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.

砗磲（亚科：Tridacninae）是帕劳共和国重要的食物与经济资源。20余年前开展的砗磲既往调查显示，海伦礁与洛克群岛南部潟湖区域分布着多样且具有局域性的种群。本研究更新了帕劳砗磲的种群结构数据，并探究了保护措施对这类重要双壳类生物的影响。我们在帕劳群岛的岸礁、堡礁、环礁及大洋礁范围内选取了11个位点开展调查，调查总面积达1650平方米。研究共测量了7个物种的831只砗磲，包括鳞砗磲（Tridacna crocea）、长砗磲（T. maxima）、诺阿砗磲（T. noae）、方砗磲（T. squamosa）、瓷砗磲（T. derasa）、大砗磲（T. gigas）及砗蚝（Hippopus hippopus）。此外，本研究首次记录了帕劳境内存在诺阿砗磲（T. noae）的案例。测量结果表明，与印度-太平洋区域的其他珊瑚礁相比，帕劳的砗磲种群属于丰度最高、分布最密集的类群之一。在本次调查涉及的所有物种中，鳞砗磲（T. crocea）的丰度最高（521只），分布密度也最大（每50平方米20.0±2.9只）。然而，受本地及国际市场高需求的影响，丰度最高的三个物种——鳞砗磲、长砗磲及方砗磲——的成体平均壳长缩短，且成体丰度有所下降。尽管面临上述捕捞压力，帕劳通过整合自下而上的传统与现代保护举措，仍对本地渔民最常捕捞的鳞砗磲产生了积极影响。在Ngermedellim海洋保护区内，鳞砗磲的补充群体及成体丰度均较高，这表明禁渔保护措施提升了该区域的种群补充率，并降低了死亡率。鉴于这类双壳类生物仍持续遭受开发，我们建议通过持续保护及实施规格限制措施，确保帕劳砗磲种群维持丰度与多样性。