| Summary: | Atlantic salmon aquaculture is a successful and growing industry, with a global production of more than 1.4 million tonnes in 2010. In the on-growing phase, Atlantic salmon are normally kept in sea cages, where both the water temperature and oxygen levels may fluctuate substantially over time and with depth. Oxygen is the main limiting factor of fish metabolism, and sufficient oxygen is therefore essential for all energy-demanding processes, including growth and basic life-supporting functions. Knowledge of the dissolved oxygen (DO) requirement of Atlantic salmon in the on-growing phase, and this species’ ability to cope with observed fluctuations in DO (30-120% O2) is however limited. The overall aim of this thesis was therefore to study the physiology and production performance of Atlantic salmon in response to changes in the oxygen availability, in order to establish knowledge that can be used to assess whether growth and welfare is compromised as a result of insufficient oxygen in the on-growing phase. Insufficient oxygen supply (environmental hypoxia) is primarily considered to be a problem in summer and autumn, and to occur in short/ frequent rather than prolonged periods. The studies of responses to oxygen fluctuations were therefore performed at 16 °C (a typical autumn temperature along the Western coast of Norway) and with hypoxia occurring in cycles corresponding to the turn of the tidal current, resembling previous observations in sea cages (2 h of hypoxia every 6 h, termed “cyclic hypoxia”). However, the oxygen requirement of fish is known to increase with temperature and with the level of activity. For the determination of the minimum DO requirement of Atlantic salmon, temperatures were therefore set to range between 6 and 18 °C, and the experimental conditions were set to induce activity levels as similar to fish in sea cages as possible. For the typical autumn temperature of 16 °C, the threshold for optimal feeding of Atlantic salmon post-smolts was ~70% O2. Feed intake, and presumably also the ...
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