In the 1970s, water management in the Netherlands resulted in numerous isolated populations of three-spined sticklebacks, which can no longer migrate from freshwater to the sea. We tested whether ~50 years of isolation resulted in reduced migratory tendencies in these resident sticklebacks. Lab-based individual testing showed behavioural divergence between residents and migrants, but also produced counter-intuitive results, especially with regards to movement tendencies. To detect differences in migration tendencies, we set up a semi-natural mesocosm, consisting of connected ponds, where movements of numerous individuals could continually be tracked at larger spatial scales. We found that wild-caught residents and migrants exhibited no differences in movement tendencies within ponds, but residents moved significantly less between ponds than migrants. Between-pond movements were consistent and the observed differences were robust across contexts (changes in water flow and group size). Our study reveals that larger-scale movement tendencies can diverge over short time scales in response to human-induced isolation, and highlights the importance of observing behaviour in ecologically relevant setups that bridge the gap between lab and field studies.

Data are collected from the Radio-frequency identification decoders, which recorded the date, time stamp, attenna of detection and the unique RFID of individual fish that is detected at the antenna. From this data, we used a self-written R script to calculate the number of crosses each individual fish made between antennas (for within-pond movement) and number of crosses fish made between ponds, through the corridor (between-pond movement). All analyses are done on these crosses.