Bioavailability and Ingestion of Microplastics by Fish Larvae in the Douro Estuary  ^†

In their early life stages, fish are highly susceptible to a wide range of biological and anthropogenic factors (e.g., habitat degradation or pollution) that can influence their growth and survival. Due to their size, microplastics (plastic particles with less than 5 mm) pose an additional threat to fish larvae since their size range coincides with their prey size. The ingestion of microplastics by fish larvae can cause gut blockage and limit food intake, and ultimately affect their growth, reproduction, and survival. This study aimed to evaluate the bioavailability of microplastics and quantify microplastic ingestion by fish larvae in an urban estuary. To this end, seasonal samplings surveys were performed in 2017 along the Douro estuary (NW Portugal). Sub-surface planktonic trawls were conducted along the estuarine horizontal gradient to collect fish larvae and microplastics. Samples were sorted, and fish larvae were identified and kept for further quantification of microplastics ingested. Microplastic bioavailability was determined using a previously optimized protocol. A total of 573 fish larvae were collected, with an average density of 14.63 fish larvae 100 m⁻³ and mostly composed of few but highly abundant taxa, such as Pomatoschistus spp. and Clupeidae n.i. A total of 609 microplastics were found in water samples, with an average density of 15.52 microplastics 100 m⁻³—namely, fibers, particles, and films. In Summer, fish larvae presented the highest values of abundance, contrary to the other three seasons when microplastic density surpassed larval fish density. Preliminary tests were conducted to identify the best protocol for the digestion of fish larvae to quantify microplastic ingestion. Additionally, in accordance with those results, fish larvae are currently being digested using H₂O₂ for a period of 7 h at 65 °C, to evaluate microplastic ingestion by fish larvae and to compare these results with the microplastics collected in the water.