This article gives a summary about MPAs. http://www.whoi.edu/oceanus/viewArticle.do?id=3782

A quick summary.

CONS:

• prevention of harvesting in one area lead to fishing in other areas

PROS:

• fish that stay in one area greatly benefit
• spillover effect: fisheries nearby benefits
• help preserve marine ecosystems and biodiversity of species not targeted by a fishery but impacted by it through trawling damage or inadvertent catch
• promote non-extractive uses of marine areas, like eco-tourism
• establish undisturbed locations for scientific studies that can further improve resource management and conservation

BENEFITS OF GEORGES BANK:

• The biomass (total population weight) of a number of commercially important fish species on Georges Bank has sharply increased, due to both an increase in the average size of individuals and, for some species, an increase in the number of young surviving to harvestable size.
• Some non-commercial species, such as longhorn sculpin, increased in biomass.
• By 2001, haddock populations rebounded dramatically with a fivefold increase.
• Yellowtail flounder populations have increased by more than 800 percent since the establishment of year-round closures.
• Cod biomass increased by about 50 percent by 2001.
• Scallop biomass increased 14-fold by 2001, an extra benefit of the establishment of closed areas primarily intended to protect groundfish.