Effects of marine reserves within their borders

The Oceans Need Our Help

Oceans around the world are becoming degraded. Evidence shows that human activities, including intense fishing around the world, are altering ocean ecosystems beyond their natural state. According to numerous scientific studies, fish, shellfish, and other important species are declining in many places. These changes are impairing the ocean’s capacity to provide food, protect livelihoods, maintain clean water, and recover from environmental stresses like severe storms.

Many people are inquiring about solutions to reduce negative impacts and foster ocean health. Increasingly, government agencies, commercial groups, non-governmental organizations, the public, and scientists are discussing the idea of establishing marine reserves to complement other efforts to restore and sustain ocean ecosystems.

Marine reserves are ocean areas that are fully protected from activities that remove animals or plants or alter habitats, except as needed for scientific monitoring. Most marine reserves are established with the goal of increasing the abundance and diversity of marine life inside the reserve. Scientific research shows that marine reserves consistently accomplish this goal.

Marine Reserves around the World

This map shows the location of 124 marine reserves that have been studied by scientists with the results published in scientific journals.

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Average changes (orange bars) in fishes, invertebrates, and seaweeds within marine reserves around the world. Although changes varied among reserves (blue dots), most reserves had positive changes.

More Fishes, Shellfish, and Other Marine Life

Considerable scientific documentation—published in peer-reviewed journals—provides a clear picture of what has happened after the establishment of marine reserves.

Scientists have studied more than 124 marine reserves in at least 29 nations and territories around the world and monitored biological changes inside the reserves. The number of species in each study ranged from 1 to 250 and the reserves ranged in size from 0.006 to 800 square kilometers (0.002 to 310 square miles).

As indicated in the graph above, the studies documented a wide range of changes inside marine reserves, but nearly all of the effects were positive. A global review of the studies revealed that fishes, invertebrates, and seaweeds had the following average increases inside marine reserves:

  • Biomass, or the mass of animals and plants, increased an average of 446%.
  • Density, or the number of plants or animals in a given area, increased an average of 166%.
  • Body Size of animals increased an average of 28%.
  • Species Density, or the number of species, increased an average of 21% in the sample area.

These increases were similar between tropical and temperate reserves, indicating that marine reserves can be effective regardless of latitude.

Heavily fished species often showed the most dramatic increases. Some fished species had more than 1000% higher biomass or density inside marine reserves. Even small changes in species diversity and individual body size are important. These two indicators have less potential for change than do biomass or density. Additionally, in some studies such as ones conducted in Florida, USA and Kenya, reserves produced greater increases than MPAs that allowed some fishing. Read more about success stories and lessons learned from marine reserves around the world and scientific considerations for designing reserves and networks of reserves.

Bigger Fish Have More Young

Fishes and invertebrates grow bigger in marine reserves than in unprotected areas. This effect is extremely important because large animals contribute much more to the next generation. They produce substantially more babies than smaller individuals. For example, a 60-centimeter (24- inch) coral trout produces 10 times more young than when it was 40 centimeters (16 inches) long. This increased reproduction can be important in replenishing nearby fished areas.

Coral trout reproduction

Global Reserves Facts


- Marine reserves cover less than 0.1% of the ocean worldwide.
- Most reserves are quite small. Half of the 124 reserves studied by scientists covered less than 3.75 square kilometers (1.5 square miles).
- A survey of 255 marine reserves conducted in 2005 showed that only 12 were patrolled routinely to prevent poaching.

Visit the PISCO Website


Visit the website of the Partnership for Interdisciplinary Studies of Coastal Oceans to learn more about the Science of Marine Reserves.  

There you can download our booklets, video series, and high-resolution figures.

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