The current study deals with the optimal design of rectangular rearing ponds of cold-fish water like trout based on hydraulic efficiency for investigating the hydrodynamic flow mixing. The hydraulic efficiency criterion is defined by the residence time distribution curve of the pond. In this study, ten artificial rectangular rearing ponds with 120 m² area and different aspect ratios (form 0.13 to 20.83) have been considered. In order to calculating the hydraulic efficiency, depth averaged hydrodynamic numerical model, Smagorinsky turbulence model and convective-diffusive equation using finite volume technique on unstructured triangular mesh have been applied. According to the findings of the study, choosing a minimum aspect ratio of 2 (hydraulic efficiency more than 50%) is essential for having an acceptable level of water mixing. However, it is recommended to choose a minimum aspect ratio of 7.5 (hydraulic efficiency more than 75%) for high level of water mixing. In cases of physical limitations for constructing rectangular pond with the minimum aspect ratio of 7.5, the minimum aspect ratio can be considered as 5 by improving the geometry of the inlet part and reducing dead zones.