Decadal variation in the Tridacna population at two marine protected areas in Rodrigues Island, Republic of Mauritius

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Abstract. Ramah S, Kaullysing D, Soondur M, Taleb-Hossenkhan N, Bhagooli R. 2025. Decadal variation in the Tridacna population at two marine protected areas in Rodrigues Island, Republic of Mauritius. Indo Pac J Ocean Life 9: 172-178. Giant clams (Tridacninae) are largely threatened worldwide, but information regarding their distribution and abundance is limited in the Western Indian Ocean, especially in the waters of the Republic of Mauritius, including the island of Rodrigues. This study provides the first decadal assessment (2015-2024) of the population density of Tridacna maxima and T. squamosa within two marine protected areas: Southeast Marine Protected Area (SEMPA) and English Bay Marine Reserve (EBMR) found in Rodrigues Island. Annual surveys were carried out by placing five permanent belt transects of 50 m × 5 m (250 m²) at both sites. The data were extrapolated to individuals per hectare (ind./ha), and Mixed-effect models were used to assess temporal and spatial differences. Across the 10 years, SEMPA consistently supported higher Tridacna densities than EBMR, particularly for T. maxima (peak 640 ind./ha vs. 360 ind./ha). Species identity and interactions were significant, with T. squamosa occurring only in SEMPA, ranging from 13 to 120 ind./ha, and was absent from EBMR throughout the study. Statistical analyses revealed significant effects of site and species identity, as well as strong site × species and year × species interactions, indicating that temporal dynamics differed remarkedly between species and between MPAs. Surveys revealed significant site- and species-specific variations in giant clam densities in Rodrigues. Mean densities of T. maxima and T. squamosa were significantly higher in SEMPA compared to EBMR (p<0.001). Both species persisted across survey years in SEMPA, whereas densities in EBMR remained consistently low with no significant increase over time (p=0.01). These results indicate spatial heterogeneity in population trajectories and identify SEMPA as a key area of persistence. The findings of this study provide baseline evidence of site- and species-specific variation in giant clam populations in Rodrigues, contributing to broader understanding of spatial heterogeneity and long-term dynamics for future comparative research in the Western Indian Ocean.

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