Abstract
The objective of the current study was to determine the extent to which planktonic sea lice (Lepeophtheirus salmonis Kroyer, 1838 and Caligus elongatus Nordmann, 1832) were present in the biofouling on open-sea net pens used for commercial rearing of Atlantic salmon (Salmo salar L.), and to assess if biofouling organisms on nets act as barriers similar to salmon lice skirts. We have examined two possible interactions of biofouling and planktonic sea lice, the first was if biofouling could function as a microhabitat for planktonic sea lice, with resuspension of sea lice during net cleaning operations. The second interaction was if biofouling may cause a retaining effect on the transport of planktonic stages out of the net pen. These interactions were investigated at different commercial salmon farms. With only one sea louse found among the biofouling on nets and cleaner fish shelters, we found no indication that sea lice utilized biofouling as a reservoir. This was further supported by the lack of impact on the proportion of samples with sea lice and the average density of sea lice in the water following the release of biofouling material during in-situ net cleaning. Furthermore, the presence of biofouling had no effect on the proportion of samples with sea lice or the average density of planktonic sea lice in the net pens. The presence of a lice skirt resulted, however, in a significantly higher proportion of samples with planktonic sea lice inside the net pen in one of the two sites utilizing lice skirts. The results of our study suggested that the presence of biofouling has no influence on the average density and proportion of samples with planktonic stages of sea lice and that planktonic sea lice do not inhabit biofouling.
Highlights
The ectoparasitic salmon louse (Lepeophtheirus salmonis, Krøyer, 1837) continues to constitute major problems for the Norwegian Atlantic salmon (Salmo salar L., Linnaeus, 1758) aquaculture industry, affecting fish health and the economy of salmon farming, while causing public concerns (Costello, 2006; Liu and Bjelland, 2014)
Lice counts for all net pens at the site at the week of sampling averaged 0.12 adult female lice per salmon, and the sampled net pen had been between 0.09 and 0.15 adult female lice per salmon and between 1.45 and 1.95 Caligus elongatus per salmon the three last weeks2
Net cleaning had no significant effect on average density of planktonic sea lice, outside the net pen, at the site NoskirtC [before: 0.92 ± 0.43 (SD) lice m−3, during: 0.85 ± 0.36 lice m−3; bootstrap two-sample t-test = 0.82, p = 0.45, Figure 2E]
Summary
The ectoparasitic salmon louse (Lepeophtheirus salmonis, Krøyer, 1837) continues to constitute major problems for the Norwegian Atlantic salmon (Salmo salar L., Linnaeus, 1758) aquaculture industry, affecting fish health and the economy of salmon farming, while causing public concerns (Costello, 2006; Liu and Bjelland, 2014). Nauplii have been shown to prefer higher temperatures, that optimize and accelerate development time to the copepodite stage (Johnson and Albright, 1991; Pike et al, 2006; á Norði et al, 2015), but have been shown to aggregate underneath thermoclines if the upper water layer is > 12◦C (Crosbie et al, 2020). Both nauplii and copepodites of salmon lice avoid low salinities and will aggregate at or under the halocline, if present (Bricknell et al, 2006; Crosbie et al, 2019). The infestive stage needs to find a host before it exhaust the energy reserves from the yolk, because salmon lice do not feed until they attach to a salmon (Piasecki and MacKinnon, 1995; Tucker et al, 2000)
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