Abstract
The present article reports the densities of planktonic sea lice (Lepeophtheirus salmonis and Caligus elongatus) in three Atlantic salmon (Salmo salar) localities, and the relationship between the abundance of adult sea lice on the salmon and the densities of planktonic sea lice stages, during a complete production cycle followed by a fallowing period. Samples were taken downstream inside and immediately outside of cages, at one locality with lice skirts and two localities without lice skirts. There were no differences in densities of planktonic sea lice in samples taken from the inside or the outside of cages for any of the localities. However, the proportion-non-zero of planktonic sea lice samples taken from inside the cage was higher during months with a temperature above 9°C (mean abundance: 0.40–2.5 individuals m–3) than months with temperature below 9°C (mean abundance: 0.02–0.21 individuals m–3, odds ratio of the proportion-non-zero: p < 0.01). Densities of planktonic sea lice correlated most strongly with temperature in the first year (τ = 0.44–0.57, p < 0.05). A significant correlation between the number of adult female lice on salmon and average density of plankton sea lice was found in the locality with lice skirts during the second year (τ = 0.43 inside cages, τ = 0.58 outside cages, both p values < 0.05). Background levels of planktonic sea lice in the succeeding fallowing period showed neither L. salmonis nor C. elongatus planktonic sea lice, suggesting that there was successful reduction of the densities of planktonic sea lice for this area during the fallowing period.
Highlights
Effective methods of controlling the infestation of the ectoparasitic sea lice (Lepeophtheirus salmonis Krøyer 1837, and Caligus elongatus Nordmann 1832, hereafter both species are referred to as sea lice) on farmed Atlantic salmon (Salmo salar Linnaeus 1758) are needed for expansion of Atlantic salmon production in Norway
This study investigated the number of planktonic sea lice inside and outside the salmon cages at three different salmon farms during an entire production cycle
The results of our study showed that the densities and proportion-non-zero (% of samples containing at least one planktonic sea lice) of planktonic sea lice found inside of the cages were similar to those outside, suggesting that planktonic sea lice disperse from cages even if they are equipped with lice skirts
Summary
Effective methods of controlling the infestation of the ectoparasitic sea lice (Lepeophtheirus salmonis Krøyer 1837, and Caligus elongatus Nordmann 1832, hereafter both species are referred to as sea lice) on farmed Atlantic salmon (Salmo salar Linnaeus 1758) are needed for expansion of Atlantic salmon production in Norway. Planktonic and Parasitic Sea Lice to a host (Piasecki and MacKinnon, 1995; Hamre et al, 2013). C. elongatus is considered a generalist parasite, since it is able to infest more than 80 different species of fish, while salmon lice (L. salmonis) is considered a salmonid specialist, mostly affecting the three salmonid genera Salmo, Salvelinus, and Oncorhynchus (Wootten et al, 1982; Kabata, 1992). Both species infest Atlantic salmon, but L. salmonis is the only species currently regulated by Norwegian authorities (Forskrift om lakselusbekjempelse, 2020a). The critical accepted upper level is, lowered to 0.2 adult female lice per salmon on average for 6 weeks in the spring (Forskrift om lakselusbekjempelse, 2020a)
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