Abstract
Monitoring of planktonic salmon louse (Lepeophtheirus salmonis salmonis) abundance and parameterization of key life-history traits has been hindered by labour-intensive and error-prone quantification using traditional light microscopy. Fluorescence illumination has been proposed as a means of improving visualization, but prior to this study adequate investigation of the relevant fluorescence profiles and measurement conditions has not been undertaken. We investigated the fluorescence profiles of L.salmonis and non-target copepod spp. with excitation and emission matrices (200-600nm) and identified unique fluorescence signals. Fluorescence microscopy using excitation wavelengths of 470±40nm, and emission wavelengths of 525±50nm, showed that after 90days of formalin storage salmon lice have a mean fluorescence intensity that is 2.4 times greater than non-target copepods (copepodid and adult stages). A 7-day heat treatment of 42°C in formalin increased the difference between salmon louse copepodids and non-target copepods to a factor of 3.6, eliminating the need for prolonged storage. Differences in the fluorescence signal and endogenous fluorophores were investigated with respect to variation in sea lice species, age, stage and host fish origin. Under the conditions outlined in this paper, the fluorescence signal was found to be a reliable means of visualizing and differentiating salmon lice from non-target zooplankters. Adaptation of the fluorescence signal would greatly expedite traditional methods of enumerating salmon louse larvae in plankton samples and could provide a means of automated detection.
Highlights
The salmon louse, Lepeophtheirus salmonis salmonis (Krøyer 1837; Skern-Mauritzen et al, 2014), is an obligate ectoparasite of salmonids and a major constraint to Atlantic salmon (Salmo salar) aquaculture
Measurements taken with the DAPI filter set showed that mean fluorescence intensity of 6 days post-hatch (DPH) L. salmonis copepodids was significantly different from the non-target copepods. (p < .00001), but they were not significantly different from C. elongatus copepodids (p = .0977)
Rather than examining all possible combinations, the analysis focused on the points at which L. salmonis copepodids exhibited the greatest fluorescence and where their fluorescence would be greater than other animals
Summary
The salmon louse, Lepeophtheirus salmonis salmonis (Krøyer 1837; Skern-Mauritzen et al, 2014), is an obligate ectoparasite of salmonids and a major constraint to Atlantic salmon (Salmo salar) aquaculture. In Norwegian waters, wild Atlantic salmon smolt migrating from rivers towards the sea are infected by L. salmonis copepodids suggested to primarily derive from infested farms (Fjørtoft et al, 2019; Kristoffersen et al, 2018), and the resulting lice loads increase their risk of mortality (Taranger et al, 2014). We used EEM measurements to explore the fluorescence profiles of the target sea lice species and non-target copepod spp., and identified the wavelengths where the greatest contrast in the fluorescence occurred. The fluorescence intensity exhibited by the animals was quantified at those wavelengths through fluorescence microscopy and analysed for statistical differences The reliability of those fluorescence signals was further examined by investigating factors that might influence them, including storage time in formalin, host fish origin, copepodid age and developmental stage
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