1. Background and rationale
The sea louse Lepeophtheirus salmonis is a key parasite of cultured Atlantic salmon worldwide, and is widely distributed throughout most farmed salmon producing countries, including Scotland. The louse feeds on the salmon, and causes multi million pound commercial losses to the salmon aquaculture industry globally (Costello, 2009). Its life-cycle includes free-living life stages, and life stages attached to fish (Costello, 2006). The sea-louse life cycle is heavily affected by water temperatures, making the louse more abundant in summer and autumn months, and thus more sensitive to climate change, which may increase management challenges in the future. Mitigation throughout the farmed salmon production period in open-sea net pens is needed to keep lice numbers at bay.
In the absence of preventative sea lice management measures, sea lice infestation can lead to reduced salmon welfare and lower productivity at farm level through low feed efficiency or growth reduction. Furthermore, the value of salmon at harvest may be reduced, and environmental costs of salmon production may increase due to inefficient resource use, greenhouse gas emissions and nutrient pollution as a result of lower productivity, or potential spread of lice through interactions with wild fish. Altogether, sea lice infestation without mitigation may reduce the overall economic performance of farmed salmon production. Sea lice control involves measurable economic and environmental costs as well as costs that are more difficult to monetise, such as costs related to fish welfare and public perceptions.
As in other salmon producing countries, lice numbers per fish are regulated in Scotland. Scottish Government regulations include a mandatory reporting when average adult female lice counts per fish are 2 or above, and have an intervention limit at 6 average adult female sea lice per fish, both during a weekly count. These are to be reported by industry to the Fish Health Inspectorate (Aquaculture and Fisheries (Scotland) Act 2007; Aquaculture and Fisheries (Scotland) Act 2013, updated in 2019). There is also a Code of Good Practice for Scottish Finfish Aquaculture (CoGP) developed by industry, which states that sea lice management measures should be guided by the build-up of preadults to prevent the development of gravid females, and it includes a suggested sea lice management measure criteria of 0.5 adult female lice per fish between 1st February to 30th June inclusive, and 1.0 adult female lice per fish between 1st July to 31st January inclusive.
Sea lice control measures can be incidental, continuous, or embedded in management. The methods investigated in this project were selected based on a review of the literature (Murray, 2015; Overton et al. 2019) and expert opinion, which include those of the members of the project steering group. The selected methods include: (i) incidental sea lice management measures such as in-feed medication (Slice®); bath sea lice management measures using licenced veterinary medicines (AlphaMax®, Salmosan Vet® and hydrogen peroxide); fresh water bath sea lice management measures; and physical removal (hydrolicer, thermolicer and optilicer); (ii) continuous sea lice management measures such as biological control using cleaner fish (wrasse and lump-sucker, which eat lice off fish); and use of physical barriers (skirts) to keep lice at early life stages out of the pen.
One of the most important characteristics of a sea lice management measure is its efficacy. Examples of factors that affect the efficacy of sea lice management measures are water temperature, salmon weight and welfare status prior to sea lice management measures (for instance thermolicer is recommended for fish up to 4 kg (Gismervik et al., 2017 in Norwegian, in Overton et al., 2018), sea lice numbers (Gautam et al., 2017b), most abundant sea lice stages prior to sea lice management measures (freshwater is less efficient at older louse stages (Wright, Oppedal and Dempster, 2016), and oxygen saturation and medicine dispersion during sea lice management measures (Treasurer, Grant and Davis, 2000), and many more. Variability in these factors makes it difficult to evaluate efficacy of sea lice management measures. In addition, lice counts, the method used to measure efficacy, are often not comparable between sea lice management measures which can lead to inaccurate estimations (Jimenez et al., 2013; Gautam et al., 2017a). Moreover, frequently used sea lice management measures may reduce in efficacy over time as susceptibility is selectively removed from the population (Lees et al., 2008; Svåsand et al. 2016 in Norwegian, in (Bui et al., 2019)). Overall, efficacy of sea lice management measures is a very complex variable that is difficult to quantify precisely.
A management strategy for a group of fish between stocking and harvesting usually includes several mitigation methods, combined to enhance efficacy of the individual methods and reduce the risk of resistance. In general, a sea lice management plan includes continuous sea lice management measures such as co-habitation of salmon and cleaner fish or sea lice management measures that are embedded in management, such as good husbandry and synchronised fallow period between sites in one area. When these are not sufficient in keeping lice numbers low, incidental sea lice management measures are used where deemed appropriate. Choosing the next sea lice management measure in the sequence or combination of methods used on a farm does not only depend on the efficacy of sea lice management measures, but also on other factors, such as method’s feasibility to producer and site, previously used methods and weather forecast.
Information about current industry practices and sequences of sea lice management measures for Scottish salmon producers is often not publicly available, mostly due to the commercially sensitive nature of some data. Companies rely on in-house assessment of the relative cost-effectiveness of sea lice management measures relevant to them. Reporting and publishing of sea lice management measures in Scotland is currently available only for licenced veterinary medicines (ScotGov, 2019). Media reports provide some qualitative information, such as Scottish producers spending less on approved medicines and more on cleaner fish and physical removal technology (SSPO, 2018). This may imply that sea lice management measures that are not based on licenced veterinary medicines are becoming more common, but such reports are not always reliable. There is variation in data availability and resolution, with differences ranging widely between salmon producing countries, and depending on the type of health management measures employed.