Flow Hydrodynamics and oxygen level distribution around and within large fish cages: Measurements

Sufficient and reliable supply of oxygen is critical for efficient and sustainable aquaculture production. Oxygen fuels production and strongly influences growth, utilisation of feed and fish welfare. Suboptimal levels of oxygen (hypoxia) induce stress, which may lead to impaired welfare and increase mortality. Flow hydrodynamics around and inside the cage influence the distribution of pellets in the cage during feeding and optimal use of pellets is important for fish farmers as feed represents the greatest cost associated with fish farming. In addition, excess feed represents an environmental problem as it increases particulate wastes originating from the farm. The key physical factors that are to be considered are water depth, current flow velocity, near field circulation, dissolved oxygen and water temperature. The flow regime inside a cage depends on the incoming current, how the flow interacts with the net structures and the effects of the fish and their behaviour. This flow situation can be divided into two components: flow through and around a net cage and a system of net cages and the effect of swimming fish on a uniform current. Different models have been used to calculate the drag forces, the wake effect behind net structures, the water exchange and oxygen flux in fish farms, but are not accurate enough to evaluate the flow field around and inside net cages, also none include any effect of the fish : A comprehensive model must incorporate the behaviour of fish within sea-cages which rarely distribute themselves randomly in the cage volume, but that their swimming depth and speed is a response to several environmental gradients.
In this paper, full scale measurements of the flow field within and in the vicinity of a large sea cage are presented. Different sensors like ADVs, ADCPs, DO loggers, echo-sounders, underwater cameras and a Microstructures profiler have been used. The results show the effect of the cage and fish on the flow.