Using citizen science and survey data to determine the recruitment envelope of the giant clam, Tridacna gigas (Cardiidae: Tridacninae)

Restocking of giant clams, particularly Tridacna gigas, aims to reestablish breeding populations that have long been decimated by overexploitation. In Camiguin, southern Philippines, T. gigas juveniles that were restocked in 2001 have reached reproductive maturity and recruits have been observed on the reefs. To help document T. gigas recruits in Camiguin, the Cantaan Centennial Multi-purpose Cooperative (CCMPC) initiated a program to encourage local fishers to report any sightings of giant clams, in the surrounding reefs. The fishers would mark the location of any giant clam recruits they encountered during their regular fishing trips using makeshift buoys. Personnel from CCMP would then visit the marked sites to collect GPS data. The giant clams would later be relocated to an ocean-based nursery at the CCMP marine protected area (MPA) to ensure their protection. A total of 26 T. gigas recruits with a mean shell length of 17.8 +/- 1.5 cm were found 0.2 km-2.3 km away from the restocking site between 2015 and 2017. Twenty-three of these individuals were reported by fishers while 3 were encountered during field surveys. It is highly probable that these T. gigas recruits are the progeny of the restocked individuals because there are no local populations of wild T. gigas in the area and no artificial spawning events had been conducted at the site. These results suggest that the reefs near Camiguin are suitable for giant clam recruitment and indicate that the recruitment envelope of T. gigas can extend up to several kilometers. Importantly, these findings highlight the value of a strong commitment by local organizations and collaboration with fishers as citizen scientists to enhance the sustainability of giant clam conservation efforts.