Free-choice exploration increases affiliative behaviour in zebrafish

Cognitive stimulation has been shown to be rewarding and capable of eliciting positive emotions in several species. In contrast to the abundant learning and exploration opportunities available in nature, captive environments can be under-stimulating-with the potential to induce anhedonia and reduce welfare. Zebrafish are now a popular scientific model, in part because of their high cognitive function and sensitivity to environmental manipulations, yet little is known regarding their response to free-choice opportunities to explore. To begin to understand the effect that cognitive enrichment may have on zebrafish behaviour, we housed zebrafish within six tanks (10 fish/110 L tank) for nine months and then removed a divider to expose 10 cm of additional novel tank space. We measured free-choice exploratory behaviour (latency and number of inspections), as well as anxiety/fear (bottom-dwelling) and social behaviour (agonistic behaviour, group cohesion and coordination). We collected video data on each of four days: the day before (baseline), the day of, the day after, and two weeks after divider removal. Using multilevel models, we found that after the divider was removed, zebrafish moved into the novel space on average within 9.7 ± 7.6 s (mean ± SD), and the number of inspections increased on each day observed (p < 0.003). We found no evidence of bottom-dwelling (p > 0.73), indicating that the manipulation was not likely to have induced a negative emotional response. Furthermore, the opportunity to explore altered social behaviour: reducing agonistic behaviour (p = 0.02), and increasing shoal cohesion (p = 0.04) and coordination (p = 0.04) for up to a day relative to baseline. In light of the fact that their natural habitats would normally include such information-gain opportunities, these results indicate that free-choice exploration may be beneficial for zebrafish welfare. This study thus adds to the growing body of literature on the role cognitive stimulation plays in welfare and indicates that zebrafish are good candidates for further cognitive enrichment research.