Design of an Enhanced Monitoring Methodology for the Trawl Fishing Fleet of Llançà: a Case Study in the Northwestern Mediterranean (NE Spain)

Accurate, auditable monitoring of fishing activity is essential for sustainable fisheries management and regulatory compliance. Recent revisions of the EU fisheries control framework introduce explicit requirements for expanded electronic monitoring and strengthened traceability, including the progressive use of remote electronic monitoring (REM) with cameras for vessels identified as high-risk, and the digitalisation of catch reporting and lot traceability. These changes create a legal and operational imperative to upgrade vessel monitoring systems beyond GPS position logging. Study data come from anonymised operational records of the active trawl fleet based in Llançà. Inputs are the Caja Azul positional records, RPM and engine CAN/NMEA1-derived signals, STW and SOG, electronic logbook entries, and bathymetric/cartographic layers. Cartographic products used are the official nautical charts and services of the Instituto Hidrográfico de la Marina (IHM) together with General Bathymetric Chart of the Oceans (GEBCO, IHO2 – UNESCO), the bathymetry of the European Marine Observation and Data Network (EMODnet) and harmonised DTM3 products for the Gulf of Lion where high-resolution official isobath charts for closures (e.g., 90 m / 100 m contours) are not available. Datasets are time-synchronised and filtered; trajectories are cross-checked against declared activity and regulatory polygons (VMS/VMS4-derived zones, national closure areas). Fishing events are detected by a multi-parameter algorithm that flags characteristic trawling signatures: low SOG, moderate STW, and stable high RPM profiles, combined with gear-sensor signals and spatial coincidence with permitted trawling grounds. Data handling, analyst review workflows, and privacy/anonymisation procedures conform with national and EU data governance expectations. The methodology is explicitly aligned with: (a) the revised EU fisheries control legal framework requiring expanded electronic monitoring and enhanced traceability; (b) Spanish national legislation that promotes sustainable fisheries management and data collection; and (c) the Catalan electronic localisation and tracking framework applicable to vessels with port base in Catalonia. The system design anticipates REM camera integration where required by risk-based rules and supports end-to-end traceability and digital catch declaration workflows. Pilot application to Llançà fleet data demonstrates that adding RPM and dual-speed metrics (STW and SOG), plus contextual cartography and logbook cross-checks, raises fishing-event detection accuracy substantially (preliminary results: more than 40% improvement versus GPS-only detection), reduces false positives in discard/gear-activity classification, and refines estimates of operational time and fuel consumption. The combined sensor and REM-ready architecture provide the metadata required for auditor-grade traceability of catch lots. Enhancing Caja Azul data with mechanical, hydrodynamic and cartographic inputs—and designing the architecture to accept REM/cameras where legally required—delivers a scalable, regulation-aware solution for small-scale trawl fleet monitoring. The method strengthens enforcement capacity, improves scientific data for effort and impact assessment, and supports EU/Spanish digitalisation and traceability objectives for fisheries control.