Status and trends of giant clam populations demonstrate the effectiveness of village-based protection in American Sāmoa

Giant clams (subfamily Tridacninae) serve diverse ecological functions in coral reef ecosystems but have experienced severe populatiaon declines across much of their native ranges. Continued overharvesting, habitat degradation, and climate change impacts reinforce the need for updated population assessments and have prompted consideration for endangered species status. Here, we report a territory-wide evaluation of giant clam populations in American Sāmoa, integrating historical data (1994/95, 2002, and 2018) with new surveys conducted from 2022 to 2024 to assess the population status of these ecologically important bivalves. Using belt transects (50 m × 2 m at 10 m depth), we examined clam densities, size-class distributions, species composition, and population trends across six islands-Tutuila, Aunu'u, Ofu, Olosega, Ta'ū, and Muliāva. This study added 264 transects to the historical dataset and showed population densities of giant clams varied among islands but have remained relatively consistent over time. Using univariate and factorial ANOVAs of giant clam abundance and size data, we assessed spatial and temporal variation across American Sāmoa, and our analyses tested for effects of island, year, protection status, and their interactions. The lowest recent clam densities (83.5 individuals per hectare in 2022) were observed on the main island of Tutuila, where 98% of residents live. Mean clam density on Tutuila has increased from 14.1/ha in 1994/95, but the island has considerable variation amongst locations. Remote islands, such as Ta'ū and Muliāva, showed higher densities, up to 812 and 1,166/ha, respectively. Most (96.7%) of giant clams found on transects were identified as , with infrequent occurrences of the cryptic species and , primarily within specific, village-managed protected areas. On Tutuila, surveys sites included a variety of jurisdictions and levels of management, with village protected areas and remote sites supporting both higher clam densities and larger individuals. Overall, inaccessible remote sites and those under traditional village enforcement significantly outperformed all other management strategies, including federally designated no-take zones. These findings suggest that empowering traditional Indigenous community stewardship may offer a viable alternative to blanket federal restrictions, and support the importance of localized, community-based management practices in American Sāmoa. We emphasize the need for more frequent monitoring across varying depths, anthropogenic influences, and management regimes to better understand the population dynamics of these valuable coral reef species. [Abstract copyright: ©2025 Marra-Biggs et al.]