Coral Fish Ecological Indices and Relationship with Seawater Physical Parameters at Nasi Island

Coral reef fishes are primary indicators of reef health and providers of essential ecosystem services, including maintaining food web balance, controlling algal growth, and supporting local fisheries. Monitoring the structure of coral reef fish communities is crucial for assessing reef conservation status, particularly in the Nasi Island ecosystem, Aceh, which has experienced coral bleaching and continues to face pressures from fishing activities and tourism. This study assessed species richness, diversity, and similarity of coral reef fishes at three stations (Deudap, Alue Riyeung, Pasie Janeng) during October–November 2024 using Belt Transect and Underwater Visual Census methods. The species richness index of coral reef fishes at Nasi Island was high (7.89–9.65), indicating that the reef ecosystem still provides complex habitats and resources that support diverse taxa. Diversity indices ranged from 2.98–3.49, categorized as moderate to high, reflecting relatively balanced community structures, although dominance by certain species may influence ecosystem functioning. Similarity indices between stations ranged from 52–60%, classified as high, suggesting ecological connectivity and uniform environmental conditions across sites. Integration of these indices confirms that the coral reefs of Nasi Island continue to function as resilient ecosystems, with the capacity to maintain biodiversity and ecological stability despite anthropogenic pressures and the impacts of coral bleaching. These findings provide an important foundation for conservation planning, sustainable resource management, and ecotourism development in the region. Keywords: Coral Reef Fish; Diversity; Similarity Index; Species Richness.