| Summary: | Atlantic salmon aquaculture is an important industry in Norway and farmed salmon is among the most economically important global aquaculture species. Despite this, the growth of the industry has stagnated in Norway and it is affected by severe challenges to fish health and welfare. This thesis presents the findings from analyses of production data from one large Atlantic salmon production company with operations in Northern Norway. The aim was to identify risk factors for total and cause specific mortality, quality downgrading, and reduced production performance. We found that smolt weight did not significantly affect subsequent growth during the seawater stage, but a higher specific growth rate (SGR) during the freshwater stage resulted in a somewhat reduced thermal growth coefficient (TGC) during the seawater stage. We found that mechanical delousing treatments were significantly associated with increased total mortalities and mortalities due to heart and skeletal muscle inflammation (HSMI) or cardiomyopathy syndrome (CMS), while bathing treatments were not. A higher smolt weight was associated with both significantly higher mortalities due to HSMI or CMS and to mouth rot. The effect on mortalities due to mouth rot is strong but not easily explainable with the available dataset and should be subject to further investigation. Mortalities attributed to mouth rot predominantly occurred during the first 7 months post seawater transfer. We found a significantly increased mortality due to mouth rot in fish transferred to seawater with lower and falling temperatures, which agrees with previously reported observations from the industry. We also found that 1+ smolts had significantly lower mortality attributed to mouth rot. The presence of ulcers was the most important cause for downgrading during primary processing in the dataset from Northern Norway, affecting 7.4 % of the processed fish. Ulcers were also a persistent cause for mortalities over several production cycles. The prevalence of ulcers, both as a cause for ...
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