Influence of Ocean Currents and Stocking Density on the Dissolved Oxygen Distribution Inside Gravity-Type Fish Cages

Abstract Aquaculture has been the world’s fastest-growing food production sector over the past 40 years. Offshore aquaculture has gained extensive attention due to its notable advantages in enhancing fish welfare, particularly through improved water exchange and efficient waste dispersal. However, high current speeds can lead to fish exhaustion and weight loss, while low currents reduce the supply of dissolved oxygen (DO), posing a challenge to maintaining fish welfare. Understanding the oxygen levels and their relationship with the current speeds at offshore sites is crucial for sustainable ocean-based fish farming. This study investigates the influence of ocean currents and stocking density on DO distribution for fish in gravity-type cages through numerical simulations. The simulations are carried out using the framework of a finite volume method in OpenFOAM. A new solver is developed by coupling with a dynamic porous media (DPM) model for the cage nets in the fluid domain and a modified extended position-based dynamics (XPBD) for the deformations of the fish cages. The solver is combined with an additional scalar transport equation of the DO concentration. Results show that the increase in stocking density leads to an increasing reduction in DO, while a high current speed can help reduce the DO reduction. The DO reduction is proportional to the stocking density and inversely proportional to the current speed. An empirical formula describing the effects of the stocking density and the current speed is proposed. The present numerical results and the proposed formula can provide valuable insights for the aquaculture industry.