Modeling volume deformation in gravity-type cages with distributed bottom weights or a rigid tube-sinker

Fish mortality caused by the shrinkage and deformation of gravity-type cage net volume during typhoons is of great concern to the marine cage aquaculture industry. To remedy this deformation problem, we developed a specially designed tube-sinker to replace the weights that are commonly used at the bottom of cage nets. Herein, we describe the features of our new cage net system, the formation of motion equations, a new approach to estimating the volume reduction coefficient, and physical model tests in a hydrodynamic wave tank. Results of the numerical simulation and the experimental measurements show good agreement. This study indicates that the tube-sinker not only improves the volume reduction coefficient by 10% for most wave conditions, but also that it significantly reduces severe fluctuations in volume deformation, thereby reducing fish fatalities due to skin abrasion with the net.