Mesocosm experiments reveal the loss of migratory tendencies in a recently isolated population of three-spined sticklebacks

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.