Trends in de-lousing of Norwegian farmed salmon from 2000–2019—Consumption of medicines, salmon louse resistance and non-medicinal control methods

Abstract

The salmon louse Lepeophtheirus salmonis has been a substantial obstacle in Norwegian farming of Atlantic salmon for decades. With a limited selection of available medicines and frequent delousing treatments, resistance has emerged among salmon lice. Surveillance of salmon louse sensitivity has been in place since 2013, and consumption of medicines has been recorded since the early 80’s. The peak year for salmon lice treatments was 2015, when 5.7 times as many tonnes of salmonids were treated compared to harvested. In recent years, non-medicinal methods of delousing farmed fish have been introduced to the industry. By utilizing data on the annual consumption of medicines, annual frequency of medicinal and non-medicinal treatments, the aim of the current study was to describe the causative factors behind salmon lice sensitivity in the years 2000–2019, measured through toxicity tests–bioassays. The sensitivity data from 2000–2012 demonstrate the early emergence of resistance in salmon lice along the Norwegian coast. Reduced sensitivity towards azamethiphos, deltamethrin and emamectin benzoate was evident from 2009, 2009 and 2007, respectively. The annual variation in medicine consumption and frequency of medicinal treatments correlated well with the evolution in salmon louse sensitivity. The patterns are similar, with a relatively small response delay from the decline in the consumption of medicines in Norway (2016 and onward) to the decline in measured resistance among salmon louse (2017 and onward). 2017 was the first year in which non-medicinal treatments outnumbered medicinal delousing treatments as well as the peak year in numbers of cleanerfish deployed. This study highlights the significance of avoiding heavy reliance on a few substance groups to combat ectoparasites, this can be a potent catalyst for resistance evolution. Further, it demonstrates the importance of transparency in the global industry, which enables the industry to learn from poor choices in the past.

Estimating cage‐level mortality distributions following different delousing treatments of Atlantic salmon (salmo salar) in Norway

“…The present data set contains a large number of treatments for the most frequently applied delousing methods. The analysis of this data set also confirms the reported national yearly reduction of medicinal treatments and paradigm shift from medicinal to non‐medicinal delousing treatments in Norway (Aaen et al., 2015; Helgesen et al., 2020; Myhre Jensen et al., 2020; Overton, Dempster, et al., 2018). The substantial increase in total number of treatments from year 2016 to 2017 (Table 3) and from year‐class 2015 to 2016 (Table 4) is mainly due to missing production data from year‐class 2015 and 2016 from one company.…”

“…Over a long time period, large‐scale reliance on a few chemotherapeutants to control salmon lice has resulted in a drift towards resistance (Denholm et al., 2002; Myhre Jensen et al., 2020). Today, salmon lice display reduced sensitivity and/or resistance towards all available chemotherapeutants, except benzoylphenyl ureas (Aaen et al., 2015; Helgesen et al., 2020).…”

“…Currently, in Norway thermal and mechanical delousing are the most frequently applied treatment methods for immediate removal of salmon lice (Helgesen et al., 2020). In 2019, about 3.3 non‐medicinal treatments occurred for each medicinal treatment event (Myhre Jensen et al., 2020), and of all immediate delousing treatments performed in 2019, 54% were thermal and 27% mechanical (Oliveira et al., 2021).…”

Farmed salmonids drive the abundance, ecology and evolution of parasitic salmon lice in Norway

“…skirts and snorkels] and selective breeding). Lice have evolved resistance to at least 4 of 5 chemical therapeutants (Aaen et al 2015, Myhre Jensen et al 2020, and while evidence is incomplete for the other louse control methods, Coates et al (2021) concluded that the evolution of resistance to non-chemical methods is a strong possibility given the variation that exists in and between louse populations (Jacobs et al 2018) on which non-chemical selection pressures could act and that this variation may have a genetic basis.…”

Losing the ‘arms race’: multiresistant salmon lice are dispersed throughout the North Atlantic Ocean

“…Instead, organophosphates, pyrethroids and hydrogen peroxide were reintroduced. The initial effect was good, but the problems with resistance reappeared quickly [ 21 ].…”