Flood control is an important issue for the Dutch coast, as about two thirds of the country is vulnerable to flooding. However, migratory fish are hampered by tidal barriers when they want to reach for freshwater habitat. Therefore, several tidal barriers are equipped with fish passages for relevant migratory species. Often, these fish passages are tested by a monitoring program using a net behind the fish pass. A large catch in these netting programs are automatically used to mark a successful designed fish passage. However, the majority of these fish passes are not tested for effectiveness in terms of local abundance, migratory delay and successful migration (%). In spring 2017, a mark recapture experiment was carried out by tagging 2187 glass eel (Anguilla anguilla) to estimate abundance and to evaluate a fish passage in a pumping station located in Scheveningen. Before fish can reach freshwater habitat they first have to pass two separated harbours, a floodgate and a pumping station. Previous netting programs behind the fish passage showed thousands of successful migrants.
Two methods were used two evaluate the effectiveness of the fish passage in the pumping station: group dyeing (0.05gr/L Bismarck Brown Y) and Visible Implant Elastomer tags. Glass eel were caught, marked and released in a harbour in two consecutive periods. In total nine eel successfully passed the fish pass showing a low migration efficiency (<1%). Based on mark-recapture assessment at least 34.000 glass eel had accumulated in front of the floodgates with a residence time of at least 20-43 days in the system. Data analysis showed that the complex is hampered by the flood gates and limited attraction flow. However, data also showed that glass eel need limited migration opportunities to migrate to the pumping station. Therefore, limited adjustments will probably improve migration efficiency of the complex.