Direct evidence of predation on endangered sea lamprey in estuarine tidal area

• Our study shows the first evidence of sea lamprey predation in a tidal estuary. • In areas with possible predation, using digestion sensors is now essential. • Predation assessment is possible with a mix use of digestion and classical tags. • This predation is likely caused by the European catfish Silurus glanis. The sea lamprey Petromyzon marinus is a anadromous species that has undergone a dramatic decline in many European rivers over recent decades, observed for instance as substantial decrease in the number of returning spawners (i.e., the reproductive stock), observed in most major river systems. Consequently, in Western Europe, its conservation status varies from Near Threatened (NT) in Ireland, to Least concern (LC) in Great Britain, Vulnerable (VU) in Spain, Endangered (EN) in France, and Critically Endangered (CR) in Italy, according to national IUCN assessments. This decline is attributed to multiple pressures, including habitat degradation, obstacles to migration, climate change, and overfishing. However, the potential impact of predation on this endangered species remains poorly documented, especially in estuarine areas. This study aimed to evaluate predation on sea lampreys during their upstream migration in the tidal oligohaline zone of the Loire estuary, using acoustic telemetry over two years. Fish were captured, tagged with acoustic tags (with or without digestion sensors), and released into the river. The predation rates (certified by digestion tag and estimated with classical tags) ranged from 8.8 to 20 %, suggesting that predation could represent a significant, yet overlooked, threat in estuarine zones. Based on indirect evidence and catfish fishing data, the European catfish Silurus glanis — an invasive and opportunistic predator increasingly abundant in the Loire system — appears to be the most likely predator responsible for these events. This study provides the first evidence suggesting sea lamprey predation by European catfish in estuarine tidal zones. It highlights the need to consider the impact of invasive species on migratory fish populations, not only in upstream sections of rivers but also from the very first stages of their freshwater migration.