Spatial and temporal behavioural patterns of the European eel Anguilla anguilla in a lacustrine environment

Conservation of the critically endangered European eel Anguilla anguilla (Linnaeus, 1758) requires knowledge of silver eel escapement across all eel-producing habitats. Describing eel production from deep waters is especially challenging, thus telemetry studies can be used to detect specific behavioural patterns and aid in designing cost-effective survey methodologies. Here, 36 and 68 European eels were monitored via acoustic telemetry in Hanningfield reservoir, UK in 2015 and 2016, respectively. The aim of the study was to assess activity rates and home range sizes of eels in relation to explanatory variables: fish length, stage, lipid level, time of day, water temperature, night duration, lunar phase, water depth and distance to shore. In addition, site fidelity and return time to previously visited locations across the study area were investigated. Time of day, water temperature and lunar phase significantly affected eel behaviour, with higher activities observed during increased temperatures and dark periods. Furthermore, their monthly displacements rates were higher over medium-ranged depths (4-7 m), with depth use affected by water temperature, time of day and lunar phase. In general, eels had large home ranges in this study, which increased further during warmer and darker periods. Despite travelling great distances, with long return times to previously visited locations, they displayed high home fidelity between months. The results of this study provide greater insight into eels' temporal and spatial utilisation of a lacustrine habitat, and survey design to collect abundance data for the assessment.