This paper employs a dynamic model for crimes that involve time and punishment to analyze the use of a net with illegal mesh size in a management regime where each community claims territorial use right over a fishery but has a discount rate that may differ from the social discount rate. The equilibrium stock and harvest levels are found to be much lower if the regulation is violated. Moreover, the optimal penalty for violation must be decreasing in the shadow cost of taking the risk to fish illegally, and increasing the risk of punishment increases the equilibrium stock level.