Many artisanal fishing communities struggle with overfishing. One potential channel to mitigate overfishing is through self-monitoring and denouncing of illegal fishing practices. In this paper, we investigate the determinants of fishermen’s willingness to report the catching of illegally small fish at a lake in Brazil. By using laboratory experiments and surveys, we provide novel evidence that impatience and pro-sociality play key roles. Fishermen who are more impatient in a laboratory inter-temporal choice experiment show a higher propensity to report misbehavior from other fishermen. This finding suggests that impatience is a driver for the punishment of resource exploitation. Moreover, we find that fishermen who are more pro-social in a laboratory public goods experiment are also more likely to report overfishing, suggesting that both time and social preferences are related to the reporting of resource overexploitation.