The screening of putative semiochemicals involved in the interaction between Lepeophtheirus salmonis copepodid and the Atlantic salmon could be used to develop methods to prevent infestation. A bioassay was designed to evaluate the attractiveness of Atlantic salmon for L. salmonis copepodids by counting the number of hooked parasites. A salmon was bathed in a tank with 60 copepodids during 45 min. The salmon was humanely killed and its body was scrubbed above a plastic bag. The content of the bag was filtered to count the number of hooked parasites. This procedure was tested at different water temperatures and for different body mass. The temperature significantly influenced the number of hooked copepodids (p < .05). Smolts presented significantly more hooked copepodids at a higher water temperature (24.6 at 13.8°C) compared to smolts at a lower temperature (18.6 at 6.1°C; p = .011). No correlation was found between the body mass and the number of hooked parasites (p = .65). This bioassay was used to successfully measure the attractiveness of Atlantic salmon for L. salmonis copepodids, making it an interesting tool to screen putative semiochemicals designed to prevent the infestation.
Lepeophtheirus salmonis is an ectoparasite causing economic concerns in Atlantic salmon farming. Salmon lice infestation management methods can be stressful and impact fish welfare. This work investigated the stress effect on the attachment of L. salmonis copepodids to Atlantic salmon through two approaches: (a) handling by netting and air exposure (acute stress), and (b) crowding with restricted surface access in a tank (chronic stress). In the first experiment, we compared the number of attached L. salmonis and cortisol levels between a group of handled salmon and a control group. In the second experiment, a group of densely packed salmon was compared to a control group based on the number of attached copepodids, cortisol levels and neutrophil:lymphocyte ratios. Handled salmon showed significantly higher plasma cortisol levels (p < .001) and more attached copepodids (p = .01) than control salmon. Conversely, the cortisol level and number of attached copepodids were not significantly different between the densely packed and control salmon (p > .05). The neutrophil:lymphocyte ratio was significantly higher (p = .0014) in the densely packed salmon than in the control salmon. Handling salmon increased their risk of infestation by L. salmonis. This has implications for reinfestation rates following delousing treatments in commercial salmon aquaculture, which often involve crowding and handling salmon.
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