The sicklefin devil ray (M. tarapacana) has a circumglobal distribution and forms seasonal aggregations at oceanic seamounts, though the drivers, demographics, and human impacts on these aggregations remain poorly understood. This study examined abundance patterns and demographics of M. tarapacana at Baixa do Ambrósio seamount (Azores) using a remote camera system (OMS) and photogrammetry. The 2024 aggregation consisted of both mature and immature individuals, with a female-biased sex ratio. Regression results indicated isometric growth across the observed size range. A comparison between measurement methods showed higher accuracy and robustness of the DOV system. The OMS deployments resulted in 2,366 five-minute videos collected across 70 sampling days in 2022 and 2024 to quantify M. tarapacana abundance in relation to environmental variables and human activity. Abundance was significantly influenced by tidal dynamics, moon illumination, and fish abundance, with peak detections during full moons, spring tides, and rising tides. The correlation with fish abundance likely reflected similar responses to environmental drivers rather than a direct causal link. M. tarapacana abundance was positively correlated with diver numbers, possibly indicating attraction to divers or a coincidental overlap between diver abundance and favourable oceanographic conditions. Overall, Ambrósio may serve as an important multi-life-stage feeding and breeding ground.
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