Drone surveys reveal strong inter- and intra-annual site fidelity of oceanic manta rays Mobula birostris to a turbid coastal bay in Fiji

Understanding the spatial ecology of the Endangered oceanic manta ray Mobula birostris is critical for robust conservation planning, yet knowledge of its habitat use across the southwest Pacific remains insufficient. Using non-invasive drone surveys, we confirm strong inter- and intra-annual site fidelity of oceanic manta rays to Laucala Bay—a shallow, turbid, riverine-influenced coastal bay adjacent to Fiji’s capital city, Suva. This habitat contrasts with the majority of prevailing research, which typically associates the species with oligotrophic, offshore environments. Between December 2018 and December 2024, we recorded 874 sightings of 22 individual oceanic manta rays from 414 unique sampling dates. Occurrence varied seasonally, with >90% of sightings recorded between May and November, peaking in September and October. Recapture percentages and multi-year recapture percentages were high (>85%) along with a mean recapture frequency per individual of 34.2 ± 4.5 (mean ± SE) recaptures, indicating strong inter- and intra-annual site fidelity. Maximum likelihood residency models indicated that manta rays spent a mean (±SE) of 53.6 ± 44.2 d within the bay annually, while annual residency index (RI) values revealed that individuals were sighted on 13.9 ± 2.0% of detectable days on average across all years in Laucala Bay. These results demonstrate that coastal riverine-influenced habitats may also play an important ecological role for oceanic manta rays and should be incorporated, alongside offshore environments, into management and conservation strategies.