Sea lice management measures for farmed salmon production: research

Executive Summary

The sea louse Lepeophtheirus salmonis is a key parasite of cultured Atlantic salmon 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. Its life-cycle includes free-living life stages, and life stages attached to fish. 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. 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. 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. 

The sea lice control measures methods investigated in this project 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. Efficacy is highly variable, and can be affected by a number of environmental factors, for example, water temperature, salmon weight and welfare status prior to sea lice management measures, sea lice numbers and most abundant sea lice stages, and oxygen saturation and medicine dispersion during sea lice management measures. 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. Moreover, frequently used sea lice management measures may reduce in efficacy over time as susceptibility is selectively removed from the population. Overall, efficacy of sea lice management measures is a very complex variable that is difficult to quantify precisely. 

A management strategy to keep sea lice at bay 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, cost, weather forecast and availability of the measure. 

For modelling purposes i.e. to create a quantitatively driven ranking, the study examined individual methods, however there is typically no individual method that achieves the desired level (no/very low numbers) of sea lice across a production cycle. Hence farm managers can and do use the wide range of methods available to them, which may include those lower ranked overall, as necessary to build the optimal sea lice management strategy.

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. Media reports provide some qualitative information, such as Scottish producers spending less on approved medicines and more on cleaner fish and physical removal technology. 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. 

The aim of this research project was to gather socio-economic and environmental information on sea lice management measures employed on Scottish salmon farms, and understand the relative cost-effectiveness of sea lice management measures from both the economic and environmental dimensions of disease control in the salmon industry. Specific objectives include assessment of the relative effectiveness of sea lice management measures; assessment of the cost of deploying sea lice management measures and provision of common measures of economic efficiency across the sea lice management measures. The project used a combination of methodological approaches to achieve the specific objectives. 

Socio-economic and environmental information was gathered on sea lice control measures employed in the salmon sector based on a review of secondary sources of information from Scotland as well as other countries. Where no Scottish data were available and data from other countries had to be used, these were inspected and selected using expert opinion (health practitioners in the Scottish salmon sector and members of the research group conducting the study) to ensure closest possible relevance to the Scottish salmon sector, and sensitivity analysis was performed to account for uncertainty in the data. Primary data collection using in-depth interviews and participatory workshop involved a small number of Scottish salmon producers (not representative by region and farm size) and processors. Primary data was combined with secondary data to avoid potential bias due to uncertainty and sample size. 

Results from the participatory workshop with stakeholders representing different stages of the supply chain were used to inform the analysis of farm-level behaviour (uptake of management measures). The analysis employed a newly developed participatory process from the system dynamics literature called group model building where stakeholders rank control measures based on their efficacy (estimated and/or perceived), and then collectively identify incentives linked to different stages of the supply chain to reduce occurrence of sea lice in primary production. The involvement of stakeholders contributed to identifying network or spill-over effects, supply chain constraints and gaps in skills and training requirements. 

Cost Effectiveness Analysis (CEA) and Life Cycle Analysis (LCA) were used to assess the relative cost-effectiveness of sea lice management measures and their impact on the economic performance (including carbon cost) of Scottish farmed salmon industry. CEA and LCA have not been used, to the best of our knowledge, in other studies focussed specifically on the control of sea lice in the salmon sector; however, they have been extensively used in studies analysing the economic and environmental impacts of control of animal disease. 

Cost-effective analysis (CEA) is a technique used to prevent or mitigate a disease where the impact cannot be measured routinely in monetary terms, which we used in this context to evaluate different sea lice management measures. The Cost-Effectiveness ratio (CE ratio) is a commonly used indicator to determine the effectiveness of an intervention. Determining the cost of the intervention and effectiveness of the intervention are essential steps to determine applicable CE ratios. The cost of the intervention includes all costs of controlling or preventing a disease. The effectiveness of an intervention is used to compare how effective different prevention or mitigation measures are, and is generally represented by an efficacy score for each measure. The Life Cycle Assessment (LCA) method considers the environmental burdens and resource use in the production and exploitation of a commodity within defined boundaries. LCA is arguably the most holistic method available for environmental impact assessment.  

The methodological framework used in this study combines LCA and CEA, with the former feeding into the latter. All sea lice management measures tested in the study were modelled as single use measures. In addition, we modelled three combinations of measures applied in sequence between stocking and harvesting that represent realistic scenarios assumed to bring down and maintain sea lice count within acceptable (regulation compliant) efficacy levels assumed not to impact fish health, welfare and productivity. Intervention cost of the sea lice management measures included cost of equipment, cost of implementation, environmental cost and cost of side effects. The accumulative cost was assessed against the reduction of sea lice on an adult salmon fish, which is considered as the efficacy score for this study (not experimentally assessed). The models were estimated using a combination of primary and secondary data, and simplified modelling assumptions were made to account for low availability of open source data on sea lice management. Detailed sensitivity analysis to substantiate the validity of results was required to account for simplified modelling assumptions, uncertainty due to limited data, absence of control farms to compare our results with, and subsequent use of expert opinion on efficacy of single use measures to obtain qualitative relative efficacy scores, and to design combination measures.  

Results of the quantitative analysis indicate that sea lice management by in-feed and long term usage of skirts to prevent sea lice from entering the pens have the highest relative cost-effectiveness. Findings from the workshop analysis indicate that, according to stakeholders’ perceptions, skirts’ relatively lower impacts on environment and fish welfare are translated into positive impacts at the retail side of the supply chain and positive consumers’ perceptions. The cleaner fish, fresh water, physical removal measures and the licensed veterinary medicines were among the second most cost-effective measures, and this is supported by their mixed and at times contradictory environmental, health and welfare impacts. The use of hydrogen peroxide (both well boat and tarpaulin) represented the least cost-effective measures among single use measures and, based on the opinions of stakeholders involved in the participatory workshop, these were also regarded as less positive methods by the public in view of their fish welfare and environmental aspects, and human health implications. As presented in the qualitative part of this study, i.e. findings from the participatory workshop, cost-effectiveness of prevention and mitigation of sea lice is not the only measure of importance as, for example, skirts were perceived to reduce oxygen flow and may have a detrimental effect on fish with compromised respiratory functions, and therefore their effectiveness concerning general fish health and welfare can be low.

To account for the wide range of expert opinion based efficacy rates for each sea lice management measure, we ran sensitivity analysis at the extreme values to identify any corresponding variation in the ranking and magnitude of measures’ cost-effectiveness. No major changes in ranking occurred under the maximum efficacy values, with the exception of physical removal (thermolicer) measure, which became less cost-effective. Under assumed minimum efficacy level, the ranking changed considerably. Skirts and in-feed measures remained as the most cost-effective measures under assumed minimum values for efficacy scores, and hydrogen peroxide remained the least cost-effective among the single use measures. Physical removal measures and use of other licensed veterinary medicines became significantly more cost-effective, while fresh water measures became significantly less cost-effective. In addition to changes in ranking, cost-effectiveness exhibits the expected changes in magnitude under both the minimum and maximum efficacy assumptions. Sensitivity analysis results indicated the variability in cost-effectiveness related to changes in efficacy levels, namely that the higher the efficacy of a measure, the higher the cost-effectiveness. 

To account for uncertainty owing to combined data sources, additional sensitivity analyses were carried out to assess the impacts of varying values for the costs of interventions on the model outcomes, such as feed conversion ratio. Changes in the costs of interventions to similar extent for all management measures did not change the overall cost-effectiveness rankings of the management measures although the absolute values of cost-effectiveness were changed. However, changing costs of single use measures showed some changes in cost effectiveness rankings. For example, a significant increase in the price of cleaner fish led to decreased cost-effectiveness of the measure per unit of effectiveness to the extent of cleaner fish becoming slightly less cost-effective than the hydrolicer measure.

Results indicate that sea lice management measures using tarpaulins were more cost-effective than measures using well boats under the whole range of efficacy values used in this study. This relates to the higher costs associated with operating well boats compared to tarpaulins. However, we assumed that efficacy was unaffected by the method of performing these bath measures, while in practice the level of control and monitoring provided when using a well boat may mean that such an approach will likely lead to improved outcomes. 

Cost-effectiveness of combination measures are not comparable to that of single use measures as these combine different measures in different sequences, with many of them repeated and as such, aspects of cost additivity apply. When comparing the three combination measures, results indicate a small difference between the most cost-effective and least cost-effective combination ranging from £1.23 per fish per unit of effectiveness to £1.67 per fish per unit of effectiveness. Depending on farm circumstances, the difference may be even smaller, as well as the magnitude of total costs, for the cases where cost additivity can be adjusted to account for cost synergies across measures. Sensitivity analysis employed to test results for combination measures indicate high sensitivity to fish mortality.

Fish welfare was taken into account in both, the qualitative workshop analysis and the quantitative models, the latter through sea lice management measure related mortalities, which affect primarily the economic performance (including carbon cost) and is, arguably, a proxy welfare indicator. Results of the workshop showed that perceived importance of both salmon and cleaner fish welfare is high, equally for industry and consumers. Where cleaner fish are used as a sea lice control method, the welfare and health of both salmon and cleaner fish are affected by any other sea lice management measures applied. The differences between these fish species, not only in size but also many other biological parameters, imply that measures optimised for salmon may affect cleaner fish differently. Results of the workshop indicate that cleaner fish are perceived as cost-effective and their welfare is key to positive consumer perceptions. 

Participatory analysis identified potential incentives for further improving control of sea lice on farm, many of these already taken into account in the Scottish salmon sector. These include: better balancing of science-based evidence and precautionary principle based policies (health, environment, welfare); public sector driven positive incentives such as subsidised access to technology; research on consumers’ willingness to pay for sustainably farmed salmon; media campaigns and education to the public on implications of disease control in aquaculture; market based incentives (price differentiation through labelling re sustainable disease control); market based incentives (traceability); development of health monitoring/preventative technologies; development of delousing technologies from product/flesh at packing; research on salmon welfare linked to sea lice control; research on cleaner fish welfare linked to sea lice control; media campaigns to maintain/improve industry image to the public; private driven stick type incentives such as higher environmental/welfare standards required under processor/retail contracts; research on efficacy of disease control; and improved collective action to sea lice control along the supply chain.

It should be noted that caution should be applied when comparing cost-effectiveness of single use measures based on the limitations previously mentioned regarding data availability and uncertainty as well as the consequent simplified modelling assumptions. Data sources are a combination of primary and secondary data, and expert opinion, moreover the geographical distribution of data sources from e.g. Norway and Canada were translated to the Scottish situation as closely as possible, but might not fully represent the Scottish salmon industry situation, more specifically as regards regional differentiation. With additional as well as more robust primary data, further ways to improve the analysis include the methodological integration of economic, biological and epidemiological modelling. The findings from the participatory workshop indicate the complexity of sea lice control not only on farm but beyond farmgate, as supply chain, regulatory and environmental effects, and the need to address it as a holistic challenge. 

As demonstrated by the limited literature on the topic, which this study adds to, this type of research will always be constrained by access to data for reasons detailed in this report. Thus, while much information has been collected on the variables included in the analyses and the robustness of results tested using sensitivity analysis, there may be other factors for which neither data nor robust proxies could be identified under this study, and this should be taken into consideration when reviewing these findings as basis for future research.