Dynamics of upstream movements of the European eel Anguilla anguilla in an inland area of the River Meuse over the last 20 years

The dynamics of upstream movements of the yellow eel Anguilla anguilla were
investigated at Lixhe on the Belgian River Meuse in an inland fish pass regularly
monitored from 1992 to 2014. Based on a constant year-to-year sampling effort, we
examined the abundance of ascending yellow eels and their body size, seasonal
movement, and the associated water temperature and flow. Over the last 23 years, the
number of ascending yellow eels has declined at an average 4.2% per year since
1992. The abundance of eels in 2014 is estimated at 4.5% of the ascending stock in
1992. We observed that some annual variations in eel abundance at Lixhe might be
related to opening fish passes downstream of the study site. The results clearly
demonstrated that long-term declining abundance of eels has resulted in increased
sizes (mean increase, 4.1 mm per year since 1992) and temperatures triggering the
upstream movement process (1.03°C per decade), with earlier dates for the last eel
passages reducing the difference between temperature extremes of eel passages
through the fish pass during the migration season. Eel movements occurred in spring
and summer at low river discharge and were mainly triggered by high-temperature
events. Eels have become larger with time because of improved feeding opportunities
and more growth habitats available resulting from the long-term reduction in
recruitment. This study highlights the importance of investigating long time spans for a
better comprehension of the changes observed in yellow eels and for the optimization
of management measures and future research.