The Hop-o’-My-Thumb white pebbles: How AIS data allow gillnet and trammel net fishing effort to be tracked in the forest of fisheries data

Dolphin bycatch represents a major conservation issue driven by interactions with passive fishing gear such as gillnets and trammel nets. Current fishing effort estimation methods, relying on vessel speed thresholds or fishing hours, inadequately reflect the reality of these static gears, where soak time, net length, and height may influence bycatch. This study integrates high-resolution geolocation data (AIS) with onboard observers’ data to propose an enhanced methodology for assessing fishing effort, incorporating these three key parameters. Applied over several years (2016–2023) in the Bay of Biscay, the length of the nets estimated show a lot of variability between 0.5 and 11 km depending on the species targeted and the vessel’s length. The height of the nets is also highly variable, ranging from 1 to 8 meters depending on the species targeted and the gear used. The fishing effort integrating all the parameters reveals significant spatial and temporal variability in fishing practices and highlights areas at risk of bycatch for common dolphins, mainly on the continental shelf and during winter, where fishing effort is the highest. The proposed approach offers a novel method for measuring fishing effort of passive gear, at a high resolution, facilitating a more precise understanding of fishing impacts and bycatch risks, and providing practical tools for managing fisheries and marine conservation. • A new method estimates passive fishing effort using AIS and observer data. • Soak time, net length, and height are combined to assess fishing time and surface. • Net dimensions vary greatly by target species and vessel size. • High-resolution effort maps reveal seasonal and spatial hotspots. • Approach improves understanding of dolphin bycatch risk in gillnet fisheries.