Scottish Animal Welfare Commission: welfare of cleaner fish used in the Scottish salmon industry - report

5. Ethical review

The aim of the ethical review was to consider the ethical issues around the use of cleaner fish as a form of sea louse control in salmon farming. We considered the situation from the point of view of the cleaner fish and the salmon, but also considered the wild populations of each species. We used an ethical matrix that considers welfare, freedom, flourishing and fairness to guide our thinking but here we present a summary of the issues.

Ethical issues:

The welfare of the salmon vs. the welfare of the cleaner fish

The farming and use of a species (cleaner fish) to improve the health, welfare and productivity of another species (Atlantic salmon) is almost unique in food animal production. The interviewed NGOs were all concerned about the welfare of cleaner fish in a system where the welfare of the salmon will be very likely prioritised over the welfare of the cleaner fish. There is a concern that, although there are costs associated with the purchase and maintenance of the cleaner fish, they are not the final saleable product of the salmon farms, so their welfare is not considered as high a priority by the salmon companies. Most companies have dedicated cleaner fish managers, who have responsibility for the cleaner fish, and there are codes of practice and welfare standards specifically for the cleaner fish, but the ultimate priority likely lies with the salmon. Some NGOs would prefer that cleaner fish were not used at all in salmon production, but recognised that until another reliable method of sea louse control is found, the use of cleaner fish is important for the welfare of the salmon and for having a limited impact on the environment around the sea pens.

Welfare benefits for the salmon

In most cases, the salmon benefit from the presence of the cleaner fish, as their activity reduces the level of infestation of sea lice. A lower parasite burden also means that the salmon have better immune function and are more resistant to other diseases. The use of cleaner fish reduces the need for more aversive treatments such as chemotherapeutics and thermal delousing treatments, which are stressful for the salmon, and may be associated with higher mortality.

Life in the wild vs. life in captivity for cleaner fish

Compared with free-living ballan wrasses and lumpfish, cleaner fish that are reared in hatcheries and deployed onto salmon sites experience a less enriched and varied physical and social environment. However, they may experience less hunger, predation, disease, parasitism and fear (of predators, etc.). Hatchery-reared fish are vaccinated, but disease pressure may be high in salmon farms, depending on the level of biosecurity, presence of vectors in the environment and management factors.

Effects on the wild population of cleaner fish

It is possible that there may be adverse effects of the removal of eggs, juvenile or mature fish on the wild populations of cleaner fish. The wild population may become less viable because of a reduction in numbers. Likewise, if numbers are reduced, the opportunity for protection against predators provided by a larger school of fish is reduced. Additionally, the sequential hermaphroditic nature of ballan wrasse, with females transitioning to males after 6 to 7 years, and males only making up 8-12% of the overall population (Pritchard et al., 2025b) means that avoiding catching these males is important for the sustainability of the population. Encouragingly, a recent study assessing data from Scottish fish landings data showed that no ballan wrasse males were caught (only one fish likely in transition), but 4.4% of fish caught were outside the size limits imposed. However, there are no data on population sizes or trends to allow us to assess the effect of fishing on the wild populations.

Effects on the wild population of salmon and other wild fish species

The wild salmon population may benefit from the presence of cleaner fish within the sea pens. If the number of sea lice within the pens is reduced, there are likely to be fewer lice moving from the sea pens into the surrounding seawater and infecting the wild salmon population. All wild fish species may benefit from the use of cleaner fish in salmon production, as there may be a reduction in the use of chemotherapeutics, which means that less of the residue reaches the wider sea environment.

However, the effects of removal of cleaner fish from the marine habitat on the parasite load of resident fish may be an issue, but there are no data to understand if there is any effect. Additionally, some species of fish caught as ‘bycatch’ during the wild-capture of ballan wrasse have potential to be vulnerable to overexploitation (Pritchard et al., 2025a), suggesting that more refined methods of capture should be implemented.

Ethical consideration of early euthanasia for cleaner fish

Ideally, the cleaner fish would be used in more than one salmon production cycle. Ethically, it seems wasteful to sacrifice healthy cleaner fish because of difficulties with biosecurity nor utilise them for some consumptive use. Some of the salmon companies are actively considering options to reduce this wastage.