First of all, I wish to thank my main supervisor, Frode Oppedal, who initiated this project and gave me the opportunity to work with it, for introducing me to the research facilities and living area of Matre, and for guidance, positivity, friendship and support all the way through this learning period. Further, I wish to thank my two co-supervisors, Rolf Erik Olsen and Professor Albert K. Imsland, for invaluable help with planning and executing the experiments, and particularly for critical revisions of manuscripts and guiding through the publication process. Further, my colleague Dr. Thomas Torgersen has been an important tutor, inspiration and partner for discussions on the effect of environmental factors on fish metabolism. There are many people in my working environment that I owe gratitude to, for their contributions to the work presented in the present thesis and for making it an enjoyable experience. The staff at Matre Research Station made the experiments possible and patiently followed my strict time schedules during fish samplings. In particular, Ivar Helge Matre and Grethe Thorsheim have provided skilful assistance with management of the advanced research facilities, and with laboratory analyses, respectively. The Animal Welfare group and the people involved in CREATE, has been important for professional discussions and for keeping me up at night..! Last but not least, my dear colleague and friend, Tone Vågseth, has been involved in every step of the way, with planning of experiments, technical assistance, data analysis and manuscript preparation. Together with all the other lovely people at Matre Research Station, she makes it fun to be at work! Finally, I wish to thank my beloved Thor Erik and Fredrik Eldor, for all the love, laughs and never-ending encouragement, and for the support and patience of family and friends while I have been focused on this challenging task.

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In order to establish hypoxia tolerance thresholds for Atlantic salmon (Salmo salar) in the on-16 growing phase, the effect of temperature (6, 12 and 18 °C) and hypoxia acclimation (33 days 17 of hypoxic periods occurring every 6 hours at 16 °C) on the oxygen consumption rate (MO 2 ) 18 and limiting oxygen concentration (LOC; referred to as the hypoxia tolerance threshold) was 19 investigated in fish were kept under production-like conditions (fed, undisturbed and freely 20 swimming fish in tanks). Further, the effects of temperature and oxygen on the relationship 21 between gill ventilation frequency (Vf) and MO 2 were studied in order to evaluate Vf as an 22 indicator of MO 2 . Both MO 2 and LOC were found to increase exponentially with temperature 23 (Q 10 =2.7 for MO 2 and 1.4 for LOC), while hypoxia acclimation resulted in a tendency for 24 reduced MO 2 , but no lowering of LOC. The mean LOC at 6, 12, 16 and 18 °C were 2.9, 3.4, 25 3.8 and 4.3 mg L -1 , respectively. A strong correlation between MO 2 and LOS (LOC given in 26 units of oxygen saturation) was found (R 2 =0.93), regardless of temperature, suggesting that 27 measurements of MO 2 can be used to estimate the LOS of post-smolts. Vf was considered a 28 reliable estimator of MO 2 in normoxic conditions, but not during reductions in oxygen, due to 29 the increasing Vf, and relatively stable MO 2 as oxygen declined towards LOC. 30 31

There is currently little information on the sea cage O 2 levels in S. aurata aquaculture, but variation in the above mentioned factors is expected to cause variable levels of O 2 in S. aurata production systems as well. In order to develop production strategies ABSTRACT: We investigated the effect of temperature on the limiting oxygen saturation (LOS) of gilthead sea bream Sparus aurata. This threshold was defined as the % O 2 saturation where fish no longer upheld their routine metabolic rate (RMR, the metabolic rate of fed and active fish) during a progressive decline in oxygen saturation. S. aurata (398 ± 10 g, mean ± SE) were kept in 3 replicate tanks and subjected to 3 changes in temperature: 16 to 20°C, 20 to 16°C and 16 to 12°C. At each temperature, fish were left to acclimatize for 8 to 10 d, before daily feed intake (DFI), the routine oxygen consumption rate (routine MO 2 , mg kg −1 min −1) and the LOS were measured. In addition, at 20°C the swimming speed was measured in fish subjected to a decline in O 2 from full air saturation to levels below the LOS (minimum of 8−10% O 2 ). For the temperature range tested (12−20°C), DFI, MO 2 and LOS increased exponentially with temperature (7.5-, 3.6-and 2.2-fold, respectively) with mean (± SE) LOS being 17 ± 1, 21 ± 0 and 35 ± 5% O 2 at 12, 16 and 20°C, respectively. A gradual decline in swimming activity was observed as O 2 declined below the LOS, indicating increasing metabolic stress and/or a 'sit-out' coping strategy which may prolong survival time in severe hypoxia. The results show the importance of temperature as an influential variable over the environmental O 2 requirements of S. aurata.

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