Where art thou louse? A snapshot of attachment location preferences in salmon lice on Atlantic salmon hosts in sea cages

Abstract

Problematic sea lice infestations on farmed Atlantic salmon (Salmo salar) have motivated extensive research and development into new methods to prevent, monitor and control sea lice. Most of these technologies require detailed information on the behaviour, spatial distribution and demography of lice on host fish. This study investigated how salmon lice (Lepeophtheirus salmonis) infestation density varies across the host's surface under sea cage farming conditions. Lice abundance, demography and attachment location were tracked over time, with repeated sampling of 300 individually tagged salmon across three replicate experimental sea cages. The data reveal clear differences in attachment locations according to sex and stage, but with an overall preference for the dorsal surface among mobile stages—dorsal head for adult females and dorsal‐posterior section for males and pre‐adults. Total lice abundance was highly variable between repeated measures of individual fish, consistent with frequent host‐switching or mortality. Total lice numbers also declined between sampling dates, likely due to handling, with lost mobile lice being almost exclusively adult males. As the distribution of sea lice on hosts is likely determined by numerous factors, future image‐based automated detection systems should be validated in settings that reflect the complex host–parasite interactions that occur in open farming systems.

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

“…The sex ratio of salmon lice is generally 1:1 (Carmichael et al., 2013), but females develop slower than males (Eichner et al., 2015), and thus, at the end of the analysis period females were newly moulted to the adult stage, whereas males had been adult for a longer period. Earlier studies have reported a higher loss of adult males than females possibly due to mating‐associated behaviour (Bui et al., 2020; Hamre & Nilsen, 2011; Stephenson, 2012). Nevertheless, no differences from a 1:1 sex ratio were detected in the present study, possibly due to the relatively short experimental period.…”

“…Salmon lice can be long‐lived and may remain on the host fish for extended periods of time under optimal conditions (Hamre et al., 2009); however, little concrete evidence is provided for the rate of loss of salmon lice after infection; it varies considerably between studies ranging from 30% to almost complete loss during development to adults (Bjørn & Finstad, 1998; Bui et al., 2018; Hamre et al., 2009; Hamre & Nilsen, 2011; Jones, Fast, Johnson, & Groman, 2007). Biological mechanisms have been explored whereby lice are likely to be lost from the host through host physiological or immune defences (Wagner, Fast, & Johnson, 2008), mate‐searching behaviours in mature stages (Stephenson, 2012), natural mortality, or some cumulative interaction of these factors (Bui, Oppedal, Nola, & Barrett, 2020). Although some Pacific salmonids display resistance towards salmon lice, the most commonly farmed salmonids—Atlantic salmon ( Salmo salar ) and rainbow trout ( Oncorhynchus mykiss )—are susceptible (Dalvin, Jørgensen, et al., 2020; Fast et al., 2002; Gjerde & Saltkjelvik, 2009; Johnson & Albright, 1992; Jones et al., 2007; Sutherland et al., 2014).…”

Evolution of salmon lice in response to management strategies: a review

“…Salmon lice move on and between hosts to find mates and avoid intra-specific competition (Hull et al 1998;Todd et al 2005). Motile louse stages are distributed differently on wild and farmed hosts, with farmed salmon having greater concentrations on the head (Jaworski & Holm 1992;Bjørn & Finstad 1998;Todd et al 2000;Bui et al 2020b). This may be due to lice choosing microhabitats based on environmental conditions, such as host density or host swimming behaviour.…”

“…Chalimi and adult female lice appear to be more resistant to these technologies than other life stages (Table 1). This is also the case during the incidental mechanical delousing that occurs when salmon are transferred between pens or handled during sampling (Reynolds 2013;Powell et al 2015;Bui et al 2020b). This selective removal of certain age and sex classes would alter the demographics of louse populations.…”

“…2000; Bui et al . 2020b). This may be due to lice choosing microhabitats based on environmental conditions, such as host density or host swimming behaviour.…”