Spatial Ecology and Foraging Behaviours of the Oceanic Manta Ray (Mobula birostris) in Aotearoa New Zealand

Abstract

Understanding the movements and associated behaviours of animals is critical for facilitating informed conservation and management decisions. This is particularly important for species such as the globally endangered oceanic manta ray (Mobula birostris), which has a highly conservative life history and is therefore more vulnerable to population disturbances. Investigating the behaviours of highly mobile animals which spend most of their time underwater can be challenging. Satellite telemetry provides a valuable means of tracking movement, and collecting environmental data, facilitating the inference of behaviours and influences of environment variables on ecological processes. This thesis focuses on the finescale spatial ecology and foraging behaviours of oceanic manta rays during their seasonal presence in northeastern Aotearoa New Zealand. Seven individuals were tracked using high-resolution SPLASH10F satellite tags, which indicated broad variation in movements along the northeastern coast, with particular fidelity to Tīkapa Moana—Te Moananui-ā-Toi—the Hauraki Gulf. Throughout their range, manta rays displayed high fidelity to surface waters < 5 metres, with occasional deeper dives. I used Generalised Additive Models to investigate the environmental influences on behaviours inferred from horizontal movements, revealing that foraging was linked to clearer waters during the day and more turbid areas at night, suggesting a potential trade-off between increased visibility facilitating prey detection and discrimination, and foraging in areas with higher prey density. Deeper dives were more frequent at night, with depth of dives related to increased moon illumination. Manta rays occupied shallower waters around dusk and dawn, and at midday. Dives likely reflect a combination of searching, foraging, and navigational behaviours. These findings provide valuable baseline information on oceanic manta rays in New Zealand—a region where they are particularly understudied. Future research should attempt to integrate horizontal and vertical movements to further inform manta behaviours, and should aim to increase sample size, tagging a range of demographic groups across a longer time period. Incorporating prey distribution into modelling will increase understanding of how this species might respond to environmental stressors, informing conservation management, and ensuring this population can persist into the future.