Population structure of giant clams (subfamily: Tridacninae) across Palau: Implications for conservation

Giant clams (subfamily: Tridacninae) are an important food and economic resource for the Republic of Palau and the greater Indo-Pacific region. However, giant clam diversity and distribution data for Palau are out of date. This study reports the species diversity and distribution of giant clams across the Palauan archipelago (total survey area of 3,300(2) m) from data collected between 2015-2017. This is the first documented finding of Tridacna noae in Palau, however, it was rare. Only four individuals were found at only two locations. Results show that Palau is home to a diverse and abundant population of giant clams. Tridacna crocea (an important food and economic resource) exhibited the highest abundance, with an average of 20.0 +/- 2.9 individuals per 50 m(2). Tridacna maxima and Tridacna squamosa were ranked next in abundance. In contrast, T. noae, Tridacna derasa, Tridacna gigas, and Hippopus hippopus were found in low numbers or only found in few locations. The density of all recruits, juveniles, and adult T. crocea and T. maxima did not differ significantly between conservation areas and open fishing sites, which suggests the possibility that ecological factors such as habitat loss, acute weather events, or changes in sea surface temperatures could be impacting replenishment or recruitment. However, one protected area was found to have a higher abundance of T. crocea recruits and adults, which suggests that protection from fishing may have increased recruitment rates and lowered mortality rates at this site. Taken as a whole, clam populations in Palau remain reasonably abundant and healthy. As demand for giant clams continues to rise in Palau and the region, local regulations should focus on sustainable fishing practices by establishing size limits, species bans, and long-term monitoring plans to maintain the diverse populations of giant clams found there.