Global microplastic pollution poses a disproportionate reproductive threat to critically endangered marine fishes.

Microplastic pollution is ubiquitous across the global ocean, threatening marine ecosystems. While environmental baselines often treat contamination as a temporally uniform stressor, the reproductive period represents the most physiologically vulnerable window in the life cycle of marine fishes. However, it remains unknown how seasonal reproductive aggregations spatiotemporally interact with microplastic accumulation hotspots and vector-driven co-contaminants at a global scale. Here we show that the spatiotemporal overlap between peak spawning and pollution intensity poses a disproportionate threat to marine fish communities, particularly critically endangered species. Integrating 6327 seawater microplastic observations and reproductive traits of 992 species across 65 large marine ecosystems, we reveal that microplastic-mediated vector effects for polycyclic aromatic hydrocarbons and perfluorooctane sulfonate peak precisely during the critical spring spawning window, amplifying synergistic bio-risks by up to 42.3%. This reproductive-period vulnerability is exacerbated by climate-driven warming and hypoxia, which synergistically maximize microplastic bioconcentration factors, leading critically endangered fishes to endure compound vector exposures nine times higher than least-concern taxa. With 7.8% of global critical spawning grounds currently exceeding ecological safety thresholds, these findings fundamentally challenge existing static conservation paradigms. Explicitly incorporating these seasonal life-cycle dimensions into global marine management is imperative to safeguard threatened biodiversity and fisheries sustainability.