ILMU KELAUTAN: Indonesian Journal of Marine Sciences
Vol 31, No 2 (2026): Ilmu Kelautan

High Local Variability in Hard Coral Juveniles Reveals Uneven Recovery Potential Across Proximate Islands in Karimunjawa, Indonesia

Muhammad Fadhilah Ramadhan (Department of Marine Science, Faculty of Fisheries and Marine Sciences, Universitas Diponegoro)
Dwi Haryanti (Department of Marine Science, Faculty of Fisheries and Marine Sciences, Universitas Diponegoro)
Yoko Nozawa (Tropical Biosphere Research Centre, University of the Ryukyus)
Diah Permata Wijayanti (Department of Marine Science, Faculty of Fisheries and Marine Sciences, Universitas Diponegoro)
Agus Sabdono (Department of Marine Science, Faculty of Fisheries and Marine Sciences, Universitas Diponegoro)
Yopi Novita (Utilization of Fishery Resources, University of Bogor Agricultural Institute)



Article Info

Publish Date
10 Jun 2026

Abstract

Coral reefs are vital marine ecosystems in the Karimunjawa Archipelago, Indonesia, but they face increasing threats from climate change and local anthropogenic stressors. Coral juveniles are widely recognized as reliable indicators of recent recruitment and reef recovery potential. In 2023, hard coral (scleractinian) juveniles (<10 cm in diameter) and benthic assemblages were surveyed at nine islands in this archipelago. At each island, 50 photo quadrats (50 × 50 cm), placed at 1 m intervals along a 50 m transect at a depth of ~ 7 m, were analyzed. The statistical approach zero-inflated Generalized Linear Mixed Model (GLMM) with AIC model selection was used to determine the factors influencing the abundance of hard coral juveniles. The highest coral juvenile density is on Sambangan Island (145 juveniles; 11.6 colonies.m-2), and the lowest is on Tengah Island (19 juveniles; 1.5 colonies.m-2). Most juveniles belonged to Montipora (49%), Fungia (20%), and Porites (6%). Hard corals dominated the benthic cover, with median island-level values ranging from 25% to 79%. Juvenile densities varied greatly among islands, with a significant positive correlation between juvenile density and hard coral cover detected at only two islands. Overall, juvenile density declined with proximity to the main Karimunjawa Island, the center of human activity. Our findings revealed unexpectedly high spatial variation in coral juvenile abundance among proximate islands, indicating differences in recovery potential and anthropogenic pressure. Our results highlight the need for island-specific coral reef conservation strategies, even across short spatial scales.

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Journal Info

Abbrev

ijms

Publisher

Subject

Earth & Planetary Sciences

Description

ILMU KELAUTAN: Indonesian Journal of Marine Sciences (IJMS) is dedicated to published highest quality of research papers and review on all aspects of marine biology, marine conservation, marine culture, marine geology and ...