Microplastics (MPs) which are small plastic particles of less than 5 millimetres are now ubiquitous in our environment. Their microscopic size and their persistence in the marine ecosystem cause them to be easily mistaken for food by a range of marine organisms which are ultimately transferred in the food chain. The Indian Ocean has been reported to have the second-largest plastic load in the world. However, the fate of microplastics in this area and their consequences on marine food resources remain relatively understudied. For instance, very few studies exist on microplastic pollution in the marine environment of the Western Indian Ocean. Scientific data on ingestion of MPs by commercial species is consequently poorly available for this geographical area, including the ecoregion of the Mascarene Islands which lie about 800 Km East of Madagascar. This PhD thesis, started in December 2023, therefore aims to detect, quantify and characterise the presence of MPs in some commercial species (Katsuwonus pelamis, Thunnus alalunga and Thunnus albacares), of the Mascarene Islands. Microplastics in fishery products: a case study of the Mascarene islands (Southwest Indian Ocean).