Abstract: Space restrictions often necessitate the incorporation of turning pools in the design of fishways. Several biological studies have identified potential problems fish have with turning pools, varying from disproportionate residence times, flow negotiating difficulties and even fish leaping out of the turning pool. Likewise, our research on fish passage times of a vertical slot fishway on the river Mosel in Germany, detected significant time lags in turning pools. A number of potential explanations have been stated in the literature including confusion associated with large vortices, turbulence and complex flows, flow characteristics exceeding the swimming abilities of the fish, or fish could be using such basins to rest. The actual activity within these turning basins remains unknown and represents an important knowledge gap. This paper presents the results of an HDX and sonar monitoring campaign, registering behaviour in a turning pool in a vertical slot fishway on the river Mosel in Germany, where increased passage times were recorded for several fish species in turning pools. Behavioural patterns like residence times, acc- and deceleration, resting and swimming tracks for both up- and downward migration are related to flow direction patterns described in a 3D hydraulic model. The results offer insights in the behavioural characteristics of migrating fish in turbulent basins, with direct implications on technical design features like rounded basins and additional baffle walls.