Scottish Animal Welfare Commission: welfare of cleaner fish used in the Scottish salmon industry - report
Report on the welfare of cleaner fish used in the Scottish salmon industry produced by the Scottish Animal Welfare Commission.
4. Outcomes of evidence gathering
Structure of the review of evidence
The lifecycles of the fish species used as cleaner fish are complex and, in some cases, lengthy. In addition, the lifecycle of sea lice is relevant to this discussion. An overview of these cycles is given for information in Appendices IV, V and VI.
It became clear that there are a number of points across the lifetime of the cleaner fish where welfare issues are more likely to arise. It was also clear that at some of these points there are different issues for the two different cleaner fish species, and for the wild-caught and captive-reared wrasse. Therefore, the analysis considered the two species separately where appropriate, as well as considering the welfare impacts of captive rearing and wild capture.
Additionally, there were some important overarching questions that were addressed, particularly with regard to the numbers of cleaner fish deployed and their subsequent losses, as well as the effectiveness of cleaner fish in reducing sea louse infestation in comparison to other methods of reducing or removing sea lice.
Finally, the ethics of the use of cleaner fish within the salmon farming industry was considered. An ethical review was conducted to consider the farming of one type of animal (the cleaner fishes) to aid the health, welfare and production of another species (the salmon). To our knowledge, this situation is unique in aquaculture and terrestrial livestock production.
The text of this report is based on the outcomes of the interviews with stakeholders as well as information from a review of the relevant literature. Published reports and literature are cited and are shown in the reference list.
4.1 Cleaner fish lifecycles – a pen picture
Ballan wrasse
The ballan wrasse (Labrus bergylta) that are used as cleaner fish in the Scottish salmon industry are either wild-caught or bred and reared in specialist facilities. Wild-caught fish are mainly fished from waters on the west coast of Scotland, the Inner and Outer Hebrides and Orkney (Scottish Government, 2022), with some fish coming from Northern Ireland and other northern areas of the UK, and deployed into the salmon sea pens. Farmed ballan wrasse are reared in specialist onshore facilities that produce large numbers of these fish for Scottish and overseas markets. Movements of fish are subject to health certification where relevant.
In the case of captive rearing, broodstock are kept in tanks at the rearing facility. The species is a protogynous hermaphrodite, with some females transforming into males from about 6 years of age (Leclerq et al., 2014). After fertilisation, eggs from the broodstock are transferred to a hatching tank, and once hatched, they are reared in a series of tanks according to wrasse size until they reach the appropriate size for deployment onto the salmon farms. Live food is provided for the larval stages, as this is important in promoting their survival (Brooker et al., 2018). The fish are between 18 months and two years old when they are ready to be used on the salmon farms.
Towards the end of the period in the rearing facilities and possibly also once at the salmon farm in a period before deployment into the salmon sea pens, the ballan wrasse undergo a period of ‘conditioning’. This involves exposing the fish to the conditions of water temperature, lighting, and food type and presentation that will be found in the sea pens. This is done to accustom the fish to the conditions found in the sea pens. The fish remain in the sea cage until the end of the salmon growth cycle. In most cases, they are not redeployed into another salmon cage, as there is a risk that they might carry disease from the previous pen to a pen of naive salmon (see Appendix V).
Lumpfish
Lumpfish are typically reared in specialist facilities. Lumpfish eggs may be collected from the wild (e.g., from Icelandic or Norwegian waters), or broodstock are captured from the wild or kept at hatcheries (Powell et al., 2017; Treasurer et al 2024). Fertilised eggs are collected and the fish are reared through their developmental stages in tanks within the hatcher. They are ready for deployment on salmon farms when they are about 10g in weight and between 6.5-7 months of age (Powell et al., 2017).
4.2 Key stages across the lifecycle
4.2.1 Capture and transport of cleaner fish from the wild for deployment in sea pens
Ballan wrasse
Some companies prefer to catch all or a proportion of their ballan wrasse from the wild rather than breed and rear them in onshore facilities (see note below on relative effectiveness of wild vs farmed wrasse in sea lice control).
Licensed fishers are used to catch the ballan wrasse ideally from sites close to the salmon farms, so that the fish are adapted to the local environmental and water conditions. In 2022, 63 derogations (an addition to their domestic fishing licence) were issued for wrasse to fishers, and 78.6 tonnes of wrasse were landed, with 80.6% of the catch being ballan wrasse. Fish were landed on the west coast of Scotland and in Orkney. Of these, 25.8% were returned as they were under- or oversized (Scottish Government, 2024b). Suitably-sized fish are deployed directly into salmon pens but some are also used as broodstock.
Fishers are covered by regulations from the Marine Directorate (Scottish Government, 2022) that specify the maximum and minimum size of fish that can be landed, the requirement to use live traps and to ensure welfare when lifting traps from the water (traps must be raised slowly to prevent the swimbladder from bursting). The size range for wild-caught ballan wrasse has been changed from 12-24 to 14-24 cm whilst additional modelling work is being carried out (Scottish Government, 2025). RSPCA welfare standards also cover the capture of wild wrasse and include stipulations on methods of capture, raising the creels or pots slowly, and maximum water temperatures at capture (high water temperatures cause greater stress). Most of the salmon companies operating in Scotland are members of the RSPCA Assured farm assurance scheme. Fishers are also audited by the purchasing company. Because cleaner fish command a good price (between £10-15/fish (Scottish Government, 2024b)), it is in the interest of the fishers to keep mortality to a minimum and deliver the fish in good condition.
Welfare concerns
The care and skill of the fishers are critical to the experience of the fish being caught. Capture, and removal from the wild, likely has a negative impact on the welfare of the captured fish. The fish that are captured, but returned due to being too large or small, will also have a negative experience or potentially incur injuries. Welfare does not appear to be formally monitored in the post-capture period using recognised indicators.
Additionally, there are concerns from conservation groups and the industry about the sustainability of the native wrasse fishery, in particular whether local populations of fish are being depleted due to removal of fish for use on salmon farms. The Marine Directorate set controls on the wrasse fishery in 2021, which aimed to restrict fishing to sustainable levels. These regulations require licensed fishers to apply for an annual letter of derogation to catch wrasse within the permitted season to allow for breeding and impose restrictions on the size of wrasse caught, number of pots (traps) set and data that must be returned (Marine Directorate, 2022). Large numbers of wrasse are also bought in from other parts of the UK, particularly Northern Ireland. The effects of removal of cohorts of animals of the size required for use as cleaner fish on the overall natural populations is currently unknown, but there is the potential for a negative impact.
4.2.2 Capture and management of brood stock
Ballan wrasse
Female wrasse used as broodstock may be caught from the wild or reared in specialist units. As noted in paragraph 4.1, the ballan wrasse is a protogynous hermaphrodite species, where some females change into males from 6 years of age (Leclerq et al., 2014). It is thought that the most dominant females change sex (Muncaster et al., 2013). Due to regulations on landing sizes (which define the upper and lower limit at which fish can be caught from the wild), hatcheries must currently keep groups of females until some have transitioned into males, as male fish will typically be larger than those that can be caught from the wild. Brood stock is kept in tanks containing hides in onshore facilities (such as at Otter Ferry Seafish). It is standard practice to establish a 1:4 male to female sex ratio (Grant et al., 2016) in these tanks.
Lumpfish
Currently, some facilities in England and Wales have captive broodstock that produce eggs that are reared to a suitable size in these sites before transfer to Scotland (Treasurer et al., 2024). However, it is likely that production at these facilities will cease due to the recent move away from the use of lumpfish. In other case, eggs are sourced from the wild in Norway or Iceland, subject to health certification, and reared in hatcheries.
Welfare concerns
It is likely that there are considerable physical and social differences between the wild and the captive environment experienced by the broodstock animals, including water quality and temperature, food types, social structure and environmental complexity. Broodstock animals kept in captivity do not have the freedom that their wild counterparts do, and tanks are less enriched than the wild environment, but predation and probably disease risk will be lower, and food provision may be better. The practice of breeding and rearing cleaner fish in these facilities is relatively new, so changes are constantly being made to their management procedures to improve fish survival and husbandry. Fish caught from the wild for use as broodstock may also experience stress during the process of capture and transport.
4.2.3 Care and management of cleaner fish during the rearing period in onshore facilities
Ballan wrasse
Fish are reared from eggs in dedicated facilities. In 2023, 8 tonnes of cleaner fish were produced, while in 2024, the total production was 6 tonnes (with no differentiation between the species – Scottish Government, 2024a)).
The different developmental stages of the fish are kept in different seawater tanks with environmental conditions appropriate to their lifestage. Lighting and temperature are controlled. Live feeding is provided for the larval stages, as this has been shown to be very important for development (Helland et al., 2014). For the older stages, food is provided, and enrichment (artificial kelp/plants for shelter) is typically used. Rearing conditions are covered by RSPCA guidelines that specify hygiene, feeding and other husbandry practices (RSPCA, 2024)
A ‘conditioning’ period is used in the month before the fish are sent to the salmon farms. The aim of this conditioning period is to acclimate the fish to the conditions that are found in the sea pens, in particular the low water temperatures, diurnal light/dark patterns and method of presentation of food. Thus, in this period the fish are moved from a constant feeding regime to being given food blocks, which is how feed is delivered in the sea pens. Hides are provided that are similar to those found in the sea pens, and the lighting and water temperature are changed to those which they will experience in the sea pens.
Fish are vaccinated for atypical furunculosis, Aeromonas salmonicida and Vibrio anguillarum.
Lumpfish
A rearing facility for lumpfish is located in Anglesey, Wales. Tanks containing seawater are used to rear the fish in an indoor facility. Acclimation pens are also reported to be used in the rearing of lumpfish. However, as noted above, the demand for lumpfish in salmon farming is declining.
Welfare concerns
As for the broodstock, there is the potential for welfare harms if all aspects of care and management, such as water quality, stocking density, food quality and provision and enrichment, are not provided or maintained at a high standard. As the keeping of cleaner fish is relatively new, there is much to learn about their husbandry, and there may be welfare compromises for fish whilst their needs are being fully researched and 'best practice guidelines’ are being established.
4.2.4 Transport of cleaner fish from rearing facility or point of wild capture to the salmon farm
Both species of fish are transported from the onshore rearing facility on a lorry carrying large seawater tanks. Wild-caught fish are also transported in tanks. Transportation is also covered by the RSPCA Assured standards (2024), which prescribe limits for stocking density, water temperature differentials between the sea and the tank in the case of wild-caught fish, and oxygen saturation in the transportation tanks.
Welfare concerns
As for any transportation event, the health and welfare of the animals depends on the use of appropriate methods and the quality of care taken during on-loading and off-loading, and conditions during travel. The water quality, oxygenation levels and temperature of the water in the transportation tanks must be the same as the seawater for wild-caught fish or the same as the rearing tanks in the case of the captive-reared animals. Stocking density must also be correct. Noise and vibration are additional causes of stress during transportation (Leggatt et al., 2006).
4.2.5 Deployment of cleaner fish on salmon farms
Acclimation period and introduction to the salmon sea pens
In some farms, the cleaner fish are introduced directly into the sea pens with the salmon. In other farms, there is a further period of acclimation on the site of the salmon farm, in a process akin to a ‘soft release’ procedure for translocation of wild animals. In this process, the fish are put into a small sea (or ‘keep’) pen that may be separate from, but contained within, the salmon pen. This allows the cleaner fish to recover from transport and become acclimatised to the ambient sea and climatic conditions, as well as the feeding and other management systems, before release into the main salmon pen.
The RSPCA has guidelines for the introduction of cleaner fish into the sea pens, which state that health and welfare checks should be made as the fish are put into the sea pens and daily thereafter, that hides are available and mortality is monitored.
Specialised staff
All of the interviewed companies have specialist staff specifically trained to manage their cleaner fish. In most cases, this was a specified individual whose sole job was to provide husbandry for cleaner fish. SalmonScotland stated that this is the case for most major salmon production companies.
Welfare checks
In many companies, welfare checks are carried out on the cleaner fish, using a set of operation welfare indicators (OWI) specifically designed for cleaner fish. OWI lists have been created specifically for the cleaner fish species used (e.g., Noble et al., 2019), but the extent of their use is not clear. One company stated that ten fish are caught for checks made on body condition, health and the presence of injuries (eyes, fins).
Mortality is monitored. One company reported that if mortality exceeds 2%/week, a vet visit is triggered.
Health management
Cleaner fish are vulnerable to disease. Wrasses are currently vaccinated against atypical furunculosis (AF) and Vibrio anguillarum in the rearing facilities. There are reports of outbreaks of other diseases (e.g., viral haemorrhagic septicaemia (Hall et al., 2013) but these are uncommon. Lumpfish are vaccinated against Pseudomonas spp. Wild-caught wrasse are not vaccinated, because the procedure requires handling, which is stressful to the fish, and there is a trade-off between the benefits of vaccination vs. the stress of handing. Ballan wrasses with AF are culled.
Treatments for other conditions may involve the cleaner fish receiving the same treatments as the salmon. Cleaner fish are tested for amoebic gill disease (AGD), which is caused by the amoeba Paramoeba perurans, and may be treated using peroxide or freshwater treatments, although it has been shown that ballan wrasse are less susceptible to AGD
Resources provided for the cleaner fish
The industry Code of Good Practice contain a number of recommendations regarding the care and management of cleaner fish including resources and staff training (Scottish Finfish Aquaculture, 2016). Cleaner fish are provided with feed blocks within the salmon sea pens. The core of the blocks floats up to the surface when the edible section has been consumed, and they need replacing. Originally it was thought that the fish would survive entirely by consuming the sea lice from the salmon, or that keeping them slightly hungry would encourage them to consume more sea lice, but more recent evidence demonstrates that providing a consistent source of food is essential. Wrasses have a ‘grazing’ food strategy, as they have no stomach or pyloric caeca and need to feed constantly (Lie et al. 2018; Norland et al., 2022). The number of blocks used depends on the numbers of fish and the season.
Hides or shelters are provided for the cleaner fish. Hides are typically plastic strips or artificial kelp (Helland et al., 2014) or may be natural kelp. Artificial kelp is typically provided in rings or in ‘corridors’, which are up to 5m in length. Corridors are used in larger sea pens and allow for better interaction between the cleaner fish and the salmon, if sited and orientated appropriately. However, studies have observed that wild-caught wrasse do not typically use the hides provided (although they stayed close to them), although hatchery-reared wrasse and lumpfish were observed to use them (Leclercg et al., 2018; Brooker et al 2020).
Handling during salmon treatment events
When the need arises, the salmon may be removed from the sea pens for treatment, grading or slaughter. This can be achieved by providing hides or creels for the cleaner fish to enter, so that they can be removed before the salmon are crowded for treatment. Ballan wrasses may also be separated out using the de-watering devices used for removing water from waste products, or the grading equipment. While the salmon company interviewees stated that they avoid gathering the cleaner fish in as well, if at all possible, from May 2025 the RSPCA standards stipulate that cleaner fish must be removed from the ‘crowding’ net or prevented from entering it, prior to any salmon treatment event. The salmon company representatives reported being concerned that separating out the cleaner fish would be difficult to fully achieve operationally.
This issue is more pertinent to lumpfish, as the interviewees stated that it is more difficult to separate the lumpfish from the salmon before any treatment. They are not attracted to the baited creels used for ballan wrasses and can stick themselves to the metal surfaces of the dewatering device. A lubricant has been developed to try to prevent this. A method of providing hides to attract them, which can then be removed from the pen before the salmon are crowded, is also an option, but the efficacy of these methods is unknown. These issues with handling lumpfish have been reported as a challenge and have contributed to a reduction in the use of lumpfish.
Slaughter vs. re-use at the end of the salmon production cycle
Currently, most of the cleaner fish are euthanised when the salmon are slaughtered at the end of their production cycle. The Scottish salmon industry’s Code of Good practice recommends that a risk assessment is done at the end of each salmon cycle, and that it considers the health status of both the cleaner fish and the salmon (Scottish Finfish Aquaculture, 2016). In many cases, the possibility of transmitting disease or sea lice from one generation of salmon to the next precludes cleaner fish being re-used in a new generation. Additionally, the need to maintain an appropriate size disparity between the salmon and the cleaner fish must be considered. In particular, the lumpfish are usually considered to have grown too large for re-use by the end of a salmon cycle. It is possible for ballan wrasses to be re-deployed, and some companies prefer to do this to allow ‘experienced’ cleaner wrasse to aid habituation and ‘training’ of naïve wrasse. However, interviewees stated that in practice re-use typically does not occur due to biosecurity concerns.
Currently, the cleaner fish are separated from the salmon and euthanised. An overdose of anaesthetic or electric stunning is typically used. The use of culled cleaner fish as a protein source in other applications, such as pet foods, is unclear.
Welfare concerns
As for most animal welfare issues, whether the cleaner fish experience good or poor welfare depends largely on the opportunity, ability and motivation of the people looking after the fish to do a good job. There are written welfare standards available, and staff with responsibility for the cleaner fish are employed, so the necessary elements are in place. The provision of hides and food blocks has been studied, but more research is likely needed to understand the best method of presentation to encourage use. The maintenance and provision of the resources also depends on the diligence of the cleaner fish managers and farm staff, and additionally on the willingness of the salmon companies to invest in the physical resources and the time and training of the staff. Disease control in cleaner fish is also an area where research is needed. There are difference levels of tolerance for disease treatments such as freshwater treatment, between the salmon and the cleaner fish, so the most appropriate treatment should only be applied to the species that requires treatment.
4.3 Other deployment issues identified
4.3.1 Encouraging interactions between cleaner fish and salmon
The cleaner fish and their resources have to be managed to promote the removal of sea lice from the salmon by the cleaner fish. This includes the placement of the hides and the regulation of food availability for the cleaner fish. The hides, where the cleaner fish locate themselves, have to be placed in the sea pens in such a way that the salmon encounter them. Some studies have shown that the diurnal movement patterns of wrasses and their preferred swimming depths is different to those of the salmon, which may limit the opportunity for them to interact (Overton et al., 2020). Ballan wrasses are agastric ‘grazers’ and need to have a constant food supply. They will turn to other food sources if sea lice are not readily available. Thus, the nets of the pens need to be kept clean of algae, as they will use this as a food source, rather than foraging on sea lice (Treasurer, 2002). More needs to be known about how to best facilitate cleaner fish-salmon interactions to facilitate sea lice removal. This is an area on which the salmon companies are actively working.
It is thought that cleaning is an innate behaviour of the cleaner fish. In support of this, neuropeptide pathways have been identified that appear to control cleaning behaviour (Soares et al., 2012). However, it is likely that learning processes refine the behaviour and make it more efficient, including learning what sea louse species are present in a locality. The discussions with the salmon company representatives suggested that the naive cleaner fish and the salmon need to learn their roles. Studies in other species of cleaner wrasse have shown that juvenile fish use social learning strategies to refine their interactions with ’clients’ (Truskanov et al., 2020), indicating that learning from experienced fish is important.
However, there is a danger that the salmon may predate the cleaner fish, or that the cleaner fish may attack the salmon (Treasurer et al., 2024). To avoid this, the size disparity must be right (salmon larger than the cleaner fish), and the salmon must be well fed.
4.3.2 Effectiveness of wild vs farmed ballan wrasse
There is some debate about the ability and motivation of captive-reared wrasses to forage on sea lice. A Norwegian study investigating lice removal rates of wild-caught and captive-reared wrasse in small experimental pens found no difference in the number of lice removed (Skiftesvik et al., 2013), However, a number of the Scottish salmon farming companies stated that farm-reared ballan wrasses are of poorer physical quality, exhibit higher levels of liver disease and have higher mortality compared to wild-caught fish. A larger study in sea pens showed that wild-caught wrasses were more active, had larger home ranges and showed more diurnal variation in behaviour than captive-reared wrasses, all of which are likely to lead to greater numbers of encounters between salmon and cleaner fish. The study also investigated the effect of a period of acclimation at the hatchery vs. two periods of acclimation, both at the hatchery and then in a ‘keep net’ within the sea pen, and concluded that only the dual acclimation strategy had any positive effect on behaviour (Brooker et al., 2020). This concurs with industry experience, as those interviewed stated that the use of an acclimation period improves the performance of the captive-reared wrasses.
Additionally, the farmed ballan wrasses are considered by the industry to be less effective at ‘working’ or removing sea lice. The interviewed salmon companies suggested that captive-reared ballan wrasses have to be stocked at higher ratios (cleaner fish to salmon) than wild-caught fish (e.g., farmed wrasses stocked at 6-8% of the salmon population compared to 2.5-4% when using wild-caught wrasses). It was stated that wild-caught ballan wrasses appear to be more effective and motivated to eat sea lice than captive-reared wrasses, at least in the initial period after introduction to the salmon pen. Producers stated that wild-caught wrasses start eating sea lice immediately upon transfer to the salmon sea pens, whereas captive-reared wrasses take some time to start to forage on the sea lice, in some cases up to 2-3 weeks after introduction. A recent study confirmed the idea that wild-caught wrasses are more active and interactive, with results showing that wild-caught wrasses show more ‘confidence’ in approaching a novel object compared to farm-reared wrasses, and that they swam in all areas of the sea pen, whilst the farm-reared fish tended to swim at the edges of the net (Brooker et al., 2021). The producers suggested that keeping wild-caught and captive-reared wrasses together allows the captive-reared wrasses to learn the cleaning behaviour, which may allow captive-reared wrasses to benefit from social learning as discussed above.
4.3.3 Changing patterns of use of the two major cleaner fish species
Through the course of the data gathering for this project (starting late 2023-mid-2025), some companies reported that they had stopped using lumpfish. This is because of problems with their robustness, separating them from the salmon and managing them during interventions. Lumpfish will stick to hard surfaces, do not voluntarily enter creels as ballan wrasses do, and their spherical conformation means that they are difficult to separate out from salmon using a standard dewatering/grading device.
Additionally, the effects of climate change mean that seawater temperatures around Scotland have become warmer, favouring the use of ballan wrasses, as least at lower latitudes. Some producers stated that lumpfish were considered to be less effective at removing sea lice, as they are considered more likely to use alternative feed sources, such as algal growth on nets. Some salmon companies still use some lumpfish, but their use is declining.
4.4 Additional issues and considerations
4.4.1 Numbers of cleaner fish deployed and losses on Scottish salmon farms
Salmon Scotland stated that 2 million lumpfish and one million wrasses are used in salmon farming. The Scottish Fish Farm Production Survey figures showed that 143,000 cleaner fish (both species) were produced on farms in Scotland in 2023, with 600,000 bought in from other parts of the UK (Scottish Government, 2024a). It was reported that these fish do not come from Southern England due to the potential for overfishing there. In addition, 19,800,000 cleaner fish were sold out of Scotland, mostly in the larval stages for on-growing outside of Scotland. At the company level, one of the major companies stated that 450,000 captive-reared ballan wrasses, 650,000 wild-caught wrasses and 1.6 million lumpfish from wild-sourced eggs from Norway were used. Otter Ferry Seafish produced 85,000 ballan wrasses for the Scottish salmon industry in 2024, and hoped to rear more if demand increased.
One farm interviewed stated that based on the number of dead cleaner fish recovered vs. the number put into the sea pen, mortality was estimated at 0.076% within the first 48h after introduction. Overall survival is 78.6% across the cycle (compared to 82% for salmon). Other companies state that losses can be high, but many do not have the figures available. High levels of loss have also been reported in research projects, (e.g., 57% for ballan wrasse and 27% for lumpfish (Geitung et al., 2020)). This high mortality rate was thought to be the result of poor food intakes, despite the use of food blocks, suggesting that the issues are not confined to cleaner fish within the salmon farming industry.
The loss of cleaner fish is one of the biggest concerns of the salmon companies and of welfare NGOs. High mortalities and incidence of disease in cleaner fish have also been reported by the industry in Norway (Nilsen et al., 2014), and high rates of mortality have been shown in experimental studies (e.g., Geitung et al. 2020). It is not clear what causes the losses or disappearance. Predation by the salmon, mortality due to handling, escapes or disease are possible causes (Overton et al., 2020). Ballan wrasses settle at the bottom of the sea pens in a resting torpor at night and may be taken by seals. Lumpfish carcasses breakdown quickly, so dead fish are not identified and counted. Cleaner fish mortalities may not be easily identifiable from the dead salmon (salmon mortality is checked weekly). One of the salmon companies described an experiment in which transponders were fitted to fish ended with the transponders being recovered from the seabed, suggesting that the fish died and the tags dropped through the net.
Welfare concerns
The mortality and the explained and unexplained losses of cleaner fish are a major concern. While some of the fish may escape through the netting of the sea pens, it is likely that the majority die and decompose, are eaten by the salmon, predated by other species, such as seals, or succumb to disease.
4.4.2 Effectiveness of cleaner fish: comparison with other methods of sea louse control
Cleaner fish are seen by many in the salmon farming industry as the first line of defence against sea lice. They are also seen as a ‘preventative’ measure, because the aim of their use is to keep the numbers of sea lice down to manageable levels, thereby avoiding the use of physical, chemotherapeutic or thermal delousing treatments. However, it was reported that there may be instances when the level of sea louse infestations can ‘overwhelm’ the ability of cleaner fish to remove the lice. At this point, some other treatment would be necessary. One company stated that about one-quarter of their sea louse control budget was spent on cleaner fish. A number of companies stated that they only used cleaner fish at certain times of year when sea lice are prevalent. Others stated that some farms only require cleaner fish to control sea lice. Patterns of mortality in cleaner fish across the saltwater phase of production may also influence the perception of cleaner fish efficacy. If mortality has been high, then there may be fewer cleaner fish to control the emerging population of the older stages of sea lice.
A review literature made by Overton et al., (2020) reported a wide range of efficacies of sea louse removal by cleaner fish from a 28% increase to a 100% reduction in sea louse numbers. Papers and reports from Scotland were included in the analysis. However, many of the reported studies were done in small experimental tanks, and none of the studies were carried out in exposed ocean sites. Higher efficacies were reported for ballan wrasses, with lumpfish being more variable in their success. A study using data from national louse counts, delousing treatments and cleaner fish stocking events from Norwegian salmon farms (which largely used lumpfish) showed that the use of cleaner fish reduced the lice populations only by a small amount, but their use delayed the use of other treatments (Barrett et al., 2020).
In comparison, a Scottish study showed that the efficacy of cleaner fish in reducing sea lice numbers was rated by experts as better than some other methods, such as in-feed medicine or hydrogen peroxide treatments (range of efficacies: cleaner fish 0.60-0.90; in-feed medicine: 0.50-0.80; hydrogen peroxide: 0.10-0.60), but lower than for Hydrolicer (0.70-0.95) or Thermolicer treatments (0.70-0.95). A combination of methods, including the use of cleaner fish, was considered to be the most effective with efficacies reported to reach 100% (Boerlage et al., 2024). These figures derived from expert opinion accord with the little published data available (e.g., up to 95% effectiveness of chemotherapeutic, but highly variable (Jimenez et al., 2013)). The use of cleaner fish as a sea louse control method has a lower impact on the welfare of the salmon. Negative impacts of cleaner fish on salmon are not reported, but significant salmon mortality can be observed when thermal, physical or chemical treatments are used. Salmon mortality during these treatments is affected by factors such as water temperature, fish size and pre-existing conditions in the fish population (Overton et al., 2019).
4.4.3 Factors affecting effectiveness of cleaner fish
It appears that the specific conditions of the site and the climatic conditions influence the effectiveness of cleaner fish in sea louse control. Factors, such as stocking density, presence of other food sources (such as fouled netting), and water temperature, affect the ability and motivation of cleaner fish to remove sea lice from the salmon (Brooker et al., 2018).
Management factors also affect cleaner fish activity. The size of the cleaner fish relative to the salmon has to be correct. The cleaner fish must be smaller than the salmon. Stress reduces the efficacy of cleaner fish. Having an appropriate acclimatisation period before the cleaner fish are put into the salmon pen is important in promoting optimal cleaner fish activity.
Genetic factors also appear to play a role. A study showed that there is variation between families of lumpfish in their sea louse foraging, indicating that there is a genetic basis to sea louse removal by these cleaner fish (Imsland et al., 2016). A study carried out by a salmon company that involved sacrificing cleaner fish showed that some individuals had more lice in their stomachs than others, indicating variation between individuals in their willingness to perform sea louse removal.
Contact
Email: SAWC.Secretariat@gov.scot