Quantifying the influence of environmental factors on elasmobranch distribution and abundance in a high-use marine...

Elasmobranchs, comprising sharks and rays, are vital indicators of marine ecosystem health but face increasing threats from anthropogenic pressures such as overfishing, habitat degradation, and tourism-related disturbances. Marine Protected Areas (MPAs) are widely promoted as conservation tools to mitigate these threats, yet their effectiveness, particularly in multi-use zones, remains uncertain. This study employed Baited Remote Underwater Video (BRUV) surveys across four MPA zones in Nusa Penida to assess elasmobranch distribution, relative abundance, and habitat associations. From 212 BRUV deployments between 2018 and 2023, we recorded 119 elasmobranch individuals across 13 species, with a sightings per unit effort (SPUE) of 0.48 MaxN per hour, reflecting only a marginal increase from the 2016 baseline of 0.39 MaxN per hour. Elasmobranch distribution varied across regions, with the highest species diversity observed in the northern and western areas where BRUV sites were associated with hard coral, rubble, and key invertebrates such as sponges, ascidians, and anemones. The southern region was characterized by repeated sightings of reef manta rays, reinforcing its function as a key aggregation site. In contrast, the eastern region recorded very few elasmobranch encounters, though the presence of endangered pelagic species suggests it may warrant further monitoring and protection. These findings suggest that biological and environmental conditions may influence elasmobranch presence independently of protection status. The limited change in relative abundance highlights the urgent need for improved enforcement, refined fisheries and tourism regulation, and habitat-based management strategies to strengthen the MPA's role in regional elasmobranch conservation.