The present work develops an automatic system that allows the counting and the measuring of a population of gray tilapia fingerlings. At the present, the processes for calculate the geometric dimensions of the fish, have a great subjectivity and spend a lot of time. In addition, the fingerling could be stressed because they are removed from the water, which it could lead to death. In this way, it searches to eradicate the manual intervention, thus a mechanical piece is elaborated which holds two fish tanks and an enclosure with illumination. A transfer of little by little of fingerlings from the first to the second container is carried out, for this purpose the grids, controlled by motors, are used to bring them closer or move them away from the orifice of the first container. Also, a pneumatic electric ball valve is used, which works as a gate that allows or forbid the passage of the fingerlings through the PVC pipe. Then, with the fingerlings in the enclosure, it proceeds to execute the counting and measuring algorithms. Satisfactory results were obtained because it has an accuracy of 97% in counting, 92% and 90% in length and width measuring respectively, endorsed by specialists. In addition, the time for the calculation of both parameters were reduced in comparison to the manual.