Population structure, residency, and abundance of whale sharks in the coastal waters off Nosy Be

Population structure, residency, and abundance of whale sharks in the coastal waters off Nosy Be, north-western Madagascar

Stella Diamant, Simon J. Pierce, Christoph A. Rohner, Rachel T. Graham, Arthur Guillemain d’Echon, Tanguy Guillemain d’Echon, Elina Sourisseau, Léonce Costika Fidiarisandratra, Gisèle Bakary, Sylvia Trélanche, Fanja Andriananrisoa, Jeremy J. Kiszka

ABSTRACT:

1. Between September and December, whale sharks (Rhincodon typus) aggregate in the coastal waters off Nosy Be, an island in north-western Madagascar. Swimming with these sharks has become an important tourism activity, but no formal protection is in place in Madagascar to protect this endangered species from the potential negative effects of tourism or other human impacts.
2. Boat-based surveys (n = 405) were conducted from tourism vessels from September to December, 2015–2019. For most sightings (98%), whale sharks were sighted while foraging for bait fish at the surface, in association with mackerel tuna (Euthynnus affinis) and seabirds (Sternidae). A total of 408 individual whale sharks were individually photo-identified over this period. All individuals were immature, and 82% of sexed sharks were male. Sharks ranged from 3.0 to 8.0 m in total length (TL), with a mean TL of 5.65 ± 0.94 m (n = 66) for females and 5.46 ± 1.09 m for males (n = 295).
3. Most sharks (72% of the identified individuals) were only identified once within the study period. Movement modelling showed an open population with a short mean residence time of 7.2 days. Resightings were recorded from up to 12 years apart (2007–2019). Ten sharks were seen in all five seasons during 2015–2019. A basic POPAN mark–recapture model estimated a total population size of 681 (608–763) sharks over the 2015–2019 period.
4. Nosy Be waters are an important foraging ground for juvenile whale sharks. Sighting data demonstrate that a high proportion of the sharks’ preferred habitat lies outside existing protected areas, but within an identified Key Biodiversity Area. National species-level protection and increased spatial management is warranted to secure the continued presence of whale sharks in this region.

Aquatic Conservation, DOI: 10.1002/aqc.3743

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