C121 Evaluate the characteristics of coral reefs from physical and biological perspectives and propose effective conservation methods

Majuro Atoll in the Marshall Islands was selected as the research field. Using in-situ observations, satellite observations, and numerical simulations, we investigated past and present changes in the marine environment and the characteristics of the current marine environment and ecosystem. The research and analysis aims were: 1) to identify priority coral conservation areas on the atoll (lagoon side), 2) to investigate the feasibility of using sand and gravel deposited in the sand apron on the atoll (lagoon side) for beach nourishment, and 3) to investigate the feasibility of using excavation sites (sites excavated from the ocean-side bedrock of the atoll for construction materials) on the atoll (open ocean side) as coral nurseries.

Field observations have been conducted twice so far: in November 2024 and February 2025. Field observations consisted of: (1) continuous monitoring of the physical environment (water temperature, water level, current direction and velocity) using thermometers, water level gauges, and current meters; (2) monitoring of the vertical distribution of the physical environment (water temperature, salinity) and water quality (turbidity, fluorescence) using a multi-parameter water quality meter; (3) observation of the underwater landscape, particularly the coral communities, through underwater surveys; and (4) observation of the topography, physical environment, and water quality of sand apron and excavation sites through surveying and other methods.

Observations conducted using a multi-parameter water quality meter in November 2024 showed a tendency for higher water temperatures on the southwest side of the lagoon, but this trend was not as pronounced in February 2025. Salinity was strongly influenced by the presence or absence of heavy precipitation immediately prior to the survey, making differences between sites unclear. Underwater surveys included observations of coral coverage at each site, the distribution and bleaching status of Acropora, and the presence or absence of predators. While coral coverage was high except in some areas of the channel and gravel bottom, coverage of Acropora species tended to be low overall.

Field observations will continue until mid-2026 to accumulate observational data on the physical environment and monitor coral growth during the survey period. The obtained physical environment data will also be used to verify the calculation accuracy of the numerical model, aiming to improve its accuracy. Subsequently, we will identify areas with a low risk of coral bleaching and mortality even under rising water temperatures. Based on the results of remote sensing data mapping and interviews regarding the spatial distribution of use and conservation, we will recommend areas within the lagoon that should be prioritized for use and conservation. We will also estimate the amount of sand and gravel deposits available for beach nourishment in the sand apron on the lagoon side and examine whether the excavation site on the open ocean side can be used as a coral farm. Additionally, in collaboration with Subtheme 1, we will conduct on-site interviews to determine the social value of the ecosystem, map social values such as fishing grounds and protected areas, and layer these values on top of each other to develop a spatial plan for ecosystem conservation and utilization.

Figure: Continuous observation points for the physical environment and installation of measuring equipment on Majuro Atoll

Figure: Horizontal distribution of water temperature and salinity in Majuro Atoll in November 2024 and February 2025

Figure: Example of calculation of flow velocity and water temperature using the flow model