A multidisciplinary approach to assess the distribution and conservation of the European eel Anguilla anguilla in an estuarine environment of central Italy

The global population of the European eel (Anguilla anguilla) has declined by over 90% since the 1980s, leading to its classification as critically endangered on the IUCN Red List. This dramatic decrease has been attributed to multiple stressors, including overfishing, habitat loss, pollution, climate change, parasitism and predation. Given the complexity and variability of these threats, understanding eel behaviour across life stages is essential for effective conservation and management. This study investigates behavioural differences between yellow and silver eels—two adult phenotypes representing distinct life stages—and examines environmental factors influencing their spatial distribution in a central Italian estuary. Between 2018 and 2021, data were collected using fyke nets and Passive Integrated Transponder (PIT) tags to track individual movements and diel activity patterns. A total of 1189 eels were sampled, comprising 1058 yellow (88.9%) and 131 silver individuals (11.1%), with a total biomass of 162.8 kg. Yellow eels were smaller (mean length: 41.6 cm; mean weight: 168.7 g) than silver eels (49.8 cm; 249.3 g). Seasonal variation revealed that yellow eels were most abundant in March and August, while silver eels peaked in October and November. Temperature and pH were key drivers of phenotypic distribution: yellow eels dominated (>99%) when temperatures exceeded 7°C, whereas silver eels were more frequent (86%) under cooler, more acidic conditions (pH < 7.4). Activity monitoring showed that both phenotypes were predominantly nocturnal. Yellow eels exhibited peaks at 2.00am, 4.00am and 8.00pm; silver eels were most active at 4.00am, 5.00am and 8.00pm. Generalized linear models confirmed significant effects of developmental stage and time of day on activity (p < 0.05).