Ecology and ethology of a population of whale shark Rhincodon typus Smith, 1828 in Djibouti

Rhincodon typus (Smith, 1828) is the largest extant fish swimming in the warm topical waters of the world. To protect and manage the conservation of this species is important to study its demography, ecology, and ethology. Whale sharks observed during the scientific expeditions organized by the Sharks Studies Center-Scientific Institute of Massa Marittima (GR) and carried out in Djibouti in 2017, 2020, and 2022 were photoidentified using the software I^3 S Classic. To date, 49 sharks are currently present in the database. Over the same 3 years period the average total length of the sharks measured through the laser-photogrammetry was 5.06 m. The frequencies of feeding behaviors were so split as follows: in 2017, passive ram feeding (54.91%), suction feeding (31.08%), and active ram feeding (14.01%); in 2020, suction feeding (56.67%), active ram feeding (23.21%), and passive ram feeding (20.12%); in January 2022, passive ram feeding (44.56%), suction feeding (35.56%) and active ram feeding (19.88%); and in November 2022, suction feeding (49.46%), passive ram feeding (25.91%), and active ram feeding (24.63%). All environmental factors were significantly correlated among them in influencing the feeding behaviors of whale sharks. Active ram feeding was generally performed when there was a higher chlorophyll concentration in the water, higher light levels, no rainfall, lower temperatures of the water, and during La Niña ENSO (El Niño Southern Oscillations) events. Suction feeding was performed when the chlorophyll concentration was from medium to high, light levels were high, there was no rainfall, lower temperatures of the water, and during La Niña ENSO events. Passive ram feeding was preferred when environmental factors respected low chlorophyll concentration’s levels, lower light levels, low rainfall, higher temperatures of the water, and during El Niño ENSO events. In conclusion, the ENSO phenomenon was not the major factor influencing the interannual sightings of whale sharks in the Djibouti area.