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<p>Prior to 2024, the coral reefs of American Samoa experienced several bleaching events in response to warming ocean temperature, but forereefs exhibited little bleaching-induced mortality. However, during the fourth Global Coral Bleaching Event (GCBE), cumulative heat stress was higher and persisted longer than previous records in this region. We use survey data (2015–2025) from the island of Tutuila to assess how the 4th GCBE affected density and partial mortality of <i>Isopora crateriformis</i>, a threatened coral under the Endangered Species Act (ESA) that can be locally abundant. Partial mortality increased threefold (from 8 to 30% overall) in 2025 compared to all previous survey years. These impacts were size-dependent, with small colonies remaining unchanged, but medium and large colonies exhibiting 18% and 46% increases in partial mortality, respectively. The extent of partial mortality in these medium and large colonies increased with the severity of heat stress measured as mean sea surface temperature (1&#xa0;km resolution) indicating that the 4th GCBE was likely the cause of the observed partial mortality. These results show that partial mortality can be a useful proxy for quantifying bleaching impacts when quantitative observations during the stress event are not possible. Furthermore, the spatial variation in heat stress around Tutuila likely shaped the distribution and condition of the <i>I. crateriformis</i> population, indicating potential spatial refuges exist where heat stress is lower. This study represents the first peer-reviewed study of climate-related impacts on an ESA-listed coral in the Pacific and provides critical information to support ESA-mandated conservation and management.</p>

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From stability to decline: threefold increase in partial mortality of threatened Isopora crateriformis in American Samoa following the 2024 Global Coral Bleaching Event

  • 期刊论文

  • 发表日期：

    2026年7月9日

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  • Kira Turnham,
  • Brittany Huntington,
  • Courtney S. Couch,
  • Edoardo Sena,
  • Kisei Tanaka,
  • Thomas Oliver

摘要

Prior to 2024, the coral reefs of American Samoa experienced several bleaching events in response to warming ocean temperature, but forereefs exhibited little bleaching-induced mortality. However, during the fourth Global Coral Bleaching Event (GCBE), cumulative heat stress was higher and persisted longer than previous records in this region. We use survey data (2015–2025) from the island of Tutuila to assess how the 4th GCBE affected density and partial mortality of Isopora crateriformis, a threatened coral under the Endangered Species Act (ESA) that can be locally abundant. Partial mortality increased threefold (from 8 to 30% overall) in 2025 compared to all previous survey years. These impacts were size-dependent, with small colonies remaining unchanged, but medium and large colonies exhibiting 18% and 46% increases in partial mortality, respectively. The extent of partial mortality in these medium and large colonies increased with the severity of heat stress measured as mean sea surface temperature (1 km resolution) indicating that the 4th GCBE was likely the cause of the observed partial mortality. These results show that partial mortality can be a useful proxy for quantifying bleaching impacts when quantitative observations during the stress event are not possible. Furthermore, the spatial variation in heat stress around Tutuila likely shaped the distribution and condition of the I. crateriformis population, indicating potential spatial refuges exist where heat stress is lower. This study represents the first peer-reviewed study of climate-related impacts on an ESA-listed coral in the Pacific and provides critical information to support ESA-mandated conservation and management.