Abstract
Mortality in Norwegian salmonid aquaculture has a major influence on fish welfare and represents economic losses for producers. We reviewed the estimated monthly mortality for freshwater farms with Atlantic salmon and rainbow trout between 2011 and 2019. We built a regression model for mortality which included the variables year, weight group, season, region and farm. Additionally, we distributed questionnaires to farmers to gather information regarding potential causes of mortalities. The analysis of data for Atlantic salmon showed that median monthly mortality increased from 0.15% in 2011 (interquartile range [IQR]: 0.06-0.39) to 0.25% (IQR: 0.1-0.67) in 2019. Mortality was highest in the North (0.27%, IQR: 0.11-0.72) and lowest in the Southwest region (0.16%, IQR: 0.07-0.4). The season with highest mortality was summer (0.24%, IQR: 0.1-0.64), while winter had the lowest (0.12%, IQR: 0.05-0.35). The smallest fish (3-12g) showed highest mortality (0.31%, IQR: 0.14-0.69) compared to heavier fish. Results from the questionnaire showed that infectious or non-infectious diseases were the most commonly reported causes of mortality. The mortality patterns described in this study identifies several important risk factors. Highlighting causal links is an important step to reducing mortality and improving welfare in the freshwater production phase of salmonids in Norway.
Highlights
The Norwegian aquaculture industry is continuously growing, and sales of Atlantic salmon (Salmo salar) smolts increased from 288 million in 2011 to 372 million in 2019
This study investigated the mortality of salmonids in 187 freshwater farms, approximately 80% (187/227) of all Atlantic salmon and rainbow trout hatcheries that reported mortality in Norway
In our additional survey we found an indication that mortality in farms using recirculating aquaculture systems (RAS) seems to be higher as compared to flow-through systems (FTS), but the survey only included seven farms with RAS, and should not be used to make conclusions
Summary
The Norwegian aquaculture industry is continuously growing, and sales of Atlantic salmon (Salmo salar) smolts increased from 288 million in 2011 to 372 million in 2019. Sales of rainbow trout (Oncorhynchus mykiss) smolts increased from 20 to 28 million in the same period (NFD, 2021b) (Figure 1). The industry is highly regulated, including animal welfare (Gismervik et al, 2020). It is still challenging to assess and benchmark fish welfare. Fish are held in an environment that is difficult for humans to inspect and fish behavioural patterns are not as intuitive for humans to understand as those of terrestrial animals. Large populations kept in large tank volumes further impairs such assessment and benchmarking. More than 60 million fish died in salmonid freshwater farms in Norway in 2019 (NFD, 2021a), and consumers and farmers have become increasingly concerned with farmed fish health and welfare
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