| Summary: | Norwegian salmonid aquaculture has become one of Norway's largest industries. With a yearly export of 1.25 million tons, Norway is the largest producer of Atlantic salmon worldwide. Norway also has a significant production of farmed rainbow trout. The production procedures for the two species are relatively similar, with both freshwater and seawater phases due to the anadromous lifestyles of the species. During the last forty years, production has developed into a highly intensified industry, with a production time frame of about two years. To accommodate the rapid production, several improvements in fish health have emerged. An important part of the production is the selective breeding of fast-growing, slow-maturing individuals with increased robustness against diseases. Additionally, the development of efficient vaccines against several infectious agents has resulted in a significant increase in fish health and welfare and a decrease in the use of antibiotics to treat bacterial diseases. The impact of nutritional contents in fish feeds, such as lipid sources and minerals, has been given increasing attention over the last decades, as the feed can severely impact fish growth and general health. While the aquaculture industry has made significant strides in addressing potential hazards impacting farmed fish, some challenges continue to impact fish health and welfare. An important quality concern for the salmon industry is the occurrence of red and melanized changes in the skeletal muscle, commonly known as red focal changes (RFC) or melanized focal changes (MFC). These changes are predominantly detected in the abdominal wall's cranioventral region at slaughter; on average, 20-30 % of farmed Atlantic salmon are affected. The finding of RFC and MFC leads to cassation of the affected area in the fillet, costing the industry great economic losses each year. As these changes cannot be detected before slaughter, i.e., no detectable symptoms, they are primarily considered a quality concern; however, RFC and MFC cause ...
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