Study of the hooking behaviour of Lepeophtheirus salmonis (Krøyer, 1837) copepodids on Atlantic salmon, Salmo salar L., using a novel in vivo test system

Abstract

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.

The effect of temperature on ability of Lepeophtheirus salmonis to infect and persist on Atlantic salmon

“…Although not directly comparable, a similar trend was detected in Trial 2 where median CISS values were at 35% at 6°C and 71% at 15°C in trial 2. The observed increased infection success/CISS at higher temperatures is in accordance with earlier studies (Delfosse et al, 2018; Skern‐Mauritzen et al., 2020; Tucker et al., 2000). The actual infection success in Trial 1 may be higher than the recorded numbers, as data in Trial 2 suggest that early counts are likely to underestimate the number of lice due to their small size and indistinguishable coloration against the salmon skin and fins.…”

Effect of handling and crowding on the susceptibility of Atlantic salmon (Salmo salar L.) to Lepeophtheirus salmonis (Krøyer) copepodids

“…In the present study, all the smolts came directly from freshwater and were transferred and reared in filtered sea water in the laboratory to avoid external infestation by L. salmonis . The mean number of attached copepodids per fish in both experiment 1 and 2 (approximately 12 and 21% of introduced copepodids, respectively) can be considered to be consistent with other studies carried out under these conditions (water temperature, 10°C) (Delfosse et al., 2018; Delfosse & Pageat, 2014; Pageat & Delfosse, 2014).…”

“…The in vivo test of L. salmonis attachment success on salmon was performed according to Delfosse et al., 2018, with slight changes due to the tank size. Briefly, 30 min after the handling procedure, 2,880 copepodids L. salmonis were introduced in the 12 remaining tanks (six handled and six controls, 240 copepodids per tank).…”

“…Three times, the entire body of each fish was scrubbed and rinsed above a plastic bag. The content of the bag was then filtered, and the plastic bag was again rinsed two times, and this liquid was also filtered, to count the number of copepodids attached to each fish using a magnifying glass, as previously validated (Delfosse et al., 2018).…”

“…The test was performed as described in Delfosse et al. (2018) in individual flat beakers equipped with valves (3.5 L, Ø 23 cm) and supplied with sea water. Briefly, at D 5 , immediately after blood sampling, 8 Atlantic salmon smolts (137.8 ± 10.6 g) from each of the two holding tanks were placed into 16 flat beakers.…”