Analysing mortality patterns in salmon farming using daily cage registrations

publishedVersio

Salmon lice should be managed before they attach to salmon : exploring epidemiological factors affecting Lepeophtheirus salmonis abundance on salmon farms

The ectoparasite Lepeophtheirus salmonis has for decades been a serious challenge to the sustainability of salmonid aquaculture and has the potential to disrupt the ecosystem of wild salmonids. To tackle this challenge, efforts have been made to develop effective lice control strategies that deter ectoparasitic infestations on salmon farms. Infestation control can be enhanced by understanding L. salmonis population dynamics that may reveal critical points at which to intervene. Here, we investigate the impact of key parameters affecting L. salmonis population dynamics on salmon farms with a view to informing L. salmonis management. We built a system dynamics model to simulate L. salmonis populations in a hypothetical Atlantic salmon (Salmo salar L.) farm. Our model focused on three key parameters in the life cycle of L. salmonis: external pressure (rate of introduction of planktonic L. salmonis from outside the farm), attachment proportion (proportion of L. salmonis that successfully infest salmon), and treatment efficacy (proportion of L. salmonis that are killed as a result of treatment). By applying various combinations of plausible values for these three parameters, their modelled impacts on L. salmonis management were evaluated in terms of the total number of required treatments and the total infestation burden during a typical production cycle. These two modelled outcomes represent the cost of treatment and a proxy for other likely costs, such as lost growth or mortality caused by the parasite. Our results indicated that overall L. salmonis infestation levels were more sensitive to changes in the proportion of successful attachment, followed by changes in external pressure, and were least affected by changes in treatment efficacy. While attachment proportion and external pressure are involved at the pre-infestation stages in the life cycle of L. salmonis, treatment efficacy is involved in the post-infestation stages. Therefore, these findings suggest that preventing infestation before lice attach to fish is a more effective L. salmonis control strategy than treating already infested salmon. Overall, the presented results provide numerical evidence for the efficiency of prevention and support the development and application of prevention measures that have been on the rise in recent years

Imperfect estimation of Lepeophtheirus salmonis abundance and its impact on salmon lice treatment on Atlantic salmon farms

Accurate monitoring of sea lice levels on salmon farms is critical to the efficient management of louse infestation, as decisions around whether and when to apply treatment depend on an estimation of abundance. However, as with all sampling, the estimated abundance of salmon lice through sampling salmon cannot perfectly represent the abundance on a given farm. While suggestions to improve the accuracy of lice abundance estimates have previously been made, the significance of the accuracy of such estimation has been poorly understood. Understanding the extent of error or bias in sample estimates can facilitate an assessment as to how influential this “imperfect” information will likely be on management decisions, and support methods to mitigate negative outcomes associated with such imperfect estimates. Here, we built a model of a hypothetical Atlantic salmon farm using ordinary differential equations and simulated salmon lice (Lepeophtheirus salmonis) abundance over an entire production cycle, during which salmon were periodically sampled using Monte Carlo approaches that adopted a variety of sample sizes, treatment thresholds, and sampling intervals. The model could thus track two instances of salmon lice abundance: true abundance (based on the underlying model) and monitored abundance (based on the values that could be estimated under different simulated sampling protocols). Treatments, which depend on monitored abundance, could be characterized as early, timely, or late, as a result of over-estimation, appropriate estimation, and under-estimation, respectively. To achieve timely treatment, it is important to delay treatments until true abundance equals some treatment threshold and to execute treatment as soon as this threshold is reached. Adopting larger sample sizes increased the frequency of timely treatments, largely by reducing the incidence of early treatments due to less variance in the monitored abundance. Changes in sampling interval and treatment threshold also influenced the accuracy of abundance estimates and thus the frequency of timely treatments. This study has implications for the manner in which fish should be sampled on salmon farms to ensure accurate salmon lice abundance estimates and consequently the effective application treatment

Seroprevalence of <it>Toxoplasma gondii</it> infection in Norwegian dairy goats

<p>Abstract</p> <p>Background</p> <p><it>Toxoplasma gondii</it> is a major problem for the sheep industry as it may cause reproduction problems. The importance of <it>T. gondii</it> in Norwegian goat herds is uncertain, but outbreaks of toxoplasmosis in dairy goat farms have been recorded. The aim of this study was to describe the prevalence of <it>T. gondii</it> infection in Norwegian dairy goats by using serology.</p> <p>Findings</p> <p>Goat serum originally collected as part of two nationwide surveillance and control programmes between 2002 and 2008 were examined for <it>T. gondii</it> antibodies by using direct agglutination test. In total, 55 of 73 herds (75%) had one or more serologically positive animals, while 377 of 2188 (17%) of the individual samples tested positive for <it>T. gondii</it> antibodies.</p> <p>Conclusions</p> <p>This is the first prevalence study of <it>T. gondii</it> infection in Norwegian goats. The results show that Norwegian goat herds are commonly exposed to <it>T. gondii</it>. Nevertheless, the majority of goat herds have a low prevalence of antibody positive animals, which make them vulnerable to infections with <it>T. gondii</it> during the gestation period.</p

Factors influencing production loss in salmonid farming

Using a unique dataset, this paper investigates factors influencing production loss in Norwegian salmonid farming. The factors can be grouped into fish-specific factors (e.g. species, genetics, and generation), input factors (e.g. vaccines and smolt quality), environmental factors (e.g. geographical location), and managerial factors (e.g. ownership). The most important result is most likely that production losses to a large extent are explainable, as our best model has an R2 as high as 0.826. This implies that it is possibile to reduce production losses significantly. For the specific factors, vaccines reduce production loss, but their effect varies by production site. Production loss also varies with which smolt plant is providing juvenile fish, indicating that there is systematic quality variation among the providers of smolt. There is also significant variation in production loss between companies and production sites, and on average production losses are lower for larger companies and sea sites holding larger numbers of fish. An important point is that while some factors explaining production loss are controlled by the individual company, others are beyond their control. Some of these external factors are related to the regulatory system.publishedVersio

Modelling the impact of temperature-induced life history plasticity and mate limitation on the epidemic potential of a marine ectoparasite

Temperature is hypothesized to contribute to increased pathogenicity and virulence of many marine diseases. The sea louse (Lepeophtheirus salmonis) is an ectoparasite of salmonids that exhibits strong life-history plasticity in response to temperature; however, the effect of temperature on the epidemiology of this parasite has not been rigorously examined. We used matrix population modelling to examine the influence of temperature on demographic parameters of sea lice parasitizing farmed salmon. Demographically-stochastic population projection matrices were created using parameters from the existing literature on vital rates of sea lice at different fixed temperatures and yearly temperature profiles. In addition, we quantified the effectiveness of a single stage-specific control applied at different times during a year with seasonal temperature changes. We found that the epidemic potential of sea lice increased with temperature due to a decrease in generation time and an increase in the net reproductive rate. In addition, mate limitation constrained population growth more at low temperatures than at high temperatures. Our model predicts that control measures targeting preadults and chalimus are most effective regardless of the temperature. The predictions from this model suggest that temperature can dramatically change vital rates of sea lice and can increase population growth. The results of this study suggest that sea surface temperatures should be considered when choosing salmon farm sites and designing management plans to control sea louse infestations. More broadly, this study demonstrates the utility of matrix population modelling for epidemiological studies

The economic impact of decreased mortality and increased growth associated with preventing, replacing or improving current methods for delousing farmed Atlantic salmon in Norway

Impacts of salmon lice is a major concern for a sustainable production of farmed Atlantic salmon in Norway. Most treatment methods for removal of salmon lice have associated increased mortality and decreased growth in a period after delousing, which affects the profitability of the farmer, and causes poor welfare and sustainability. In addition, the variance in mortality and growth, especially after non-medicinal treatment methods, is high, which makes it hard for a farmer to decide which control measure to apply to keep lice levels below the legal limit. In this study, we have applied a stochastic partial budget approach to assess the economic impact of reducing mortality and increasing growth of farmed Atlantic salmon by preventing, replacing and improving current delousing methods in Norway. We have simulated a production cycle of two different smolt-groups to find the outcome (harvested biomass, average end weight of the salmon, number of dead fish and feed consumption) of production cycles without or with two, three or four delousing treatments in the on-growing phase at sea. The results suggest that accounting for the biological losses associated with lice treatments is important when making choices of delousing strategies. The biological costs of increased mortality and decreased growth associated with especially non-medicinal treatments are expected to be high, but varies substantially. Therefore, the economic benefit of preventing or improving can also be high. The calculations imply that salmon producers could invest a considerable amount in measures for prevention or improvement of thermal treatments before break-even. For example could a farmer use on average 535,313 €/cage/ 1-yearling production in measure to prevent four thermal treatments before it is no longer economical beneficial. Depending on the performance of the four thermal treatments a farmer could use from 319,196–737,934 €/cage/ 1-yearling production on measures of improvement. Replacing one thermal treatment with another immediate treatment method has a minor economic benefit. The results further shows that sales value and feed consumption constitutes the largest share of the change in profit between different treatment regimes. The results from this study also show that not taking into account the risk of mortality and reduced growth associated with the different treatment methods of delousing, could lead to underestimating the benefit of improving, preventing and replacing treatments.publishedVersio

A stepwise integrated risk-assessment framework in aquaculture: the case of sea lice tolerance to freshwater treatments on salmon farms.

Aquaculture studies are often faced with data limitations when carrying out a quantitative risk assessment. Consolidating results from a literature search of potentially applicable methods, we propose a stepwise integrated methods approach that incorporates foundations from an antimicrobial resistance framework, the Office International Epizooties risk model, quantitative microbial risk assessment and infectious disease transmission models. We suggest that an initial ranking profile can be used to prioritize more in-depth qualitative and quantitative risk assessments, when data are available. The ranking method was done using a software that provides practical and interactive graphics for visualizing the impact of different factors and their respective weights on the likelihood of undesirable events (hazards) occurring. For this step, we illustrate how to include available data to obtain ranking results for decision makers using information from a recent sea lice freshwater tolerance literature review (Groner et al. 2019) that identified a gap in quantitative data. In our case example, for copepodid sea lice life stages, hypothetically changing how much experts believe that location and time are important factors revealed the most impact on the ranking for different degrees of freshwater tolerance evolution (no evolution, various partial options, known evolution). The factors ‘location’ and ‘time’, as well as ‘freshwater treatment’, have the greatest impact on the ranking for the preadult sea lice life-stages model. Results from our proposed ranking method can help to drive decisions around interpreting the various factors as they apply to mitigation planning and prioritizing those that should be included in further research. Additionally, we identify where quantitative data could be incorporated, as they become available, into a full risk assessment model with suggested models for a freshwater tolerance risk analysis framework

Analysing mortality patterns in salmon farming using daily cage registrations

publishedVersio

Evaluating the potential for sea lice to evolve freshwater tolerance as a consequence of freshwater treatments in salmon aquaculture

Increasing usage of non-medicinal methods (NMMs) to control sea louse infestations on salmon farms has raised questions about whether sea lice may be able to evolve tolerance of NMMs. Of particular concern is the potential for sea lice to evolve freshwater tolerance as a result of freshwater treatments. Wild trout and some juvenile salmonids swim into freshwater to control infestations and regain ionic balance after disruption by sea lice; freshwater tolerance would compromise this potentially adaptive behavior. Here we evaluated the potential for freshwater tolerance to evolve in the sea louse Lepeophtheirus salmonis. When exposed to low-salinity water, parasitic stages of sea lice are able to osmoregulate through the host, while larval planktonic stages are not. Transcriptomic work suggests that sea lice mount a costly polygenic stress response when exposed to brackish water. The population structure of sea lice is panmictic in both the Pacific and Atlantic, making it conducive to rapid evolutionary responses. It is unknown how much heritable genetic variation these panmictic populations have for freshwater treatments. While usage of freshwater treatments on wellboats is increasing, it is unclear whether the freshwater itself is a strong selective force; during the freshwater exposure, sea lice can die from physical disruption during pumping and filtration on the wellboat. Future studies are advised to quantify the heritable variation in freshwater tolerance in sea louse populations, characterize mechanisms for freshwater tolerance in planktonic and attached sea lice, and assess the risk of freshwater tolerance evolution under different management strategie