oa Tracking whale shark movements in a southern Maldivian atoll: An opportunistic feeder or reliant on the local fishing industry?

Background Whale sharks (Rhincodon typus) are generally sighted during daytime. In the southern atolls of Maldives they are instead sighted primarily at night inside the atoll. Tuna fishermen attract baitfish with a strong halogen lamp, creating substantial plankton concentrations. Whale sharks opportunistically feed near the surface under fishing boats anchored in a 40 m deep lagoon, often disturbing bait collection. This unique occurrence is well known locally but not internationally and it has yet to be the focus of a targeted research program, due in part to the remoteness of the area. This study aims to provide preliminary answers for the following question: Are whale sharks simply moving through Thaa Atoll or do they remain inside for an extended period? Approach Two sharks were tagged in December 2015, while feeding at night under the light of fishing boats inside Thaa Atoll. Wildlife Computers SPOT 5 satellite tags, secured via non-invasive fin clamps, were deployed on the first dorsal fin of two sharks to monitor their geographic movements. This pilot tagging project was undertaken in conjunction with a photo identification study (using Wildbook for Whale Sharks), to compare to other photo ID data collected during other sighting events before and after tagging. Results The first shark (M-109) retained the tag for a period of 25 days, and appeared to remain inside the atoll during this period. Frequent movements within the atoll were recorded, up to 20 km over a 24 h period. M-109 was regularly recorded at the northeast area within the atoll, near a consistent baitfish collection ground where this individual was originally tagged. However, on occasion, M-109 was recorded far from bait collection areas and/or during rough nights with no fishing activity, suggesting natural surface feeding behavior. Despite no data forthcoming from the second tagged shark (likely because of tag dislodgement through interaction with fishers nets), this individual was resighted (confirmed through photo-identification) feeding under fishing nets, at a similar location to the first sighting, 22 days later. Conclusions 1) Satellite data, for M-109, and photo-identification data, for the second shark, confirmed both sharks inside the atoll over a 25 and 22 day period respectively, leading to the possibility of an extended stay throughout the Maldivian dry season. 2) Tagged whale shark M-109 was confirmed via satellite uplinks inside Thaa Atoll on 24 of 25 days for which we have data, suggesting it did not leave the atoll during the 25 day monitoring period. M-109 showed extensive movement within the atoll, far from bait collection areas, suggesting that local fisheries may not be the only source of food exploited by whale sharks. While the results are preliminary, the next phase of research in this potential seasonal whale shark sanctuary will focus on: establishing the frequency of fisher/shark interactions within Thaa Atoll; productivity/plankton studies inside the atoll; an increased number of whale sharks satellite-tagged; expansion of photo-identification work. This study represents the first whale shark tagging study officially recognized and permitted by the Maldivian government and it was performed under governmental supervision. It represents a milestone for the future of whale shark tracking research in the country.