Structural Response and Volume Change Characteristics of Tuna Cages Equipped with External Egg Collection Nets

The installation of an egg collection net in the upper section of a Pacific bluefin tuna (Thunnus orientalis) cage (diameter 25 m × height 15 m) raises concerns regarding the potential compromise of cage stability due to the fine mesh size. This study addresses two primary questions: (1) How can the egg collection net be deployed effectively without undermining cage stability? (2) What are the effects of the egg collection net on the cage volume and shape under varying current conditions? To investigate these questions, a mass–spring interaction model was developed to simulate the contact behavior between net structures, and numerical simulations were performed under various current speeds and sinker weight conditions. The results indicate that optimal deployment is achieved when a sinker weight of 78.5 N per meter is applied along the lower perimeter of the egg collection net. The additional volume reduction induced by the egg collection net was minimal (0.01–0.54%), falling within the natural range of flow-induced fluctuations. These findings lay the groundwork for the development of more robust and efficient bluefin tuna aquaculture systems.