Zooplankton assemblage diversity in the whale shark Rhincodon typus aggregation area of Nosy Be (Madagascar)

Zooplankton assemblage was studied during seasonal aggregation of the whale shark Rhincodon typus in Nosy Be Island, a hotspot area in the Mozambique Channel. Two different sampling sites were identified in the outer neritic zone: a Feeding area, where whale shark was commonly sighted, and a Control area, never frequented by R. typus. The aim was to investigate zooplankton variability, in terms of composition, biomass and size spectra, in the two areas. Mesozooplankton was collected vertically form 10 m depth to the surface, in November-December 2018-2019 in Control and Feeding areas. A total of 36 zooplankton samples were collected during both campaigns; two different size classes, <= 2 mm and >2 mm and biomass (dry and wet weights) were analysed. Taxonomic composition was performed on 12 samples collected each year. In Feeding and Control areas mesozooplankton composition was similar and mainly represented by Copepoda. Wet and dry weights were higher in Control area compared to the Feeding, with a significant contribution of organisms >2 mm. A decrease of the total number of individuals was recorded in 2019, compared to 2018, characterized by a decrease in copepod abundance (-10%). Biomass decreased of 36.4% in 2019 with respect to 2018, mainly due to a decrease of organisms >2 mm. Similar mesozooplankton assemblage was recorded in the Control and Feeding areas of whale shark, whereas a significant increase in mesozooplankton size in Control area, evidenced by the highest biomass of organisms > 2 mm, could be due to the absence of predators. A decrease of biomass observed in 2019 needs to be further monitored, but the lack of significant differences between mesozooplankton assemblage in Control and Feeding areas suggests a possibility of multiple prey sources. Analysis of prey/predator interaction is recom-mended for understanding feeding habits and abundance of whale sharks in tropical regions.