Movements and habitat use of satellite-tagged whale sharks off western Madagascar

Abstract

Whale sharks Rhincodon typus, the world's largest fish, are routinely sighted off the northwest coast of Madagascar, particularly off the island of Nosy Be. Dedicated whale shark tourism has been developing in the area since 2011. During our first dedicated survey, from September to December 2016, we photo-identified 85 individual whale sharks ranging from 3.5 to 8 m in total length (all juveniles). None had been previously identified from surrounding countries. We tagged 8 sharks with tethered SPOT5 tags in October 2016, with tracking durations of 9 to 199 d. Kernel density plots showed that the main activity hotspot for tagged sharks was around the Nosy Be area. Three individuals were resighted back at Nosy Be in late 2017 after having lost their tags. A secondary hotspot was identified off Pointe d'Analalava, 180 km southeast of Nosy Be. Five sharks swam off the shelf into the northeastern Mozambique Channel, between Madagascar and Mayotte, and one of these continued to near the Comoros islands. Two sharks swam to southern Madagascar, with minimum track distances of 3414 and 4275 km. The species is presently unprotected in Madagascar, although a small proportion of the high-use area we identified in this study is encompassed within 2 marine protected areas adjacent to Nosy Be. Whale sharks are globally endangered and valuable to the local economy, so there is a clear rationale to identify and mitigate impacts on the sharks within the 2 hotspots identified here.

Multi-method assessment of whale shark (Rhincodon typus) residency, distribution, and dispersal behavior at an aggregation site in the Red Sea

“…Other R . typus aggregations are dominated either by immature males [7, 9, 15, 19, 22, 23, 30, 31, 58, 59, 60] or mature females [12, 60]. Three explanations have been proposed for the relative absence of immature females at most sites [60].…”

“…While R . typus is still frequently described as enigmatic, the discovery of high density, predictable aggregations has sparked a rapid expansion in research on this species [2–15]. In addition to their value as study sites, these aggregation areas have often become an ecotourism attraction and an economic boon to local communities [16–18].…”

“…First, dedicated search efforts are largely confined to known aggregations. Outside of these areas, researchers have had to rely on encounter records from pelagic fishermen [29] or satellite tracking data from relatively small samples of tagged sharks [3, 13, 15, 23, 30–38]. Second, even within aggregations, boat-based surveys are often restricted to the surface and the ability to reliably find sharks declines significantly at night, in rough seas, or when the targeted animals are at depth.…”

Photo-ID and telemetry highlight a global whale shark hotspot in Palawan, Philippines

“…reef sharks 34 , wobbegong shark 35 ; humpback whales 36 ; green turtles 37 ). In whale sharks specifically, citizen science and data mining contributions have aided the understanding of their habitat use and connectivity across different countries, demographics and life-history traits 15,27,38 . Data can also be extracted by mining historical social media posts (e.g.…”

Changes in diving behaviour and habitat use of provisioned whale sharks: implications for management

“…The whale sharks at Oslob that feed at the surface for extended periods of time (often 6 am to 1 pm) are exposed to higher temperatures and more direct sun exposure than would be typical (e.g. 30,[42][43][44][45] ). The prolonged sun exposure on shallow waters with a sandy substrate reflects sunlight which might cause skin darkening on the www.nature.com/scientificreports/ ventral side of the animals (GA, pers.…”