| Summary: | Farming of Atlantic salmon have been in rapid growth since the 1970's and is now an important industry in many countries around the North Atlantic as well as Chile. Since 2000, all countries with farming of Atlantic salmon and rainbow have faced challenges with the development of drug resistant sea lice. The use of drugs and the cost of production has increased, this has undermined the aquaculture industry's profitability and reputation. In Norway, the rapid growth of salmon farming was arrested from 2012, mainly because of increasing problems with drug resistant salmon lice (Lepeophtheirus salmonis). Norwegian authorities have issued new farming licenses with the purpose of encouraging fish farming companies to solve the most important environmental challenges. Increased production of salmon in land-based facilities, the development of offshore aquaculture and different varieties of closed containment systems (CCS) have been proposed. When this thesis was developed (2012-2015), the knowledge about fish health and welfare in commercial scale CCS was limited. It was important to assess if the use of untreated deep water could provide sufficient protection against sea lice and if introduction of lice could lead to sea lice reproduction and sustained infestations in the closed cages. Furthermore, it was necessary to investigate growth rates, mortality rates, mortality causes and fish welfare in CCS. Our studies showed that CCS with water intake at a 25 m depth provided effective protection against sea lice copepodites (Lepeophtheirus salmonis and Caligus elongatus). Adult Caligus elongatus were observed occasionally and at low abundancies. When sea lice were introduced into CCS, we observed no signs of reproduction or sustained infestations. Without sea lice, there was no need for treatments. This reduced the environmental impact and improved fish welfare with production of salmon in CCS, compared to traditional net-pens. Mean thermal growth coefficient (TGC) for post-smolt in CCS was close to 3.0, ranging between ...
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