Acoustic telemetry is a widely used technique employed to better understand fish movement patterns across seascapes. Traditionally, surgical acoustic transmitter implantation is conducted at the surface, resulting in a high degree of uncertainty as to the post-release survival of the fish and the validity of the results attained from these experiments. Few studies have conducted In situ tagging, where the capture, tagging, and release are completed entirely at the depth in which the fish occurs naturally. Through the use of closed-circuit rebreather (CCR) technology, this study outlines the first known practical application of the methodology performed at mesophotic depths. In six dives conducted at depths between 40 and 50 m, a total of 10 Nassau grouper were tagged at a spawning aggregation off the west coast of Puerto Rico. The total time (time divers arrived at the trap to time of release) for each procedure was approximately 12 min, after which all fish were released and observed without indication of stress or physiological impairment. Short-term tracking of tagged fish revealed a 100% post-surgery survival rate with maximum detection of 347 days post-surgery. Survival rates of this nature have not been quantified or reported from other tagging studies, allowing the researchers to conclude that this methodology, coupled with the efficiency provided by CCR at these depths, enhanced survivorship and bias for studies utilizing acoustic telemetry.