Microplastic exposure in whale shark feeding habitats of Djibouti.

Microplastic exposure in filter-feeding megafauna is commonly assessed using seawater sampling, despite growing evidence that biological aggregation processes may strongly influence particle availability. This study quantified microplastics associated with bulk plankton-net samples collected in Djibouti, an important whale shark (Rhincodon typus) aggregation area in the western Indian Ocean. Using a multi-year dataset (2016 - 2025), microplastic abundance, spatial and temporal variability, and potential exposure for whale sharks were investigated. Mean microplastic abundance was 18.1 ± 21.9 MP m-3, while concentrations expressed on a biomass basis ranged from 0.07 to 1.42 MP mg-1. Fibers accounted for 64.4% of all particles. Most fragments were <100 μm (90%), whereas fibers were mainly within the 100 - 500 μm range (59%). No significant spatial differences in microplastic concentrations were detected among sampling sites, indicating widespread distribution across the Gulf of Tadjoura. Microplastic abundance was strongly correlated with sample biomass (Spearman r = 0.756, p < 0.0001), indicating a close association between particle availability and biomass within the feeding habitat. Exploratory analyses revealed lower zooplankton biomass and whale shark abundance in more recent sampling years, suggesting that ecosystem productivity may influence both particle availability and the intensity of whale shark aggregations. By integrating microplastic concentrations with bulk plankton-net sample biomass and species-specific filtration rates, whale shark exposure was estimated at approximately 5900 MP h-1 (∼4 × 104 MPs day-1). These values fall within the range reported for large planktivorous vertebrates and suggest that seawater-based assessments could underestimate microplastic exposure in filter-feeding species by overlooking biologically mediated pathways.