Elusive and Vulnerable: Evaluating Spatial and Temporal Variation in the Distribution of the mobulids around O'ahu Using Environmental DNA

Grace Ann Tuthill-Christensen^*Michelle JungbluthKaren D. Crow^*

• San Francisco State University, San Francisco, United States

Reef mantas (Mobula alfredi) around the Hawaiian Islands are vulnerable to extinction due to low genetic diversity and resident populations that exhibit limited migration between islands. While known resident populations occur off Maui and Hawai'i, little is known about their occurrence or distribution around O'ahu or other islands across the archipelago, partly due to the rarity of sightings using conventional methods such as photo or drone surveys. This is the first study to characterize patterns of mobulid distribution, habitat usage, and prey assemblage around O'ahu using environmental DNA (eDNA). We employed a novel quantitative PCR (qPCR) approach using plankton net samples to quantify manta eDNA, and detected both spatial and temporal variation in manta eDNA distribution around O'ahu, with most detections occurring off the East shore of O'ahu, in Kane'ohe Bay. We also detected reef mantas off the West and North shores of O'ahu with no detections in the South, where human activity is the highest. We observed increased occupancy in summer/early fall and no detections from late fall to early winter across sites. There were no significant differences in plankton assemblage or abundance associated with manta presence, consistent with insights indicating that mantas primarily feed on offshore mesopelagic assemblages. Applying our plankton-net-based eDNA sampling and qPCR assay represents a non-invasive, viable, and effective approach for tracking patterns in reef manta distribution that is independent from biases associated with physical sightings of these elusive and vulnerable animals.