Salmon lice treatments and salmon mortality in Norwegian aquaculture: a review

Abstract

With the expansion of Atlantic salmon aquaculture, the economic and ecological impacts of salmon lice (Lepeophtheirus salmonis) has increased. Norway battles this problematic parasite with various control and preventative methods within farms. We analysed two national-level databases to examine the number of operations reported each year from 2012 to 2017 and salmon mortality rates attributable to each operation type. From 2012 to 2017, 1.4 times more operations were registered, despite only limited increases in biomass produced across this period. We detected a rapid and recent paradigm shift in the industry's approach to lice control from chemotherapeutant to non-medicinal operations. Chemotherapeutants (azamethiphos, cypermethrin, deltamethrin and hydrogen peroxide) dominated operations from 2012 to 2015 (>81%), while mechanical and thermal treatments dominated in 2016 and 2017 (>40% and >74%, respectively). Thermal operations caused greatest mortality increases (elevated mortality for 31% of treatments), followed by mechanical (25%), hydrogen peroxide (21%), and azamethiphos, cypermethrin and deltamethrin (<14%). Temperature, fish size and pre-existing mortality rates all influenced post-treatment mortality outcomes. For chemotherapeutants, mortality increased as sea temperature increased. For mechanical and thermal treatments, mortalities increased at low (4-7°C) and high (13-16°C) temperatures. Fish with high pre-existing mortality (0.25-1.0% mortality the month before treatment) experienced increased mortality after treatment, and large fish (≥2 kg) were more susceptible to increased mortality than small (<2 kg). Generally, thermal, mechanical and hydrogen peroxide operations performed better in 2017 compared to 2015 and 2016, as the percentage of mortality observations were lower. With mechanical and thermal treatments now predominant, future research and industry development should prioritise reducing mortality and improving post-treatment outcomes.

How are the salmon lice (Lepeophtheirus salmonis Krøyer, 1837) in Atlantic salmon farming affected by different control efforts: A case study of an intensive production area with coordinated production cycles and changing delousing practices in 2013–2018

“…While there have been multiple analyses of salmon lice records (Heuch, Revie, & Gettinby, 2003;Kristoffersen et al, 2014;Lees, Gettinby, & Revie, 2008;Overton et al, 2018), a systematic understanding of how the salmon lice situation is affected by our efforts to control it is still needed to get an in-depth understanding of larger trends. This study systematically reviews data for salmon lice, delousing efforts and number of salmon in a large area in Norway, which practices coordinated production cycles of salmon.…”

Sterilization of sea lice eggs with ultraviolet C light: towards a new preventative technique for aquaculture

“…Promising new approaches can reduce the likelihood of new infections by minimizing encounters between parasites and hosts, but regular delousing treatment is still necessary to reduce lice densities on farmed salmon. Delousing is costly and most methods are stressful for the salmon, including mechanical, thermal and chemical treatments . Moreover, some common delousing methods raise concerns for animal welfare, environmental impacts and growing treatment‐resistance .…”

“…Delousing is costly 8 and most methods are stressful for the salmon, including mechanical, thermal and chemical treatments. 9 Moreover, some common delousing methods raise concerns for animal welfare, environmental impacts and growing treatment-resistance. [10][11][12][13] New chemical-free approaches with minimal handling of salmon and low environmental impacts are needed for ongoing suppression of lice populations at salmon farms and prevention of new infestations.…”

Snorkel technology to reduce sea lice infestations: efficacy depends on salinity at the farm site, but snorkels have minimal effects on salmon production and welfare

“…outbreaks are often dealt with through prophylactic, chemotherapeutant-infused feeds (Stone et al 1999), topical bathing treatments (Roth et al 1993), biological control (e.g. cleaner fish; Groner et al 2013), or through mechanical and thermal delousing (Overton et al 2018) once infestations have occurred. Thermal treatments are currently the most commonly used delousing operation administered in Norwegian salmon aquaculture (> 61% registered thermal treatments in 2017; Overton et al 2018).…”

“…cleaner fish; Groner et al 2013), or through mechanical and thermal delousing (Overton et al 2018) once infestations have occurred. Thermal treatments are currently the most commonly used delousing operation administered in Norwegian salmon aquaculture (> 61% registered thermal treatments in 2017; Overton et al 2018). Thermal, mechanical, and chemotherapeutant treatments can lead to poor post-treatment outcomes for Atlantic salmon (Salmo salar; Overton et al 2018).…”