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HAVBRUK2-Stort program for havbruksforskning

Parasite- and host-driven characteristics of infestation success in salmon lice (Lice-IS)

Alternative title: Parasitt- og vertsspesifikke egenskaper for smittesuksess hos lakselus (Lice-IS)

Awarded: NOK 1.8 mill.

Salmon lice are parasites that affect Atlantic salmon and other salmonid species. With the expansion of the salmon aquaculture industry, salmon lice abundance has risen in parallel. This has a negative impact on the sustainability of production systems, particularly on farmed salmon welfare, environmental impacts, and wild salmonid population fitness. This project aims to investigate parasite- and host-driven characteristics in this system, by focusing on the biology and behaviour of lice that can influence their infestation success, and broader-scale factors related to lice population dynamics. Understanding aspects of both the host and parasite will improve preventative measures and predictive models. Focusing on biological characteristics of salmon lice, we successfully mapped the development rate of lice through their life-history stages over temperatures from 6-21°C, and a model was developed for male and female lice that described increasing growth rate with warmer temperatures. In a separate tank study, we found that age of the copepodid lice stage affects its ability to successfully infect a salmon host, generally with younger copepodids being more active and better at attaching to a host. But this was influenced by temperature, where infestation success was higher overall at 10 and 15°C compared to 5°C. Infection success in tanks can range from 20-50%, and is strongly correlated with temperature with the highest success found at 10°C. In this study, no attrition of lice was recorded between 6 and 21 °C, indicated that they can survive and persist on the host under ideal conditions. Temperature also influences daily mortality rate of free-swimming copepodids, with lower rate at 8°C (~7%) than at 12 or 16°C (~7 - 12%); this matches their known temporal pattern of survival duration. Daily intrinsic mortality rate was comparable to what the dispersal model currently uses (17%), which includes an estimate for predation. We were also interested in the behaviour of copepodids in the water column that will influence infection success. In controlled column tests, salmon louse copepodids can use pressure cues to position themselves shallow in the water column; there is also an indication that there is a genetic basis for this behaviour. At a larger scale, a study was conducted to test a cage modification for lice prevention (using the snorkel cage) at two sites, where fish in sea cages were unable to access the top 3 m surface waters except via an impermeable tube. Theoretically, these fish would then avoid the depths where infective copepodids are most likely to be, however we found that this prevention approach only worked at the site with no brackish layer. When there was some low-salinity water present, copepodids are likely to be distributed below the halocline, and during these periods or at the inner-fjord site, there was little difference in infection between control and snorkel cages. The last work package contained tasks to understand lice infection pressure on a broader scale. Using historical data, the ratio of wild salmonids to farmed Atlantic salmon present in coastal waters over the last decade was calculated. Early estimates indicate that there have been between 97-265 times more salmon held in farms compared to wild salmonids, not including escapees. In 2017, salmonids in farms accounted for 99.6% of available hosts and 99.3% of adult female salmon lice in Norwegian coastal waters. This indicates that modelled estimates of infestation pressure can safely be based on data of lice populations in farms alone. A framework was developed to validate the dispersal model using lice counts on 5 commercial sites in the same fjord system; this approach needs further development, data from more sites and more detailed information about lice management strategies over the target period to build a robust validation. This project also aimed to validate the risk assessment of salmon mortality with infection: hatchery-raised, newly-smoltified wild salmon were infected with copepodids and challenged to swim their migration distance. Infection intensities ranged up to 2.5 lice/g, and based on the raw data, almost all fish with > 0.4 mobile lice/g during the study period. 76.5% mortality occurred in fish with 0.3-0.4 mobile lice g/g, whereas 85% of salmon with less than 0.3 mobile lice g/g survived. Mortality thresholds of infection intensity were higher when considering their loads at the chalimus 2 stage, rather than the mobile stage. In summary, this project explored aspects of salmon lice biology and behaviour, and physiological impacts on the host. Further, it investigated the efficacy of commercially relevant pest management strategies and their impact on salmon welfare and behaviour, and set up foundations for model validation through biological input and data on commercial cages.

The results from this project and its individual tasks will shortly lead to significant improvements to the salmon lice dispersal model and validation of the mortality risk assessment for wild salmonids, with both providing foundational evidence for the national management of salmon aquaculture via the traffic light system. The outcome and impact of such contributions will include more robust evidence for providing advice to authorities and policy-makers. Furthermore, the multidisciplinary nature of the results from biological studies have led to improved understanding of the host-parasite system, so that future studies may be conducted effectively and without unknown bias. The outcomes of this project include stronger collaborative relationships within research groups of IMR, and with overseas institutions such as the University of Melbourne. Lastly, it has significantly advanced the research profile of the postdoc Dr. Bui.

The operational salmon lice dispersal model provides vital information to management authorities that regulates the growth of the Norwegian salmon aquaculture industry. The biophysical basis of the model is continuously revised, which enables updated or new information to be incorporated to improve accuracy and robustness. This project aims to fill some known knowledge gaps of the biology and behaviour of the salmon louse, particularly in relation to its interaction with salmon hosts. Specifically, characteristics of parasite- and host-driven infestation success will be investigated. Testing larval biological factors such as senescence and mortality rate with temperature, will complimented by studies of behavioural aspects of infestation, such as depth dispersal of planktonic larvae in the presence of hosts, and depth preferences of host with infestation status. These results will be input as updated biological parameters in the model. Depth distributions and behaviours of the infective planktonic lice and salmon will facilitate the development or improvement of passive prevention strategies, particularly depth-based cage modifications. In parallel, the importance of Norwegian wild and farmed salmonids in propagating lice populations will be quantified. Lastly, model validation will be achieved through using long-term case studies of multiple commercial sites to verify model assumptions and develop the ability to predict infestation pressure in coastal waters. The output will be a new framework for model validation and comparison that is greatly needed for this system, to facilitate robust scientific advice in a sensitive management landscape.

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Funding scheme:

HAVBRUK2-Stort program for havbruksforskning