Abstract: Closing-off the estuary Zuiderzee with a 32 km dam (Afsluitdijk) had severe consequences for fish migration between Wadden Sea and rivers IJssel/Rhine and Vecht. Plans are proposed to build a large fish passage (‘Fish Migration River’) next to the discharge sluices in the Afsluitdijk at Kornwerderzand. This passage of several km length is unique in also allowing incoming tide, thus facilitating fish using selective tidal transport. To come to an optimal design and placing of entrances that efficiently will attract diadromous fish, several studies were carried out in spring 2014 to study behaviour and abundance of migratory fish. The seaward side of the sluices were monitored with lift nets, mainly targeting flounder larvae, three-spined stickleback, smelt and glass eel. DIDSON observations were made to determine vertical distribution of fish. An acoustic telemetry experiment (18 VEMCO receivers) was carried out to study searching behaviour and passage success of sea lamprey, houting and sea trout. Abundance estimates for the different species varied from several tens to hundreds of millions. The spatial dynamics of small fishes in time in the Kornwerderzand discharge basin were large. Numbers of small fish in the discharge basin were high, but the highest concentrations occurred at different locations in time. Lower numbers of glass eels and sticklebacks after highwater than before upcoming tide are in accordance with Selective Tidal Transport, although smelt numbers were similar after high water, suggesting more active swimming behaviour. Telemetry data showed large dynamics and intensive searching of the whole discharge basin by the fish. Passage success of sea lamprey appeared to be limited to 12-25%. In addition, substantial delay of sea lamprey migration was indicated. Passage success of houting and sea trout appeared to be larger than 50%. In this, discharge events, tidal current, wind and active swimming all play a role.