The ecology of the whale shark in Djibouti

Within the Gulf of Aden, Djibouti is an important site where whale sharks (Rhincodon typus Smith 1828) aggregate seasonally; however, up to now, only few studies have been carried out in the area (Rowat et al. in Environ Biol Fishes, 2007. https://doi.org/10.1007/s10641-006-9148-7; Mar Freshw Res, 2011. https://doi.org/10.1071/MF10135; Rezzolla and Storai in Cybium 34:195–206, 2010). From 2015 to 2018, whale shark photo-id, skin biopsies, and zooplankton samples were collected to investigate the ecology of this species in Djibouti. Of the 190 photo-identified individuals, the majority were juvenile males, confirming a significant male-based aggregation (X2 = 65, df = 1, p < 0.0001). Within each season, the maximum number of times an individual was re-sighted was 11, while the inter-annual re-sighting rates varied between 11 and 32%. Animals were feeding on surface zooplankton, consisting primarily of copepods (85%); however, Chaetognatha and Ctenophora appeared to be important taxa as well, since the areas most visited were also those with higher concentration of these organisms. Feeding zooplankton patches had a mean biomass of 42.2 ± 31.9 mg m−3, almost 1.5 higher than control stations, indicating that the abundance of this species was positively related to food availability. Furthermore, whale sharks exhibited a clear temporal distribution associated with the variation in zooplankton biomass due to the monsoon winds. Sharks aggregated in October–February, with a peak in sightings in November, and dispersed outside the upwelling season. δ13C values suggested that whale sharks might migrate to other habitats or shift their activities to deeper areas outside the aggregation phase. Little intra-specific variation in δ15N and δ13C was found between animals, suggesting a similar habitat use.