<p>This paper presents a simplified yet accurate numerical approach for the hydroelastic analysis of a modular fish pen collar. The method integrates the Finite Element Method (FEM) and the Boundary Element Method (BEM) to solve the fluid–structure interaction problem. The structural model is developed using FEM based on Mindlin plate theory, while the hydrodynamic model employs BEM derived from linear wave theory. The proposed approach uses only planar plate elements for the FEM structural model, and the same mesh is adopted for the BEM hydrodynamic model. This unified meshing strategy reduces the complexity compared to a previous method that modeled the fish pen collar as a 3D shell structure with different element meshes for FEM and BEM. The accuracy and stability of the proposed modeling approach are validated through comparisons with experimental and numerical results from the literature. Numerical case studies further demonstrate that the connection stiffness between fish pen modules has a significant influence on the hydroelastic response of the collar. A numerical optimization framework based on Differential Evolution is presented to minimize the collar’s hydroelastic response by optimizing the connection stiffness. The framework can handle both single and multiple stiffness variables, and it incorporates constraints on the allowable moments at the connections.</p>