Sea cage used to trial a distributed sensor network. Photo by Magnus Oshaug Pedersen
Another interesting TNA project carried out in AQUAEXCEL2020 focused on “Testing a Distributed Sensor Network for Measuring Flow Field in a Sea Cage”, and was carried out by Maarja Kruusmaa, in the SINTEF ACE facilities (Norway).
Nowadays, sea cages are used to grow a variety of fish in the open sea. In order to keep the fish healthy, minimum flows through the cage must be met to keep the cage oxidised and clean from waste. As the fish are usually circulating in the cage in a rotational swarm, and the cages are moving with the sea level, conventional measuring techniques (ADV, ADCP) will not work as the precision is decreased by the background motion and also by the swarm of fish swimming in the field of view of the profiling measuring techniques. In addition, the flow inside of the cage can vary and ADV or ADCP typically used for a point measurement do not give information about the flow field distribution along the cage. Therefore, a vertical array of sensors was tested in a fish cage (50m x 27m) to measure flow through the fish cage at various depths. The sensors were developed in Tallinn University of Technology for measuring flows at the seabed and near obstacles where acoustic technology usually fails. These sensors are based on converting mechanical vibration of sensor stems into electrical signals and thus are passive and only measure local flow field. In contrast to ADCP or ADV, they are not as sensitive to disturbances of the flow (for example, swimming fish). These sensors are also cheaper than ADVs and ADCPs. Placing several of these sensors in a network allows us to measure the distribution of flow. This means that if the flow in one part of the cage is higher than in another, this could be detected.
The project demonstrated that the installation enables longterm continuous online observation of flow, and that the distributed flow sensor network gives information about distribution of flow in the cage. The initial results show that the velocity maxima appear with the rise and drop of the sea level as expected. It can also be seen that the currents are stronger closer to the surface and diminish with increasing depth. The results will need to go through additional data processing and validation (comparing with the ADV measurements). For future long-term deployments, better ruggedized cables and data loggers are needed.
The AQUAEXCEL2020 TNA programme enables external teams to access project partners’ facilities via submission of research proposals, which are funded based on an independent evaluation. Access is offered to 39 research infrastructures of participating institutes, with experimental costs, travel and subsistence covered by AQUAEXCEL2020. Available facilities cover the entire range of production systems, environments, scales, fish species and fields of expertise. Access is available to EU and Associated States’ research teams, industry, and small and medium-sized enterprises (SMEs), based on the scientific excellence of proposals and relevance to the aquaculture sector. To find out more visit: https://www.aquaexcel2020.eu/transnational-access/tna-overview