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

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.