Field measurements of cage deformation using acoustic sensors

As aquaculture continues to supply an increasing share of the worldwide seafood demand, it will become critical for farmers to maximize their efficiency. Presently, the majority of marine finfish are produced in gravity type net pens which can deform when they are subjected to currents. The water velocity loading affects the overall net shape which results in net cage volume loss and consequently, increases fish stress and decreases growth rates.In this study, an acoustic method is utilized to monitor the deformation of a small-scale fish cage deployed in currents. Twelve acoustic sources and four hydrophones were deployed on and around a small scale net pen for 60 days to monitor the net cage movement and volume. Local current velocities were recorded using two current meters, one inside and one outside the net pen. Three volume approximation techniques were examined, using the positions of the acoustic sources to predict net chamber volume as it responded to the currents. A numerical model of the system was then configured, set with loads under similar water velocities and results between field measurements and the model were compared.The use of acoustic sources and hydrophones to monitor cage deformation was shown to accurately monitor net deformation. Field measurements compared well to numerical model predictions, with errors ranging from −3.8% to 32%, depending upon the number of acoustic sources employed in the volume calculations. At low water velocities, six acoustic sources were found to accurate predict the net pen volume. In higher currents, a minimum of nine acoustic sources was recommended.