Fish cage and mooring system dynamics using physical and numerical models with field measurements

Abstract

As wild fish stocks decline, marine aquaculture is expected to play an increasing role in satisfying the global need for seafood. Since the expansion of near-shore aquaculture is becoming more difficult because of multi-use issues and environmental impact concerns, the feasibility of moving aquaculture into the open ocean is being studied. To enable the optimum design and evaluation of fish cage and mooring performance in the energetic open ocean, physical and numerical modeling techniques are being utilized. To validate these methods, the dynamics of a fish cage and mooring system deployed in the Gulf of Maine are simulated with physical and numerical models and compared with field observations. Assuming that the system can be modeled as a linear system, a stochastic approach was used to analyze the motion response (heave, surge and pitch) characteristics of the fish cage and the load (tension) response in an anchor line to wave forcing. Transfer functions were calculated from field observations for storm events and used to understand the system dynamics and to validate the models for the deployed system. Fish cage heave and surge motions were found to be overdamped, while pitch exhibited a resonance at low frequencies (less than 0.1 Hz). Transfer functions for anchor line tension were consistent with observations over most of the wave frequencies. The physical model clearly revealed the pitch resonance, while the numerical model was better at predicting mooring line tension. Results also provided insight concerning dynamical processes that require further study, including fluid–net panel interaction, transfer function amplitude dependence and the nonlinear relationship between steady current and wave fluid velocity on drag and its effect on system geometry and therefore response.