| Summary: | Infections with marine tapeworms have been reported as an increasing problem in aquaculture of Atlantic salmon (Salmo salar L.) in Norway over the last few years. These are caused by the cestode Eubothrium sp. Little is known about the infection dynamics of this species and there is a need for knowledge regarding when these infections occur and how they develop. Four cohorts of Atlantic salmon were followed for their first half year at sea and examined for tapeworms. Two of these were launched in autumn 2017 and two in spring 2018. Sampling was planned to be carried out at one month, three months and six months after sea-launch, but some variations occurred due to practical reasons. A total of 531 salmon were examined during the study. Eubothrium sp. was found to follow a seasonal pattern of infection, with the infection pressure being highest in summer-autumn. Cohorts launched to sea in spring were initially exposed to a lower infection pressure. Abundance of the parasite was negatively correlated with fish size at the time of infection, suggesting that smaller fish have a higher risk of infection. This was supported by early stages of the worms mostly being found in fish under the length 35 cm. Little evidence was seen of infections occurring in fish over this size, perhaps due to reduced feeding on the zooplankton that serve as intermediate hosts of Eubothrium sp. This suggests that effective treatment after the fish has passed this size could lower the chances of reinfection, suggesting that treatment would only be needed once. Masteroppgave i fiskehelse FISK399 MAMN-FISK
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