Abstract

In 2019, it was estimated that more than 50 million captive Atlantic salmon in Norway died in the final stage of their production in marine cages. This mortality represents a significant economic loss for producers and a need to improve welfare for farmed salmon. Single adverse events, such as algal blooms or infectious disease outbreaks, can explain mass mortality in salmon cages. However, little is known about the production, health, or environmental factors that contribute to their baseline mortality during the sea phase. Here we conducted a retrospective study including 1627 Atlantic salmon cohorts put to sea in 2014–2019. We found that sea lice treatments were associated with Atlantic salmon mortality. In particular, the trend towards non-medicinal sea lice treatments, including thermal delousing, increases Atlantic salmon mortality in the same month the treatment is applied. There were differences in mortality among production zones. Stocking month and weight were other important factors, with the lowest mortality in smaller salmon stocked in August–October. Sea surface temperature and salinity also influenced Atlantic salmon mortality. Knowledge of what affects baseline mortality in Norwegian aquaculture can be used as part of syndromic surveillance and to inform salmon producers on farming practices that can reduce mortality.

Highlights

  • In 2019, it was estimated that more than 50 million captive Atlantic salmon in Norway died in the final stage of their production in marine cages

  • Our study population consisted of Atlantic salmon put to sea from 1627 cohorts produced on 642 different farms

  • We present the descriptive results of salmon mortalities and its putative determinants in plots (Fig. 2) and in data summaries (Table 1)

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Summary

Introduction

In 2019, it was estimated that more than 50 million captive Atlantic salmon in Norway died in the final stage of their production in marine cages This mortality represents a significant economic loss for producers and a need to improve welfare for farmed salmon. In Norway, previous research found large variations in mortality patterns between geographically separate areas, between years of sea transfer, and at different time points during the production c­ ycle[2]. The average monthly mortality in the main Atlantic salmon producing regions of Chile was between 0.38 and 1.78% in 2­ 0181 Building on these studies, which describe the mortality patterns in Atlantic salmon farms, the step is to understand the main determinants of mortality that could be targeted to mitigate deaths. Higher temperatures appear associated with lowered IPNV prevalence within f­arms[16]

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