Despite there is a growing interest in studying the presence and effects of microplastics (MP) in fishes and other aquatic species, knowledge is still limited in tropical areas. In this study, we examined the presence of MP in the gastrointestinal content of 30 filter feeders of thread herring, Opisthonema complex (Clupeiformes: Clupeidae) from the Central Pacific coast of Costa Rica. We detected the presence of MP in 100% of the individuals with an average of 36.7 pieces per fish, of which 79.5% were fibers and 20.5% particles. To our knowledge, this is the first study in Costa Rica that demonstrates the presence of MP in planktivorous fishes. The effects of microplastics ingestion by O. libertate and its transit through aquatic food webs should be studied in greater detail, with greater number of sampling points at different times of the year. However, our work confirms that contamination by microplastics is having direct effects on the marine life of Costa Rica.

Despite there is a growing interest in studying the presence and effects of 18 microplastics (MP) in fishes and other aquatic species, knowledge is still limited in tropical 19 areas. In this study, we examined the presence of MP in the gastrointestinal content of 30 filter 20 feeders of thread herring, Opisthonema complex (Clupeiformes: Clupeidae) from the Central 21 Pacific coast of Costa Rica. We detected the presence of MP in 100% of the individuals with 22 an average of 36.7 pieces per fish, of which 79.5% were fibers and 20.5% particles. To our 23 knowledge, this is the first study in Costa Rica that demonstrates the presence of MP in 24 planktivorous fishes. The effects of microplastics ingestion by O. libertate and its transit 25 through aquatic food webs should be studied in greater detail, with greater number of sampling 26 points at different times of the year. However, our work confirms that contamination by 27 microplastics is having direct effects on the marine life of Costa Rica.28 29