Overestimating management progress - modelled vs. monitored silver eel escapement in a North Sea draining river

The success of European efforts towards the recovery of the European eel (Anguilla anguilla) population will rely on accurate assessments of local stock status for the implementation of conservation measures. Yet, direct and continuous monitoring of the escapement of potential spawners (“silver eels”) is unfeasible in most habitats. Therefore, population models are widely used to estimate local silver eel escapement, but require input information on recruitment, demographic characteristics, and mortalities that are often estimated with great uncertainties. We conducted a combined mark–recapture and acoustic telemetry study across two migration seasons to quantify the actual silver eel escapement in a sub-catchment of the German river Ems. Results were compared with predictions from the demographic model used to provide stock parameters in Germany according to the EU eel-regulation. Mark–recapture results suggested an annual female silver eel escapement of ∼15–17 tons, while the demographic model predicted 90–98 tons, indicating a considerable overestimation. Our results suggest that realistic prediction of silver eel escapement is hardly feasible without high-quality input information and highlight the need for site-specific model calibrations against monitoring data. Overestimations of local stock sizes are problematic if they obscure the necessity for adequate conservation measures, hindering their implementation.