Skip to main content


In the Wadden Tools – Swimway Wadden Sea project we aim to identify the bottlenecks that fish encounter in the Wadden Sea and work out management measures to address these. We study how fish use the Wadden Sea and evaluate existing and new management measures. The results of our research will support improvements for monitoring and management, the ultimate goal being to improve the living conditions for fish in the Wadden Sea.

Declining fish stocks
Fish have always used the Wadden Sea as a spawning area, nursery, feeding ground, resting area and a place to overwinter. Similar to the stepping-stone function of the Wadden Sea for migrating birds on their route between breeding and wintering areas, the Wadden Sea plays an essential role in the life cycle of many fish species. However, habitat changes caused by anthropogenic activities such as dike and dam constructions, fisheries and dredging, and activities that have contributed to climate change, have compromised the functions the Wadden Sea provides for fish, and over the past decades fish numbers have declined drastically. The large fish species have declined, and the mean size of many remaining species has also declined.

In recent years considerable investments have been made to improve connectivity between the Wadden Sea and the hinterland. Especially for diadromous migratory species that need to migrate from salt to fresh water and vice versa, the many hard borders between the Wadden Sea and the freshwater bodies form a barrier that prevents them from completing their life cycle. Along the entire Wadden Sea coastline, the connections between the tidal flats and the bodies of fresh water on the land side of the dikes have been improved by creating inlets and constructions that enable fish passage. Furthermore, large efforts have been put into protection and restoration of shellfish beds. How, when and for which species these beds provide essential habitats is still largely unknown. In addition, areas have been closed to the various kinds of fishing that use gear that is dragged over the seabed (e.g. for brown shrimp and mussel spat). In addition, large investments have been made to increase breeding habitat for (fish-eating) seabirds. Whether these newly built breeding spots are in the right place from a ‘fish-as-food-for-birds’ perspective is not clear. Lastly, along the edges of the Wadden Sea efforts are being made to create new salt marshes and improve the management of existing salt marshes. However, as the function of this habitat for fish is not well understood, it is not yet clear how all these past measures affect their living conditions. The Wadden Tools – Swimway Wadden Sea project is addressing these knowledge gaps.

Life-cycle approach
The Swimway Wadden Sea project is based on the life-cycle approach. All fish pass through different life stages, starting as an egg, via larvae and juveniles to the adult stage. In each of these stages they require specific habitats and conditions in order to survive to the next stage. For a healthy population, individual fish must pass through all these stages successfully and the habitats required in each stage must be interconnected. The whole chain is important, and the strength of the links determine the population development of a specific species. Read more about the life-cycle approach.

Five questions
The project is divided into five sub-projects, each of which addresses an important question and knowledge gaps related to fish in the Wadden Sea:

Project partners
In the Swimway Wadden Sea project, scientists, managers and conservation organisations work together as a team. The project was initiated by the Royal Netherlands Institute for Sea Research (NIOZ), the University of Groningen (RUG), Wageningen Marine Research (WMR) and the Waddenvereniging conservation organisation. Rijkswaterstaat and Sportvisserij Nederland are also involved. The project receives financial support from the Waddenfonds, the Ministry of Agriculture, Nature and Food, and the provinces of Groningen, Friesland and Noord-Holland.

Wadden Tools
Swimway works closely with a large number of projects on fish migration and habitat restoration. The project is one of the four Wadden Tool projects that are working on optimising the management of the Wadden Sea. The other three projects study and test management measures for the floor of the Wadden Sea and for migrating and breeding birds.

  • ‘Wadden Tools – Swimway Waddenzee’ (Swimway Wadden Sea) studies bottlenecks for fish in the Wadden Sea and the effect of management measures on fish stocks.
  • ‘Wadden Tools – Waddenmozaïek’ (Wadden Mosaic) focuses on nature in and on the bottom of the Wadden Sea and studies measures that will contribute to rich biodiversity in the tidal flats.
  • ‘Wadden Tools – Wij en Wadvogels’ (Us & Wadden birds) works to strengthen the Wadden Sea region as a habitat and breeding area for birds through concrete measures designed to increase and improve breeding, feeding and resting areas.
  • ‘Wadden Tools – Waakvogels’ (Bio-indicator birds) tests the extent to which management measures contribute to the recovery of the Wadden Sea as a refuelling station and breeding habitat for migratory birds along the East Atlantic flyway.

Swimway also works with fish migration projects in the area such as Ruim baan voor Vis 2.0 (Pathways for Fish 2.0), Vissen voor Verbinding (Fish for Connection), Eems-vissen in Beeld 2.0 (Eems fish in view 2.0). and the Vismigratierivier (Fish Migration River). These projects examine the obstacles fish encounter on their way from fresh to salt water and vice versa. In cooperation with these projects, we will map the migration routes and life cycles of fish as comprehensively as possible.