Effect of ectoparasite infestation density and life-history stages on the swimming performance of Atlantic salmon Salmo salar

-To overcome sustainability obstacles and improve operations, the Atlantic salmon farming industry is testing novel approaches to production. Redistributing farm sites to offshore locations is one such solution; however, tolerance to high-current velocity sites must be considered, particularly if fish health status is compromised by parasites. We tested the effect of parasite density and life-history stage on the swimming performance of Atlantic salmon Salmo salar using a swim flume. Salmon with 3 different salmon lice Lepeophtheirus salmonis densities (0, 0.02 ± 0.01 and 0.11 ± 0.01 lice cm-2 [mean ± SE]) were tested across the 4 major life-history stages of lice (copepodid, chalimus, pre-adult and adult) for critical swimming performance (Ucrit). Salmon Ucrit declined slightly by a mean of 0.04 to 0.10 body lengths s-1 with high parasite densities compared to uninfested and low densities, across the lice stages, while progression through the parasite life-history stages had little effect on swimming performance. Our results suggest that increasing infestation density of salmon lice incurs negative fitness consequences for farmed Atlantic salmon held in high-current velocity sites, with little difference in costs associated with attachment by different life-history stages of the lice

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

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.publishedVersio

Part of CAC2016G Vindsvik: Integrated lice management -test of various combinations of preventive measures against lice.

Source at https://www.hi.no/en/hi/nettrapporter/rapport-fra-havforskningen/2018/45-2018_assessment_of_long-term_implementationSalmon lice prevention strategies are steering towards passive implementation, and this study aimed to monitor these strategies in commercial cages over time, to determine the efficiency of these approaches and their effect on welfare. Four strategies were tested at a commercial scale at the Centre for Aquaculture Competence (CAC; funded by CAC and FHF), over a 13-month period. The additive effect of multiple treatments was established in 12 cages, which were assigned to a prevention strategy of either cleaner fish only (A group), cleaner fish and functional feed (B group), the previous two factors plus deep attractant lights and submerged feeding (C group), or the previous three factors plus a lice skirt (D group). Environmental profiles and school swimming depth were monitored throughout the study period, and sampling events occurred every 3 –5 weeks to assess the infestation and welfare status of salmon. The rate of infestation fluctuated with season, however the group with all prevention strategies (D) maintained a lower rate of new infestations compared to the groups with in-cage prevention (A and B groups). However, even with strong differences in new infestations and swimming behaviour, the level of mobile lice was similar among all groups, thus incurring a similar frequency of delousing events. This is potentially due to unrepresentative sampling of salmon or reduced cleaner fish efficiency in treatment cages. There was no overall effect of these prevention strategies on welfare status of salmon during the study period. Thus, the tested prevention strategies have promising potential for commercial implementation; however, improved and consistent efficiency is likely to be achieved with flexible operation that changes with specific environmental conditions.I de senere år har det vært økt fokus på økt bruk av er en rekke forebyggende strategier mot lakselus i oppdrettsmerder. Dette studie dokumenterer hvordan ulike tiltak gjennom en produksjonssyklus egner er seg for bruk, med fokus på den lusereduserende effekt og laksens velferd. Forsøket ble gjennomført i kommersiell skala ved «Centre for Aquaculture Competence» (CAC, finansiert av CAC og FHF 901243), ved lokaliteten Vindsvik i Rogaland og varte i 13 måneder. Den adderende/ økende effekten av flere samtidige tiltak ble undersøkt i 12 merder. Forebyggende tiltak besto av enten bare rensefisk (gruppe A), rensefisk + funksjonelt fôr (gruppe B), rensefisk + funksjonelt fôr + dype lys og fôring (gruppe C) eller rensefisk + funksjonelt fôr + dype lys/ fôring + skjørt (gruppe D). Hver 3.-5. uke ble det tatt ut fisk for lusetelling av alle stadier og skåring av fisk for å vurdere laksens velferd. Miljøprofiler ble tatt daglig og ekkolodd ble benyttet i lange perioder. Påslag av lakselus varierte betydelig med sesong. Fiskegruppen som ble gitt samtlige forebyggende tiltak (D) hadde konsekvent lavere påslag enn gruppene som kun hadde rensefisk og funksjonelt fôr (A og B). Selv om det var store forskjeller i lusepåslag og svømmedyp var nivået av bevegelige lus likt mellom gruppene, og dermed var behovet for avlusing det samme. Potensielle årsaker kan være sammensatt og mangesidig, eksempelvis at rensefiskens effektivitet er redusert eller at representativiteten i uttakene ikke har vært lik mellom gruppene som svømmer på ulike dyp. Overordnet velferd hos laksen var lik mellom gruppene. De ulike forebyggende tiltakene hadde kommersielt potensial for å bli benyttet i forhold til velferd og produksjon. Men, det er behov for å etablere og vise mer stabil og bedre effekt mot lusepåslag. Muligens kan dette oppnås ved å kombinere bruken av tiltakene i takt med de varierende miljøforhold, med fokus på brakkvannslag, og laksens valg av svømmedyp

Sea lice prevention strategies affect cleaner fish delousing efficacy in commercial Atlantic salmon sea-cages

Over the last 2 decades, cleaner fishes have been employed to remove external sea lice parasites from Atlantic salmon Salmo salar in sea cages. Norway, Scotland, Ireland, and the Faroe Islands combined now use ~60 million cleaner fish per year. While small-scale experiments demonstrate the efficacy of cleaner fishes, industrial-scale sea cages have multiple structures and conditions that create different environments, which may impact cleaner fish efficacy and welfare. Here, in commercial sea cages, we investigated if 4 different anti-lice strategies impacted the delousing efficacy, physical condition, and behaviour of cleaner fish (corkwing wrasse Symphodus melops). The strategies tested were: (1) cleaner fishes only; (2) cleaner fishes and functional feed; (3) cleaner fishes, functional feed, and deep lights and feeding; and (4) cleaner fishes, functional feed, deep lights and feeding, and lice skirts. Corkwing wrasse were sampled from 3 cage-level replicates of each anti-lice strategy 3 times over 2 mo. Lice levels on salmon were recorded every 3 to 4 wk. Only 11% of corkwing wrasse had salmon lice in their gut, with individual wrasse having up to 72 lice in their stomach. Wrasse in cages encircled by lice skirts consumed one-ninth as many lice as those in other anti-lice treatments and had less overall impact on the number of lice per salmon. Fin, skin, mouth and eye condition, K factor, and observed cleaning behaviours of corkwing wrasse were similar across all anti-lice strategies. Our results demonstrate that different in-cage anti-lice strategies altered the magnitude of lice consumption in corkwing wrasse at this site and for this production period. Moreover, while a small proportion of corkwing wrasse appear to target lice as prey, most individual corkwing wrasse were ineffective biological control agents in a full-scale farm setting.publishedVersio

Salmon lice survive the straight shooter: A commercial scale sea cage trial of laser delousing

Ectoparasitic salmon louse (Lepeophtheirus salmonis) infestations are costly for Atlantic salmon (Salmo salar) farmers in Norway. As a result, there is a strong desire for solutions to prevent and control infestations, and new technologies are typically developed and commercialised rapidly, without rigorous validation. Here, we tested the efficacy of a new commercially available control measure—delousing by underwater lasers—using a replicated design at full commercial scale. Laser delousing was used in combination with a preventive method (snorkel cages), with laser nodes deployed in 3 of the 6 sea cages at the site. The trial ran for 54 days, after which time there was no difference in infestation density of mobile salmon louse stages (pre-adult, adult male or adult female) in cages with or without laser nodes installed. By the end of the trial, adult female lice numbers in all cages were close to the legislated trigger for mandatory delousing (0.5 adult female lice per fish). The laser nodes delivered a large number of pulses relative to the number of lice in the cages, indicating that a lack of lethality rather than a lack of target detection was the limiting factor. If all pulses had been effective, they should have removed between 4–38 % of mobile lice each day. There was no effect on salmon welfare indicators such as skin condition or eye status. Our results highlight the importance of rigorous validation of new technologies across a range of conditions before widespread implementation by industry.publishedVersio

Assessment of long-term implementation of sea lice prevention technologies: efficiency in reducing infestations and impact on fish welfare

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Disentangling the key drivers of salmon louse Lepeophtheirus salmonis fecundity using multiyear field samples

Planktonic salmon louse Lepeophtheirus salmonis salmonis larvae produced at sal - mon farms spread to and infect both wild and farmed salmonids. Understanding and forecasting the production and distribution of these larval stages from farms is critical to aquaculture management. However, model forecasts are based on available data and therefore include parameters with limited empirical support. This investigation examined salmon louse fecundity with a focus on batch egg clutch size by collecting lice from farmed Atlantic salmon Salmo salar at multiple farms and from wild Atlantic salmon and sea trout S. trutta captured at field sites throughout Norway. The data were analyzed with mixed effects models and total length of female lice was identified as the primary determinant of clutch size. Further analysis revealed that female louse total length is partially explained by temperature at sampling. However, if the temperature at sampling is spatially or temporally disconnected from rearing temperature, it may not be possible to predict the total length of a louse using temperature. The fecundity investigation further found that 66% of female lice on farmed salmon were sexually mature, and 10% of these were not egg-bearing. In comparison, 73% of adult female lice on sea trout were sexually mature, and 40% of these were not egg-bearing. Our results indicate that salmon louse production forecasts would be improved by incorporating female louse sexual maturity and a clutch size parameter that is related to total length of female lice.publishedVersio

Salmon lice evasion, susceptibility, retention, and development differ amongst host salmonid species.

With different ecological characteristics amongst salmonid species, their response to parasitic infestation is likely to vary according to their spatial and temporal overlap with the parasite. This study investigated the host–parasite interactions amongst three species of salmonids and the ectoparasitic salmon louse, Lepeophtheirus salmonis. To determine any variation in infestation parameters amongst salmonids, single population groups of Atlantic salmon (Salmo salar), chinook salmon (Onchorhynchus tshawytscha), and previously-infested and naïve sea trout (Salmo trutta) were exposed to a controlled infestation challenge. We found that chinook salmon and both sea trout groups were more susceptible to acquiring lice than Atlantic salmon. Behavioural responses during infestation were more pronounced in Atlantic and chinook salmon. Parasite development was similar in lice attached to Atlantic salmon and sea trout, but hindered on chinook salmon. At 16 days post-infestation, chinook salmon had reduced lice loads to the same level as Atlantic salmon, whilst sea trout retained their lice. These results demonstrate differences in interactions with L. salmonis amongst these species, and highlight the vulnerability of sea trout to infestationpublishedVersio

In situ sea lice egg sterilization with UVC light

Sea-cage salmon farming creates ideal conditions for population growth of the salmon louse Lepeophtheirus salmonis, potentially leading to poor welfare and mortality in farmed salmon and nearby wild salmonids. Frequent delousing treatments are necessary, but current treatments have drawbacks in terms of financial cost, stress to stock, and/or environmental impacts. We tested whether 254 nm ultraviolet-C light (UVC) could function as a new treatment to reduce production of infective copepodids in infested sea-cages. In Experiment 1, we removed mature egg strings from female lice and exposed the egg strings to precise doses of UVC light. A total dose of 0.008 J cm-2 reduced copepodid production by 5 %, while a 95 % reduction occurred at 0.09 J cm-2. In Experiment 2, we exposed salmon with attached adult lice to UVC light while they swam freely in tanks over a 6 day period, achieving a dose of ~0.1 J cm-2. The treatment resulted in a 99 % reduction in copepodid production relative to control groups. However, UVC negatively impacted fish welfare, causing higher rates of cataracts and skin irritation. In Experiment 3, we tested the sensitivity of fish (without lice) to increasing doses of UVC light, and found that minor skin injuries occurred at >60 % effective doses, while cataracts began to develop at very low doses. We conclude that UVC should only be used with caution, either for treatment of waste water to prevent louse eggs and larvae entering the environment (e.g. after delousing), or for short periods of time in-cage to suppress lice reproduction until fish are harvested.publishedVersio

Delousing Efficacy and Physiological Impacts on Atlantic Salmon of Freshwater and Hyposaline Bath Treatments

A pilot study was conducted to investigate the delousing efficacy and physiological effects of freshwater and hyposaline water in Atlantic salmon ( Salmo salar ) for various treatment durations. There were four primary treatment groups: salmon lice ( Lepeophtheirus salmonis) infected fish held in seawater (procedural controls), infected fish treated in freshwater (buffered with 1% seawater), uninfected fish treated in freshwater, and infected fish treated with hyposaline water. The ion-modified, hyposaline water (~5 ppt salinity) was created through nano-filtration technology which desalinated seawater. Salmon were held for a period ranging from 30 minutes to 48 hours in their designated treatment bath, lice counts were made before and after treatment to measure efficacy of delousing, and blood samples were taken to investigate the physiological effects on fish. The freshwater treatment had no apparent delousing effect when compared with the seawater group, but just 5% of lice remained on fish after a 4-hour hyposaline treatment. The osmotic balance of fish in the freshwater and hyposaline treatments shifted towards a lower internal concentration of ions with longer bath durations, but the fish recovered once transferred back to seawater. Indicators of stress reflected a less clear pattern with bath duration which may be due to the additional handling stress associated with moving fish between tanks and an elevated stress response triggered by transition back to a freshwater environment.Delousing Efficacy and Physiological Impacts on Atlantic Salmon of Freshwater and Hyposaline Bath TreatmentspublishedVersio