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

Abstract

This retrospective descriptive study estimates cage‐level mortality distributions after six immediate delousing methods: thermal, mechanical, hydrogen peroxide, medicinal, freshwater and combination of medicinal treatments. We investigated mortality patterns associated with 4 644 delousing treatment of 1 837 cohorts of farmed Atlantic salmon (Salmo salar) stocked in sea along the Norwegian coast between 2014 and 2017. The mortality is expressed as mortality rates. We found distributions of delta mortality rate within 1, 7 and 14 days after all six delousing treatments, using mortality rate within 7 days before treatments as baseline. The results show that we can expect increased mortality rates after all six delousing methods. The median delta mortality rates after thermal and mechanical delousing are 5.4 and 6.3 times higher than medicinal treatment, respectively, for the 2017 year‐class. There is a reduction in the delta median mortality for thermal and freshwater delousing from 2015 to 2019. There is a wide variability in the mortality rates, in particular for thermal delousing. Our results suggest that the variability in delta mortality for thermal delousing has been reduced from the 2014 to 2017 year‐class, indicating an improvement of the technique. However, a significant increase in the number of thermal treatments from 14 in 2015 to 738 in 2018 probably contributes to the overall increased mortality in Norwegian salmon farming.

Analysing mortality patterns in salmon farming using daily cage registrations

“…Looking at the final model of ‘harvest mortality’, the number of salmon lice treatments drives the mortality and none of the significant fixed effects in the early mortality model remained significant at this stage. Treatments have been reported from several other studies to be the main cause of mortality in the Norwegian salmon farming during the last years (Bang Jensen, Qviller, et al., 2020; Overton et al., 2019; Sommerset et al., 2021) as well as a recent study showing increased mortality especially from non‐medical treatments when investigating lice treatments in detail (Sviland Walde et al., 2021). Our study supports these findings of high mortality associated with lice treatments, also by identifying ‘handling’ as the most prevalent group of mortality causes in terms of number of dead fish.…”

qPCR screening for Yersinia ruckeri clonal complex 1 against a background of putatively avirulent strains in Norwegian aquaculture

“…In recent years, however, the disease has also become more common in larger sea‐farmed fish, often manifesting within a couple of weeks following non‐medicinal delousing. Such procedures, introduced in the face of increasing development of salmon‐louse resistance to chemotherapeutants, may often cause acute mortality, skin damage and poor fish welfare (Folkedal et al, 2021; Nilsson et al, 2019; Overton et al, 2019; Sviland Walde et al, 2021), and are undoubtedly extremely stressful to the subjected fish. In this study, we found eDNA from sea water equally suitable for Y. ruckeri detection by PCR as compared to freshwater, with unambiguous detection of the bacterium by both developed assays as far as 100 m downstream of a salmon sea‐cage experiencing an active Y. ruckeri CC1 infection (Figure 6).…”

Principles and Methods of Counteracting Harmful Salmon–Arthropod Interactions in Salmon Farming: Addressing Possibilities, Limitations, and Future Options

“…There are no published studies on fish welfare and mortality, but losses of scale, gill bleeding, wounds and salmon mortality have been recorded (Gismervik et al, 2017;Holan et al, 2017;Hjeltnes et al, 2019;Westgård, 2020). A recent study has assessed the mortality of various delousing methods and concluded that thermal and mechanical delousing have the highest overall median mortality (Walde et al, 2021).…”