Data report: Glass eel and yellow eel migration in the Rhine Meuse estuary : A data report of a mark recapture study spring 2022

Summary

Connectivity between sea and freshwater inland habitats is heavily obstructed by anthropogenic barriers leading to a high pressure on diadromous fish populations including European eel (Anguilla anguilla).Nevertheless each spring millons of juvenile eels (glass eel) arrive at European coasts to access freshwater habitats. Coastal glass eel migration behaviour highly depends on tidal currents including in estuaries. However, this behaviour is severly hampered in regulated water systems. Therefore a better understanding of glass migration behaviour in highly regulated water systems is needed to take proper mitigation actions. Therefore a mark-recapture experiment was conducted in spring 2022 by releasing 21.424 tagged glass eels along the coast (Haringvliet and Nieuwe Waterweg) and 4.870 glass eels at inshore locations of the Nieuwe Waterweg, Hollandsche Ijssel, Haringvliet and adjecent rivers. In addition 1.097 young yellow eel were tagged at multiple inland locations of the study area.The results showed that (untagged) glass eels were caught throughout the study area up to at the very eastern side of the study area 70km inland. The number of tagged glass eels released along the coast were enough to answer the research questions (abundance and distribution). Multiple recaptures(n=121) were present at the west side of the study area: Rozenburg, Westland and Zaaijer, but also at the very east side of the study area at Krimpenerwaard. Therefore, using the data collected in 2022, an overall abundance estimate can be made and how this relates to inland local abundance estimates.For the Haringvliet multiple recaptures (n=1980) were found at the coastal side of the Haringvlietdam,Zuiderdiep and the Goereese sluis (Stellendam) to estimate the coastal abundance at the Haringvliet. Additionally one tagged glass eel was found in the hinterland at the pumping station Putten.In addition to the eastwards dispersal of glass eels, tagged glass eels showed also dispersal along the coast. Glass eels released at Haringvliet and Nieuwe Waterweg were recaptured at Katwijk (and viceversa). Tagged glass eels showed also dispersal between Nieuwe Waterweg and Haringvliet.Multiple locations showed increased catches of young yellow eels (elvers) during the season. Similar to the local abundance estimates of glass eels using tagged an locally released glass eel, a young yellow eel (elver) local abundance estimate at relevant locations can be made.This report gives an overview of the fieldwork procedures, catches and recaptures. This report is anticipating final results, which will be incorporated in a final report to be published in 2023. The research will be continued in 2023 when multiple fish passages will be studied on efficiency.