The Journey of Fishing Gear in the Southern Bay of Biscay: From Source to Sink and Vice Versa

Understanding the pathways of fishing gear in the ocean is essential for effective management and mitigation, yet remains challenging due to the complex interaction of ocean currents, diverse debris sources, types, and environmental factors. The Southern Bay of Biscay, a dynamic marine ecosystem supporting key economic activities like fishing, serves as an ideal case study for exploring these pathways. This study combines process-based numerical modeling and observational data of fishing activity and beached litter to assess the movement of fishing debris, providing insights into its origin, dispersal patterns, and fate. We used MARS-3D hydrodynamic model outputs as ocean-forcing inputs for the Lagrangian particle tracking model TrackMPD. Data on fishing gear deposited along eight beaches, combined with fishing activity records, were used to retrospectively and prospectively estimate the trajectories of floating litter (e.g., net fragments from trawler repairs and losses at sea) and settling litter (e.g., fibers from the fragmentation and degradation of set gillnets), and establish source-to-sink relationships under winter and summer conditions in 2023. Retrospective simulations indicated that, during summer, particles washed up on the beaches of the Spanish Basque Country followed similar pathways, originating from the western Bay of Biscay and offshore the Gironde estuary. Plastics along the French coast sourced from nearby estuarine and coastal systems. In winter, strong surface currents from the far western Bay of Biscay emerged as the main drivers of washed-up particles along the coast. Prospective simulations identified the Arcachon Bay area as a main hotspot of litter from fishing activity in winter. In summer, this zone remained affected, with additional accumulation observed along the eastern shoreline of the Spanish Basque Country. Simulations of sinking fibers suggest that they stayed within the shelf region, although ongoing simulations of different resuspension scenarios will further clarify their potential for remobilization and transport under varying seasonal hydrodynamic conditions. These findings provide valuable insights into the transboundary movement and fate of ocean plastic pollution, highlighting the interconnectedness of coastal regions and the challenges of mitigating pollution across national boundaries.

 

* Funded by PNM-BA, the French Water Agency Adour-Garonne, the Nouvelle Aquitaine Region, the French OFB, the Spanish MCIU (FIERA project PID2021- 128600OB-I00), the Basque Government (consolidated research group IT1743-22) and Euskampus Fundazioa (PLASFITO project and the Laboratory for Transboder Cooperation LTC AquEus).