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Heidrikur Bergsson:
Optimizing fish growth and animal welfare in aquaculture sea cages by profiling oxygen and water current

Date: 01-04-2021    Supervisor: John Fleng Steffensen

In Danish marine aquaculture, production is limited by discharge, based on the amount of food administered. Additionally, feed not consumed by the fish, due to suboptimal conditions within sea cages, increases the impact on the surrounding environment and results in a financial loss for the fish farmer. If suboptimal environmental conditions, such as hypoxia, can be predicted before they occur, feed waste can be reduced, with the benefits of increasing production or decrease the environmental nutrient load.

The aim of this Ph.D. thesis was to create a model that can predict oxygen conditions within a sea cage based on fish physiology and the physical/chemical parameters that affect oxygen consumption of the fish and oxygen conditions within a sea cage. The model was created and tested using a three-step process that involved laboratory work, field measurement, and computer modeling. The laboratory work consisted of determining different aspects of fish physiology such as swimming energetics, swimming preferenda, and the oxygen demand due to digestion, all used in the model. The fieldwork was performed on an aquaculture site and consisted of measurements of chemical and physical parameters inside a sea cage during the farming seasons. The acquired parameters were also used in the model to get environmental limits of what can be expected in a sea cage. Finally, by combining field measurements with fish physiology determinations, a model was created that predicts oxygen conditions within an aquaculture sea cage. The measured physical and chemical parameters from within the sea cage were compared with the open and free marine data from the Copernicus Marine Service to estimate the suitability of modeled data on a local scale as an alternative to field measurements. With minor adjustments, the modeled data could be used for further analysis. In this case, the benefits of using modeled data are that Copernicus provides a six-day forecast of the parameters, and by combining forecast data and the oxygen prediction model, a six-day forecast of predicted oxygen condition within the sea cage is achieved. The applicability of oxygen condition predictions inside aquaculture sea cages is vast since they can inform the fish farmer of potential decreases in oxygen conditions. Using the predictions, the farmers can plan feeding durations and quantity, especially if oxygen is predicted to decrease below the threshold for suboptimal digestion. The model can also be used as a tool for planning the location and maximum reasonable size of aquaculture cages. With an optimized feeding plan, the fish farmer can reduce feed waste, reduce impact on the local surrounding environment, decrease financial loss, and improve general fish welfare. Additionally, with decreased discharge, production can be increased.