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

Informations

Funding country

Norway

Acronym

-

URL

-

Start date

1/1/2019

End date

12/31/2020

Budget

215,373 EUR

Fundings

Abstract

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.