‘Snorkel’ lice barrier technology reduced two co- occurring parasites, the salmon louse ( Lepeophtheirus salmonis ) and the amoebic gill disease causing agent ( Neoparamoeba perurans ), in commercial salmon sea-cages

Repeated sublethal freshwater exposures reduce the amoebic gill disease parasite, Neoparamoeba perurans, on Atlantic salmon

“…Salmon likely entered the enclosure as it was the only place within the snorkel cage that they could reach the surface and refill their open swim bladder by swallowing air at the surface for buoyancy control (Dempster et al., ). Use of the freshwater surface layer coincided with reductions in amoebic gill disease levels (Wright, Stien, et al., ), which suggest that fish were utilizing the layer for >3 min daily exposures for swim bladder reinflation and due to other motivational factors (Oppedal et al., ) or that daily exposures of ≤3 min were over longer periods than the current trial. This commercial example may open the way for implementation of similar strategies for other AGD‐affected marine fish species where freshwater exposures are practical.…”

“…While repeated exposures are difficult to achieve using traditional bathing practices such as a tarpaulin lined cage or well boat, they are possible with recently developed in situ bathing methods. A freshwater layer within a surface enclosure in a “snorkel” cage reduced an outbreak of amoebic gill disease in commercial sea‐cages for Atlantic salmon in Norway (Wright, Stien, et al., ). Salmon likely entered the enclosure as it was the only place within the snorkel cage that they could reach the surface and refill their open swim bladder by swallowing air at the surface for buoyancy control (Dempster et al., ).…”

“…Atlantic salmon may also move into surface layers for extended periods at night and during feeding or resulting from other motivational factors (Oppedal, Dempster, & Stien, ). In commercial snorkel cages, continuous filling with freshwater to create surface salinities down to 4–5 g/L has coincided with reduced AGD levels (Wright, Stien, et al., ). Developing these methods for AGD management requires a more detailed understanding of how freshwater affects N. perurans populations.…”

“…Novel approaches to parasite treatment that use fish behaviour and cage environment manipulations to minimize handling and fish welfare impacts are gaining traction in Atlantic salmon sea-cage farming for other parasitic diseases (Bui, Oppedal, Sievers, & Dempster, 2017). For example, freshwater or low salinity surface environments have been created over long timeframes in sea-cages, which may reduce freshwater sensitive parasites, depending on the salinity changes that salmon experience (Wright, Stien, et al, 2017). Physostomous (open swim bladder) fish such as salmonids (Fahl en, 1971) frequently refill their swim bladder by surfacing for air through such a surface environment (Dempster, Kristiansen, Korsøen, Fosseidengen, & Oppedal, 2011).…”

Sea lice infestation levels decrease with deeper ‘snorkel’ barriers in Atlantic salmon sea‐cages

“…Commercial-scale snorkels at present are 90 m in circumference and 10 m deep within sea-cages 160 m in circumference and 30-50 m deep. 60 At these scales, current flow and oxygen flux will likely differ from our trial results, and therefore full commercial-scale testing of the technology in different production environments is essential to determine the practical implications of operating these cages.…”