How Does the Microplastic-to-Fish Larvae Ratio Change Across Years in an Estuarine Nursery Area? ^†

Abstract

Introduction: The early stages of fish represent a critical phase for survival and recruitment, as they are highly vulnerable to both biotic and abiotic factors, as well as anthropogenic pressures. To enhance survival, many marine species use estuaries as nursery areas. However, these ecosystems are increasingly exposed to contaminants such as microplastics (MPs; plastic particles < 5 mm) that can cause several direct or indirect negative impacts on fish larvae, namely impairing their development or survival. Objective: This study aimed to quantify and compare temporal changes in the ratio of microplastics (MPs) to fish larvae (FL) (MP:FL) in the Douro estuary (NW Portugal), assessing how exposure to MPs varies across years and seasons. Methodology: Seasonal sampling campaigns were conducted in the Douro estuary during 2021/2022 and 2025. Multiple stations along the estuary were sampled using plankton tows with a 0.5 mm mesh size. In the laboratory, fish larvae were sorted and identified, and the remaining material was processed to isolate and quantify MPs. The recovered MPs were subsequently characterized according to type, size, and color. Results: Data from 2022 indicated that Clupeidae, Gobiidae, and Gadidae were the most abundant fish families, while colorless and blue fibers between 2 and 3 mm were the dominant MP types. Data from 2025 showed that Gobiidae, Labridae, and Atherinidae were the most abundant families, with similar MP types observed in water in 2022. The ratio of MPs:FL in summer and autumn of 2021/2022 was 36 and 65 MPs:1 FL, respectively, whereas in 2025 it was 0.26 and 3.80 MPs:1 FL, respectively. Conclusions: These preliminary results indicate a decreasing trend in the ratio of MP:fish larvae over time. Although further data analysis is ongoing, the observed interannual differences highlight the importance of long-term monitoring of estuarine nursery areas to better understand contamination dynamics and their potential effects on early fish life stages.