Causes and consequences of hypoxia in Atlantic salmon aquaculture
Sometimes, the desire for simplification can lead us astray. In marine aquaculture cages, where fish welfare and production performance are constantly challenged by tangible threats, it is easy to dismiss invisible and difficult to monitor dissolved O\(_2\) (DO) as unimportant. But to do so is to ig...
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| Format: | Thesis |
| Language: | English |
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2018
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| Online Access: | https://eprints.utas.edu.au/31950/ https://eprints.utas.edu.au/31950/1/Oldham_whole_thesis.pdf |
| Summary: | Sometimes, the desire for simplification can lead us astray. In marine aquaculture cages, where fish welfare and production performance are constantly challenged by tangible threats, it is easy to dismiss invisible and difficult to monitor dissolved O\(_2\) (DO) as unimportant. But to do so is to ignore the fundamental importance of O\(_2\) for survival and its underlying role in overall health and wellbeing. In this thesis, I demonstrate that while DO conditions in cages rarely threaten the survival of farmed salmon, hypoxic DO levels which negatively impact production performance and welfare, defined here as the quality of life as perceived by the animal, are a common occurrence. In field experiments conducted in Norway and Tasmania, I found that DO conditions in commercial salmon cages are highly dynamic, and vary vertically, temporally and horizontally across the cage width. Several factors correlated with DO variability, including time of day, cage size, fish behavior and current speed. Using animal-borne DO sensors, I found that the variability of DO in the cage environment is mirrored in the conditions experienced by fish. Moreover, in a manipulative experiment salmon proved capable of detecting and avoiding hypoxic DO conditions, but other environmental and social factors can supersede hypoxia avoidance. And while the study design of these in- situ trials was not optimal due to the limitations of working at large scale, it is exactly that nature of this body of work which provided novel and industrially relevant insights. Given the variable and heterogeneous nature of DO in marine cages, I also investigated the implications of exposure to low DO on the production performance and welfare of salmon in controlled experimental trials. Overall, I found that metabolic O\(_2\) demand and cost of transport decrease with fish size, and increase exponentially with swimming speed. Exposure to 50% DO saturation significantly reduced aerobic scope for activity and swimming performance across a wide size range from 0.2 to 3.5 kg; and, cyclic, short-term exposure accelerated the progression of Amoebic Gill Disease compared to salmon maintained in normoxic conditions. As a whole, this thesis demonstrates that the problem of hypoxia in aquaculture cages is not intractable, but that there are no simple solutions or quick fixes. Maximizing the production performance and welfare of farmed fish necessitates a systemic approach based on a site specific understanding of the forces acting to replenish and drawdown DO, and optimizing farming strategy to match the needs of both the site and fish. |
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