Temporal and spatial drivers of elasmobranch diversity and relative abundance in Lhaviyani Atoll, Central Maldives

The Maldives is widely recognised as a hotspot for shark and batoid diversity in the Indian Ocean, yet regional multispecies patterns of elasmobranch occurrence remain poorly characterised. This study analyses a seven-year (2017-2024) opportunistic dive-log dataset comprising 12,732 SCUBA surveys and 142,994 observation records of sharks and batoids collected across 94 dive sites in Lhaviyani Atoll, central Maldives. In total, 28 species (14 sharks and 14 batoids) were recorded, including 23 species listed as threatened on the IUCN Red List (4 Critically Endangered, 12 Endangered, 7 Vulnerable). Elasmobranch relative abundance (sightings per hour of dive effort) and diversity peaked during the late southwest monsoon (August–September) and declined during the transitional period into the northeast monsoon (December–March), following the reversal of regional circulation and productivity fronts. Community composition shifted after 2021 towards greater diversity and evenness, while overall relative abundance declined. Effort-standardised relative abundance was modelled as a function of environmental and geomorphic variables using generalised additive models (GAMs). Results revealed that elasmobranch relative abundance was primarily driven by sea surface temperature, salinity, and surface current velocity (zonal and meridional components), with geomorphic complexity enhancing occurrence along reef slopes and sheltered slope habitats. For sharks, dissolved oxygen and chlorophyll a were also significant, whereas batoids’ relative abundance was influenced mainly by temperature, oxygen, and current velocity. Spatial kernel-density maps identified four persistent northern-rim elasmobranch activity hotspots, with sharks concentrated along exposed and semi-sheltered slopes and channels, and batoids distributed more broadly within lagoonal habitats. By characterising these spatial and environmental patterns, this study strengthens the scientific basis for targeted conservation and management at a time when national and international management frameworks for sharks and batoids are rapidly evolving.