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

2. Background and definition of area of analysis

Prevention and control methods for sea lice

Cleaner fish are used as biological control agents for sea lice. There are two main species of sea louse found in Scottish waters: Lepeoptheirus salmonis and Caligus elongatus. Sea lice feed on the mucus, skin and blood of the fish. Moderate to high infestations cause damage to exposed surfaces of the fish, and subsequently osmoregulatory failure, immunosuppression and chronic stress (Grimnes and Jakobsen, 1996, Bowers et al., 2000). Infestation also facilitates infection by other pathogens leading to disease (Cai et al., 2024). See Appendix VI for a diagram of the life cycle of Lepeoptheirus salmonis.

To combat sea lice, the salmon industry uses a suite of preventative measures including the use of physical barriers to prevent ingress of the lice, feeding the salmon at or below the depth that lice typically inhabit and choosing locations of the farms where sea lice are less prevalent (Barrett et al., 2020). Once salmon are infested, a number of treatments can be used to reduce the numbers of lice, including chemotherapeutics, hydrogen peroxide, thermal and physical treatments, and cleaner fish (Overton et al., 2019). Chemical or fresh/brackish-water treatments can be used for the larval, sessile and mobile (pre-adult and adult) stages, while physical methods (such as the Hydrolicer) and thermal delousing methods (e.g., the Thermolicer) can be used for the sessile and mobile stages (Overton et al., 2019). Cleaner fish are used only to control the mobile stages (Barrett et al., 2020). See the glossary for definitions and Appendix VI for a description.

History of the use of cleaner fish in the salmon industry

In the 1990s, a number of species of cleaner fish were trialled for use in reducing sea louse infestations, but due to an increase in availability of chemical control methods and biosecurity issues with the species of cleaner fish used in that period, the use of cleaner fish declined (Treasurer et al., 2024). However, since about 2008 (Treasurer, 2018), there has been a resurgence of interest in the use of cleaner fish, because of stricter controls on the use of chemotherapeutics and reduction in efficacy of these treatments due to adaptations by the sea lice, plus the growing concern over the increase in mortality in the salmon after the use of the thermal and physical treatments (Overton et al., 2019). The use of a biological control agent is also likely to be more compatible with consumer expectations.

Because the use of cleaner fish is relatively new, knowledge on the best way to rear and manage them is constantly being generated through continuing research and practical experience within the industry, research institutes and universities. As such, the picture is constantly evolving. This project presents the situation as it was when the data were gathered from late 2023 to mid-2025.

Species of cleaner fish used

There are two main species used as cleaner fish in Scottish aquaculture: ballan wrasse (Labrus bergylta) and lumpfish (Cyclopterus lumpus) (See Appendices IV and V for further details). Both species are ‘facultative’ cleaners, which show flexibility in their foraging choices between cleaning and use of other food resources, rather than being ‘dedicated’ cleaner fish that only eat the external parasites of specific hosts. Although various other species of cleaner fish were trialled early in the establishment of salmon farming in Scotland and have been used (mostly other species of wrasse), our evidence gathering revealed that the two species named above are now the most common. Salmon companies may use both or just one species. The two species may be used on one farm, because they are adapted to different thermal conditions of the water. Lumpfish are more effective in cold water and so may be used at higher latitudes or during the winter months, while wrasses are more effective in warmer conditions.

Wild caught vs. captive-reared cleaner fish

Currently, the ballan wrasse that are used as cleaner fish may be captured directly from the wild or may be reared onshore in tanks in specialist rearing facilities. The broodstock held in the rearing facilities may also be wild-caught or captive-bred (see Section 4.1 for more details). The different rearing environments mean that the two populations of fish are vulnerable at different points in their life cycles. For instance, the strong water currents, low water temperature and variable light conditions of the salmon sea pens may affect the captive-reared cleaner fish more than the wild-caught individuals. Accordingly, we consider welfare effects on the fish from each background separately, where necessary, in the following analysis. Additionally, because the wild-caught fish are essentially translocated from their origin sites, the principles outlined in SAWC’s ‘Wild Animal Translocations: Animal Welfare Risk Assessment Guidance’ document (2025) are considered where appropriate.

Industry dynamics

During the course of the evidence gathering in this report it became apparent that use of lumpfish was declining, or had stopped altogether, and that the majority of Scottish salmon producers were focused on the use of ballan wrasse. Therefore, we will focus predominantly on ballan wrasse and their welfare in this report, as this reflects the current Scottish situation. For completeness, some mention of the situation with regards to lumpfish is included. Where the welfare of lumpfish is likely to be considerably different from ballan wrasse, we will also mention those specific issues.