Abstract
AbstractWith 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.
Highlights
Since its conception in mid-Norway in the late 1960s, the Atlantic salmon aquaculture industry has grappled with the pathogenic marine parasite: the salmon louse Lepeophtheirus salmonis (Costello 2006; Torrissen et al 2013)
The overall number of registered delousing operations reported in the Norwegian Food Authorities database chemotherapeutant bathing, hydrogen peroxide bathing, mechanical treatment, thermal treatment, and ‘bath other’ treatments(lines), and total number of reported delousing operations from 2012 to 2017 in all production zones in Norwegian Atlantic salmon aquaculture
From 2012 to 2015, bathing with chemotherapeutants dominated production zones 9 to 13, with more than 80% of all treatments registered as chemotherapeutants (Fig. 3a)
Summary
Since its conception in mid-Norway in the late 1960s, the Atlantic salmon aquaculture industry has grappled with the pathogenic marine parasite: the salmon louse Lepeophtheirus salmonis (Costello 2006; Torrissen et al 2013). Salmon lice have the greatest economic impact of all parasites affecting aquaculture, but infestations in farms negatively affect wild stocks via spillback effects (Torrissen et al 2013; Vollset et al 2017). Controlling this parasite is troublesome, expensive and important, to. The main source of lice infestations originates from farms, where regular delousing operations constantly place selection pressure on resistance development (Torrissen et al 2013). The development of coordinated production zones (Fig. 1), synchronized fallowing and synchronized treatments throughout Norway are other control measures (Norwegian Ministry of Trade, Industry and Fisheries 2012). The development of in-cage technologies that prevent infestations through environmental manipulation has become increasingly popular, especially skirts around the cages (Grøntvedt et al 2018; Stien et al 2018), snorkel cages (Oppedal et al 2017; Wright et al 2017) and deepwater feeding (Frenzl et al 2014)
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