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Effects of salmon lice infection and salmon lice protection on fjord migrating Atlantic salmon and brown trout post-smolts

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Abstract

Effects of artificial salmon lice infection and pharmaceutical salmon lice prophylaxis on survival and rate of progression of Atlantic salmon (n = 72) and brown trout post-smolts (n = 72) during their fjord migration, were studied by telemetry. The infected groups were artificially exposed to infective salmon lice larvae in the laboratory immediately before release in the inner part of the fjord to simulate a naturally high infection pressure. Groups of infected Atlantic salmon (n = 20) and brown trout (n = 12) were also retained in the hatchery to control the infection intensity and lice development during the study period. Neither salmon lice infection nor pharmaceutical prophylaxis had any effects on survival and rate of progression of fjord migrating Atlantic salmon post-smolts compared to control fish. Atlantic salmon spent on average only 151.2 h (maximum 207.3 h) in passing the 80 km fjord system and had, thus, entered the ocean when the more pathogenic pre-adult and adult lice stages developed. The brown trout, in comparison to Atlantic salmon, remained to a larger extent than Atlantic salmon in the inner part of the fjord system. No effect of salmon lice infection, or protection, was found in brown trout during the first weeks of their fjord migration. Brown trout will, to a larger extent than Atlantic salmon, stay in the fjord areas when salmon lice infections reach the more pathogenic pre-adult and adult stages. In contrast to Atlantic salmon, they will thereby possess the practical capability of returning to freshwater when encountering severe salmon lice attacks.

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Acknowledgements

We would like to thank Karin Boxaspen and Bernt Martinsen for applying salmon lice larvae and the lice protection, respectively. Further, Svein Erik Gaustad, Espen Holthe, Petter Sira, Bjørg Anne Vike and Torbjørn Utigard are thanked for assistance during the project. The study was financed by the European Commission contract no: Q5RS-2002-00730, AquaNet Canada (RSM), the Norwegian Institute for Nature Research and Statkraft Energy AS. The experimental procedures conform to Norwegian ethical requirements.

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Correspondence to Bengt Finstad.

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Sivertsgård, R., Thorstad, E.B., Økland, F. et al. Effects of salmon lice infection and salmon lice protection on fjord migrating Atlantic salmon and brown trout post-smolts. Hydrobiologia 582, 35–42 (2007). https://doi.org/10.1007/s10750-006-0545-x

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