Fishing regulation methods, first-catch sizes, and sexual maturity of the longarm river prawn Macrobrachium tenellum were estimated through monthly biological and fishery sampling of commercial catches, obtained using the "tapo-bolso" capture system in Mexcaltitán Lagoon and cast nets with 1.0- and 1.5-inch mesh sizes at Camichin Mouth, Nayarit. The results indicated the first-catch size (fcs) for females was 91 mm, for males 112 mm, and the overall population size was 102 mm. The average size at sexual maturity (mmt) was 75 mm for females, 96 mm for males, and 78 mm for the population, providing fishing reference points to suggest a minimum capture size (mct) of 85 mm for M. tenellum in Mexcaltitán Lagoon and Boca de Camichín, Nayarit. The sex ratio of 1.0:0.57 (females:males) showed statistically significant differences, X2= 0.000068, p<0.05, indicating reproductive implications for the population, where 43.0% of females were more mature, potentially increasing the reproductive biomass of the M. tenellum stock. These results offer guidance on the minimum capture size for M. tenellum, aiding in improved fishing management and the conservation of this species in Mexcaltitán Lagoon.