minimum of every seven days for all cages in each facility. This is either done manually by catching the fish in a landing net or automatically by underwater camera technology. If done manually, the fish are anaesthetised in tanks and controlled individually. A representative selection of fish is taken from each cage. Lice are counted in the following categories in terms of stages:

1) Adult female (with and without egg strings)

2) Mobile pre-adults (including adult males)

3) Attached juveniles

An average figure is calculated by totalling the number of all lice from all fish (plus lice in the tank for counting) and dividing this number by the total number of fish studied. The weighted average for the farm is calculated based on sea lice counts and the number of fish in each pen.

Infestation of wild fish by lice from Norwegian fish farms is calculated regionally by the Institute of Marine Research, by counting lice on wild salmon and using modelling. To read the Institute's report for 2023, use the following link: Lakselus – risikovurdering og kunnskapsstatus 2023 | Havforskningsinstituttet (

Target and results

Target 2024:

  • No exceedances of limits set by the authorities
  • Number of red spots: 0

Limits for levels of adult female lice per fish set by the authorities varies throughout the year and differ depending on region. This is described in detail in the Group´s Policy for control of sea lice.

Norwegian regulations require that the average number of adult female lice per fish per farm must be below the maximum limit of 0,5 at any time. In addition, during spring where smolts typically migrate from the rivers to the ocean, the average number of female lice per fish per farm must be below 0,2.

This period is defined as:

Week 16 up to and including week 21 in Trøndelag County and further south.

Week 21 up to and including week 26 in Nordland County and further north.


  2023 2022 2021 2020
Average number of adult female lice per fish 0,18 0,18 0,18 0,16


  2023 2022 2021 2020
Number of cages treated for lice ( number)  1772 1853 1576 1428


  2023 2022 2021 2020
Volume of delousing agents used via bath (kg active substance) 26626 1553 3171 28
Alphamax 0 1,42 0,83 0,64
Azasure 0 0 16,8 20,78
Salmosan 67 44,5 103,4 6,3
Ectosan 3131 1 507 3 050 0
Hydrogenperoxide 23428* 0 0 0
Volume of delousing agents via feed (kg active substance) 9,1 14,1 23,3 30,4
Slice 9,1 14,1 23,3 30,4

* included AGD treatment

Action taken due to results

Lerøy’s efforts to control salmon sea lice shall continue to follow the principles of an IPM strategy to control salmon sea lice and keep the numbers at a low level in the long term. The goal is to reduce the negative impact on the environment and the need for active interventions. Below is a description of the overall principles underlying this control strategy.

Preventive measures: Prevention is the first line of defence, aiming to control the levels of salmon sea lice by making use of passive control mechanisms. The methods utilised depend on local prerequisites and entail use of one or more of the following measures: Structural measures relating to use of locality, zone collaboration, fallow periods in between production cycles, post smolt strategy and different types of physical barriers.

Biological control: cleaner fish represent a method for treatment prevention, and the goal is to reduce the need for active measures.

Non-medicinal methods: These methods are based on different types of measures that do not include the use of medicines. Lerøy makes use of methods within the categories for fresh water, flushing and temperate water.

Medicinal methods: These methods are based on the use of medicines. In cases where medicines are utilised, these shall be 1) prescribed by authorised fish health personnel and 2) evaluated with regards to the risk for fish welfare, food safety, environment and resistance, and 3) where only medicines approved by Norwegian medicines authorities can be used.

New semiclosed technology: Lerøy has made significant investments (>500 000 000 NOK) in coastal production technology recently. A combination of submersible and semi-closed cages is now being rolled out at selected locations in Lerøy Midt and Lerøy Sjøtroll (13%  of total biomass)). We believe this will have a positive impact on fish health, and reduce the lice load. This is because the cages are lowered below lice level in the sea, thus protecting the fish from lice.

The aim is for 30 percent of the salmon to be in shielding technology at the start of 2025.

Policy: Control of salmon lice