| Summary: | Exposing fish to moderate water currents as a form of exercise is often perceived as positive for fish welfare, growth, muscle development and heart condition. This thesis examines what effect rearing Atlantic salmon (Salmo salar) post-smolts in a flow through semi-closed raceway system (Preline) has on growth, mortality, feed conversion, and development of white skeletal and heart muscle. The experiment consists of three phases: 1. Smolt in freshwater 2. Post-smolt in seawater (Preline vs. control facilities) and 3. On-growing in seawater. A total of 321.412 smolts (101 g SEM ± 4.2) were distributed in two groups. Fish in the Preline system was exposed to a 10-20 cm/s water current and a traditional open cage system were used as control. Temperature, salinity and oxygen varied between the two systems, as this was a large-scale experiment and water conditions were difficult to control. After four months in the systems, the control fish had a significantly higher weight, length and condition factor compared to the Preline group. This was linked with higher temperatures at the control site. The Preline group had a 2.44 times higher frequency of small muscle fibres in the white skeletal muscle at the end of the post-smolt phase. This was associated with the Preline fish being exposed to a constant water flow, promoting aerobic exercise. Furthermore, the increased recruitment of white muscle fibres might increase the growth potential for further production. There were significantly higher elevated mRNA levels of MEF2C and GATA4, indicating cardiomyocyte hypertrophy, in the Preline group at the end of the post-smolt phase. This was also associated with aerobic exercise and can possibly help the fish cope with stressful situations they might be exposed to in a farming environment. Masteroppgave i biologi BIO399
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