Surface environment modification in Atlantic salmon sea-cages: effects on amoebic gill disease, salmon lice, growth and welfare

Dw Wright,E Karlsbakk,L Geitung,T Oldham,Lh Stien,F Oppedal,V Nola,T Dempster

doi:10.3354/aei00269

Dw Wright, E Karlsbakk + Show 6 more

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https://doi.org/10.3354/aei00269

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Abstract

Surface environment modification is a potential parasite control strategy in Atlantic salmon sea-cage farming. For instance, a temporary low salinity surface layer in commercial-scale snorkel sea-cages has coincided with reduced amoebic gill disease (AGD) levels after an outbreak. We tested if a permanent freshwater (FW) surface layer in snorkel sea-cages would lower AGD and salmon lice levels of stock relative to snorkel cages with seawater (SW) only and standard production cages with no snorkels. Triplicate cages of each type with 2000 post-smolts were monitored in autumn to winter for 8 wk and sampled 4 times. Lower proportions of individuals with elevated AGD-related gill scores were registered in SW and FW snorkel cages compared to standard cages; however, these proportions did not differ between SW and FW snorkel cages. Individuals positive for AGD-causing Paramoeba perurans were reduced by 65% in FW snorkel relative to standard cages, but values were similar between SW snorkel cages and other types. While total lice burdens were reduced by 38% in SW snorkel compared to standard cages, they were unchanged between FW snorkel and other cage types. Fish welfare and growth were unaffected by cage type. Surface activity was detected in all cages; however, more surface jumps were recorded in standard than snorkel cages. Overall, fish in FW snorkel cages appeared to reside too little in freshwater to consistently reduce AGD levels and salmon lice compared to SW snorkel cages. Further work should test behavioural and environmental manipulations aimed at increasing freshwater or low salinity surface layer use.

Highlights

Sea-cage Atlantic salmon Salmo salar farming produces more than 2.3 × 106 t yr−1 (FAO 2017)
Even though it is well established that snorkel cages reduce salmon lice levels (Stien et al 2016, Oppedal et al 2017), we examined cage type effects on salmon lice infestations
We stocked each cage with 2000 post-smolt Atlantic salmon, naïve to both amoebic gill disease (AGD) and salmon lice exposure, in a randomized block order from 26 to 28 October 2016

Summary

Introduction

Sea-cage Atlantic salmon Salmo salar farming produces more than 2.3 × 106 t yr−1 (FAO 2017). This new and constant availability of large numbers of hosts has led to an increased scale of salmon parasite outbreaks in many marine ecosystems (Nowak 2007). The expansion of AGD outbreaks to all major salmon farming regions has caused mass mortality events and a surge in AGD treatments (Shinn et al 2015, Oldham et al 2016). Innovating parasite controls to reduce both salmon lice and AGD could safeguard the ecological sustainability and future expansions of the salmon farming industry (Wright et al 2017)

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