Marine parks prevent the exploitation of large areas of the ocean from recreational and commercial fisherman. Although this does reduce the total area in which people can go fishing, it has extremely positive impacts for fisherman.
Marine Parks and Reserves are not placed randomly. They are often found covering the most pristine reefs and key fish sanctuaries/breeding grounds. Fish are not static and do not remain in one place. Fish thrive in protected areas, and eventually move into unprotected areas. Marine Parks and Reserves ensure that areas where fishing is legal, still have fish, and that the populations of many species do not get over fished.
Fisherman are the biggest advocates for protecting our oceans!