Know your Scottish Salmon

Chemicals in salmon farming – an update on SLICE® (emamectin benzoate)

A long-awaited review of the insecticide emamectin benzoate (EMB), better known as SLICE®, was  published in 2022. The  in-feed treatment used to treat parasitic sea lice in Scottish salmon farming, it is the only application for which it is used in the UK. This is despite being intended for use in agricultural settings on a variety of crops and to treat some tree diseases.



High time to limit the use of SLICE®

A previous Fidra blog[i] looked at the use of SLICE®, and the impacts of EMB getting into the environment through uneaten food and faeces of farmed salmon.  It is found in the sediment underneath, and the water around, the open net pens of the salmon farm.  This has long been of concern due to EMB being toxic to mammals, birds, fish, invertebrates, crustaceans and aquatic plants[ii]As an insecticide it is currently only approved for use in the UK as a veterinary medicine for the treatment of sea lice in farmed Atlantic salmon

The Scottish Government is supportive of growth in the salmon farming industry, which may happen in several ways, such as increased production through larger farms or lower mortality rates. Certainly some of the increasing salmon production is likely to be in open net pens, and also likely to lead to an increase in sea lice numbers. If a farm has persistent problems with lice and is reliant on harmful treatments to manage infestations, it is not a viable farm location and is not sustainable into the future.  This is particularly true if Scotland want to be viewed as an exemplar of best practice in aquaculture.


New proposed guidelines for emamectin benzoate

In 2019, recommendations on new environmental standards for EMB were made to the Scottish Government by the UK Technical Advisory Group (UKTAG), a partnership of the UK environment and conservation agencies, and consulted on.  The resulting UKTAG Environmental Quality Standards (EQS) Recommendation for EMB were finally published in 2022[iii]. EQS are legally binding limits set in legislation, in the form of concentrations of individual substances. An EQS is for specific substances in specific environments (i.e. the marine environment), which sets concentration thresholds below which, no adverse impact on that part of the environment can occur. Long term and short term thresholds (EQS) have been set for EMB in sediment and water (Table 1).


Table 1: Summary of the proposed EQS with previous EQS for comparison

Previously two marine sediment EQS were derived, one protective of “near field” effects and the other protective of “far field” effects, in line with the approach taken to regulating the substance in Scottish fish farms before more recent changes under SEPA’s sector plan[iv]. This approach has been dropped and a single sediment EQS has been derived, following the EQS Technical Guidance[v] and designed to be protective of the marine environment as a whole.  The sediment EQS for EMB was originally set at 131 ng/kg/dwt but following intervention from Salmon Scotland indicating a potential error in calculation, UKTAG has changed the recommendation to 272 ng/kg/dwt.

Chemical use needs to go down, not up

According to the UKTAG report, EMB use at Scottish fish farms per unit biomass and application had on average increased from around 26 μg/kg biomass/year in 2002 to a peak of 67 μg/kg biomass/year in 2015. There was also an increase of average application rates from around 1.4 applications/site/year in 2002 to 2.68 applications/site/year in 2016[vi]. While the amount of biomass in Scottish fish farms doubled between 2002 and 2015, the total mass of EMB used in Scottish fish farms increased six fold over the same period.

The new EQS being set into legislation will need to be applied to existing as well as new salmon farm sites. However, there continues to be ongoing research, such as a  study on Salmon Parasite Interactions in Linnhe, Lorn and Shuna (SPILLS)[vii]. As research increases knowledge of the impacts of sea lice in and around salmon farms, and consequently the need for and impact of medicinal treatments, the EQS are likely to require revision in the future. In addition to this, the development and implementation of the Sea Lice Risk Assessment Framework should lead to salmon farms being sited in areas with lower risk of high sea lice infestation, resulting in a lower need to use EMB.

Risks of resistance

Resistance to delousing chemical treatments is increasing. Causes may be over-use of the product, and/or farms applying low doses that are not actually effective, but which enable the build-up of resistance. The long half-life of EMB has been considered to offer additional protection, but may also increase resistance by staying in the fish at levels below the therapeutic range for a long time. EMB resistance is believed to most probably have evolved in lice from a single farm source, which was then dispersed to lice throughout the North Atlantic in under 11 years[viii]. Studies on EMB and other treatments suggest this parasite is highly capable of developing and dispersing resistance quickly. Salmon from all parts of the North Atlantic can be infected with resistant lice in the open ocean where they meet and so bring resistant lice back to their countries. As a result, a large fraction of this species is exposed to chemotherapeutants over time and the life span of any given chemical treatment is likely to be limited. Researchers have as a result called for management plans of new and emerging chemotherapeutants to be agreed upon internationally[ix], an approach which should be applied to treatments that are already well-established, such as EMB.

What now for emamectin benzoate in Scotland

SEPA currently has an interim position while it waits for Scottish Ministers to update their Directions on environmental standards[x]. Under this, it will not change permits to discharge EMB authorised under the original standard of 763 ng/kg of sediment (wet weight). Permits for discharges granted under the previous interim standard of 23.5 ng/kg sediment will be varied.

With Scotland’s marine environment under increasing pressures from pollution, and the current biodiversity and climate crises, implementation of the new recommended standard and a decrease in EMB use cannot come soon enough.  Fidra is asking MSPs to direct the adoption of the new standards on Emamectin Benzoate recommended by UKTAG as soon as possible, across all existing as well as new or expanding farms, and commit to continual monitoring and regular review of the limits.





[iii] WFD-UKTAG (2022) UKTAG Environmental Quality Standards Recommendation for Emamectin Benzoate

[iv] Scottish Environment Protection Agency (2018) Finfish Aquaculture Sector Plan. Available at

[v] EU (2018) European Commission, Directorate-General for Health and Food Safety, Technical guidance for deriving environmental quality standards, European Commission (2018).

[vi] Scottish Environment Protection Agency (2017) Review of Environmental Quality Standard for Emamectin Benzoate, WRc report reference UC12191.03, February 2017. Available at

[vii] Moriarty, M. et al (2023) Salmon Parasite Interactions in Linnhe, Lorn and Shuna (SPILLS) Final Project Report. Available at

[viii] Besnier, F. et al (2014) Human-induced evolution caught in action: SNP-array reveals rapid amphi-Atlantic spread of pesticide resistance in the salmon ecotoparasite Lepeophtheirus salmonis. BMC Genomics, 15, 937.

[ix] Fjørtoft, H. B. et al (2020) Aquaculture driven evolution: distribution of pyrethroid resistance in the salmon louse throughout the North Atlantic in the years 2000–2017. ICES Journal of Marine Science, 77: 1806–1815.


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