Factors associated with baseline mortality in Norwegian Atlantic salmon farming

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.

Mortality patterns during the freshwater production phase of salmonids in Norway

“…Hence, the cumulative mortality during the hatchery period cannot be calculated for each fish group. Moreover, it is not possible to describe and model mortality using mortality rates for individual fish groups, which would be the preferred way to calculate mortality (Houe et al, 2004;Oliveira et al, 2021). Instead, we calculated mortality rates for each of five specific weight groups, allowing us to address mortality at different stages of the freshwater phase.…”

“…Observations of mortality patterns could be used for surveillance in aquaculture. Indeed, in a recent project, a model including different risk factors has been built to describe mortality patterns in the marine phase of Atlantic salmon farming (Oliveira et al, 2021).…”

Quantitative Genetics of Smoltification Status at the Time of Seawater Transfer in Atlantic Salmon (Salmo Salar)

“…This agrees with the findings of Kristensen et al (2012) who surveyed mortality in the first 90 days post seawater transfer in commercial production in Norway and found increased mortality in 1+ compared with 0+ smolt despite a trend in increasing size of 1+ smolt. A more recent analysis of mortality in the Norwegian Atlantic salmon industry further confirmed higher mortalities in 1+ compared with 0+ smolts as well as an adverse effect of increasing body weight of smolts at seawater transfer ( Oliveira et al, 2021 ).…”

Drug and pesticide usage for sea lice treatment in salmon aquaculture sites in a Canadian province from 2016 to 2019

“…Sums are used as proxies that can only be partially linked to a number of lice as they are also associated with therapeutic efficiencies and resistance. In the absence of access to site specific datasets and similarly to other authors 14 , 51 , 63 we have used regional SST and proxies for salinity measurements and water currents. These environmental parameters are indicative of the relevant features that could influence sea lice but cannot replace actual salinity and current measurements at sites 74 and have a very coarse resolution.…”

“…The high demand for effective methods of sea lice control results in rapid deployment of new strategies (including chemicals) often before extensive research into their long-term effects is completed 12 , 13 . Reviews and analyses of data on therapeutic approaches and technologies are important to anticipate short and long-term effects 6 , 9 on resistance mechanisms and fish welfare including increased mortality 14 , as well as potential effects on non-target organisms. This highlights the importance of adequate data collection to document treatment approaches including potential link with changing environmental conditions.…”