This makes it difficult to determine the optimum size and location of marine reserves and to predict direct fishery benefits in advance. Many fear that the replenishment benefits provided by poorly designed reserves would not outweigh the costs of displacing fishermen. Marine reserves displace fishermen. Closing fishing grounds will displace some fishermen. In addition, displacement is likely to increase competition and allocation battles in fishing grounds outside reserve areas and contribute to overcapacity problems in fisheries that are not protected under a limited access system.
Marine reserves offer incomplete protection.
Marine reserves cannot protect fish from the effects of natural environmental variability or other activities that are out of the hands of fishery managers. In this way, even perfectly sited reserves involve a certain degree of risk. Marine reserves may be expensive to enforce. Increases in species abundance, diversity and size inside reserve boundaries are likely to create a strong incentive for poaching, and poaching has been reported as a problem in some fisheries where marine reserves have been applied. Rigorous enforcement may require the use of costly electronic vessel monitoring devices, particularly for offshore areas that are more difficult to reach.