Integrating eDNA and Visual Surveys With Ocean Drift Models to Monitor Marine Mammals in Tropical Waters

ABSTRACT Environmental DNA metabarcoding (eDNA) is emerging as a pivotal tool for assessing and monitoring marine biodiversity, exhibiting significant promise for the detection of marine mammals. The primary objective of this study was to evaluate various protocols for eDNA sampling of seawater from a small boat in tropical environment, under conditions devoid of cold chain storage or laboratory facilities and constrained by limited financial resources. Our focus was on optimizing the capture of eDNA and the subsequent detection of marine mammals in a replicable way. This investigation involved a comparative analysis between marine mammal detections via eDNA metabarcoding and traditional visual monitoring. Sampling was primarily conducted in close proximity to marine mammal sightings, off Réunion Island to evaluate the performance of eDNA detections. Réunion Island is located in the tropical western Indian Ocean and serves as a relevant model for this study, where long‐term monitoring of cetaceans has been conducted since 2008, thereby enabling a robust comparison between visual sightings and molecular detections. Two sets of primers designed to target the hypervariable regions of mitochondrial 12S rRNA genes for vertebrates and mammals were used. Positive eDNA detections were identified in seven of the nine samples associated with visual sightings of one or more cetacean species. Marine mammal DNA was successfully amplified for three families (Balaenopteridae, Delphinidae, and Kogiidae) and found to be almost ubiquitously present for Delphinidae. Additionally, we investigated the potential influence of particle drift on the dispersal of eDNA. To better understand the spatial dynamics and persistence of eDNA in the marine environment, the Lagrangian model ICHTHYOP was used to simulate particle drift and assess how oceanographic processes might influence eDNA dispersal patterns around Réunion Island. Our study explores the potential of utilizing eDNA for monitoring cetaceans in tropical regions offering a valuable comparison to traditional visual surveys, and provides recommendations for further enhancements in future eDNA studies.