Population structure of giant clams (sub-family: Tridacninae) across Palau: implications for conservation

Giant clams (Sub-family: Tridacninae) are an important food and economic resource for the Republic of Palau. Previous surveys of giant clams conducted over 20 years ago found diverse, localized populations across Helen Reef and the Rock Islands Southern Lagoon. This study updates population structure data for Palauan giant clams and investigates the impacts of conservation on these important bivalves. We surveyed eleven sites within fringing, barrier, atoll, and oceanic reefs across the Palauan archipelago (total area = 1650m2). A total of 831 clams were measured across seven species (Tridacna crocea, T. maxima, T. noae, T. squamosa, T. derasa, T. gigas, and Hippopus hippopus). In addition, this is the first documented case of T. noae in Palau. Our measurements show that giant clams in Palau are among the most abundant and densely distributed compared to other Indo-Pacific reefs. T. crocea exhibited the highest abundance (521 individuals) and density (20.0 ± 2.9 per 50m2) of all species in this survey. However, high demand from local and international markets may have resulted in low average shell lengths and reduced abundances of adult clams within the most abundant species: T. crocea, T. maxima, and T. squamosa. Despite these harvesting pressures, Palau's incorporation of bottom-up traditional and modern conservation initiatives has positively impacted T. crocea, the most targeted species by local fishers. Within the Ngermedellim Marine Sanctuary, T. crocea exhibit high abundances of recruits and adults, suggesting that protection from fishing has increased replenishment rates and reduced mortality within this conservation area. As these bivalves continue to be exploited, we suggest that continued protection of and the implementation of size-limits will enable giant clams to remain abundant and diverse in Palau.Details for each dataset and code file are in the readme.txt file. Funding provided by: National Science FoundationCrossref Funder Registry ID: http://dx.doi.org/10.13039/100000001Award Number: 134315