This is the main conclusion of a new study by the ICM-CSIC, which reveals how urban rainfall washes waste into fishing grounds within weeks, whereas river discharges push it toward deeper waters.
A multidisciplinary team involving specialists in geology, physics, and biology of the Institute of Marine Sciences (ICM-CSIC) and ICATMAR, has successfully quantified for the first time the direct relationship between rainfall events and marine litter accumulation on the seafloor. Published in the prestigious journal Environmental Pollution, the study examinesthe impact of urban runoff and river flow on fishing grounds in the northwestern Mediterranean.
The research was based on systematic monitoring conducted between 2019 and 2024, analyzing a total of 236 bottom-trawl fishing samples collected by ICATMAR along the Catalan coast.
By testing 54 combinations between weather stations and fishing grounds, scientists evaluated the impact of post-precipitation periods ranging from 2 to 20 weeks alongside increases in river flow. The results clearly confirm that rainwater associated with urban environments, together with combined sewer overflow activity, is a decisive factor in the direct input of plastics and other waste to the seabed where the fishing fleet operates.
"Until now, we knew that rain mobilized land-based litter toward the sea, but establishing a precise quantitative relationship between precipitation and the waste that actually ends up deposited on fishing grounds was a highly complex scientific challenge," explains ICM-CSIC researcher Xavier Garcia, lead author of the study. "This interdisciplinary approach—integrating marine geology, fluid physics, biology, and resource management—has allowed us to understand that not all origins or rainfall events act in the same way, and that the type of hydrological connection is key to predicting where waste will accumulate," he adds.
Two opposing dynamics
Data analysis enabled the team to identify two distinctly different behaviors. In so-called Type A ports, urban rainfall triggers a direct increase in marine litter on fishing grounds, an effect that typically peaks between 2 and 5 weeks after the rains. This rapidity confirms that deficient urban drainage and sewer overflows during intense rainfall are the most immediate and impactful entry pathways for bottom-trawl fishing grounds.
Conversely, Type B ports exhibit a completely different river dynamics. Instead of bringing more waste to regular fishing grounds, major increases in river discharge primarily exert an emptying or flushing effect (washout), sweeping pre-existing litter into much deeper areas beyond those where fishing vessels operate. In five out of the six river-connected ports analyzed, high-flow events reduced the presence of waste between 13% and 100% for every 100 m³/s increase in river discharge. The sole exception was detected in Vilanova i la Geltrú, where accumulation originates from transport from the Llobregat river mouth (12%), combined with a dominant contribution (88%) from local rainfall.
Furthermore, the study emphasizes that the choice of the weather station recording rainfall data is a critical factor in determining its impact on waste accumulation. For the same municipality, selecting different weather stations yielded waste variation estimates ranging from -62% to +88% per 10 mm of rain, demonstrating that the mere geographic proximity does not guarantee a true hydrological connection.
A paradigm shift in coastal management
These findings have direct implications for how public administrations should design marine protection policies and sanitation infrastructure. The research demonstrates that management cannot be uniform; instead, solutions must be tailored to each coastal area. In regions dominated by Type A urban impacts, top priority must be given to improving drainage infrastructure, stormwater retention, and combined sewer overflow management to prevent plastics and other marine litter from entering the seabed areas closest to the coast.
"Our results show that management strategies must necessarily be local and adapted to the reality of each point along the coast," concludes the ICM-CSIC and ICATMAR researcher Eve Galimany. "While in certain areas the absolute priority must be upgrading sewer networks and urban drainage to prevent rain from washing litter onto fishing grounds within weeks, broader waste prevention actions must generally be promoted. Understanding these mechanisms is an essential tool for protecting our marine ecosystems and ensuring the sustainability of the fishing sector."