In this paper, we analyze how public monitoring and enforcement (M&E) efforts affect the success of a collective property right. We develop a bioeconomic model to generate several theoretical predictions, which we test empirically by assembling and analyzing novel data on public patrolling and fishing activity in the Chilean abalone fishery. Consistent with our model, we find robust evidence that patrolling increases abalone stocks and harvest for nearby fishers' organizations. In our preferred (conservative) specifications, a 10% increase in patrolling increases stock density by 0.95% and harvest by 1.2%, which translates roughly to an increase in annual revenues of 6770 USD on average within a port captainship jurisdiction. Our work provides new empirical evidence on the determinants of success for collective property rights regimes, revealing the pivotal role that public M&E can play in helping sustain these institutions.