'El Blanquizal' was one of the most important Nassau grouper (Epinephelus striatus) spawning aggregation sites (SAS) in the Mexican Caribbean. However, the characteristics of the population that still uses this site for reproduction remain unknown. The objective of the present study was to monitor and evaluate the Nassau grouper spawning aggregation at El Blanquizal using passive acoustics and visual counts during 2022 and 2023. Based on the sound pattern related to reproductive behaviour (SRRB) and visual monitoring, we determined that the Nassau grouper spawning aggregation season at El Blanquizal occurs from January to March, with up to three reproductive events (one per month). Each event is synchronized with the lunar cycle (the highest reproductive activity occurs between the 9th and 12th days after the full moon), a specific temperature range (26.9-27.7°C) and a particular time of day (28-60 min before sunset). Visual counts estimated an abundance of at least 1700 groupers during the reproductive season, with 80% being adults (TL >
50 cm), reproductive behaviour (courtship) and the identification of colouration phases associated with the spawning event (bicolour phase). These observations confirm that passive acoustics is a highly promising complementary method for monitoring Nassau grouper spawning aggregations and that the El Blanquizal is one of the few sites where Nassau grouper reproduction currently occurs. Its abundance during the spawning aggregation likely places it among the two most important aggregation sites for the species in Mexico, highlighting the priority of continuous monitoring and the implementation of protective measures based on the present results.