Effects of Marine Reserves: what the data show

Marine reserves studied around the world


Most marine reserves are established with the goal of increasing the abundance and diversity of marine life inside the reserve. Considerable scientific evidence--published in peer-reviewed journals--shows that marine reserves consistently accomplish this goal, providing information for effective marine reserve design. The Science of Marine Reserves Project has synthesized the published data for many of these reserves and has developed detailed case studies.
The map (below left) shows the location of 124 marine reserves that have been studied by scientists with the results published in scientific journals. Click on the blue dots at each site to learn what happened in that reserve.
The chart (below right) shows the percent changes observed in fishes, invertebrates, and seaweeds for four key biological measures: population density, biomass, individual size, and species richness. The orange bars show the average changes for each measure across all 124 reserves. The blue dots show the change observed in each reserve. Although changes varied among reserves, most reserves had positive changes.
There are other marine reserves around the world that have not yet been studied. Although over 400 marine reserves have been established, they cover less than 0.1% of the world's oceans. Learn about global marine reserves here.



Effects of marine reserves within their boundaries


A global review of the scientific information about marine reserves revealed the following general trends:
Fishes, invertebrates, and seaweeds increased 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.
Increases were similar between tropical and temperate reserves. This indicates 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, and even small changes are important (see below for more information on the effects of increasing body size).
No-take reserves may produce greater effects than MPAs that allow some fishing. See examples in studies conducted in Florida, USA and Kenya.
Why are bigger animals inside reserves so important? Coral trout reproduction Larger animals produce disproportionately more young than smaller individuals, so they contribute much more to the next generation. This increased reproduction inside marine reserves can also help to replenish nearby fished areas (see the section below on effects of marine reserves beyond their boundaries) For example, a 50% increase in length of coral trout from 40 to 60 centimeters (16 to 24 inches) results in a 1000% increase in the number of young produced!
Read case studies from marine reserves around the world and lessons learned about the effective design of reserves and reserve networks.

Effects of marine reserves beyond their boundaries

Increases inside a marine reserve can lead to changes outside when young and adults move out of the reserve.
Movements of adults and juveniles
As fishes and invertebrates become more abundant inside a marine reserve, some adults and juveniles may leave the marine reserve to live in less crowded areas elsewhere. They also may leave because they need a different habitat as they grow or because they reproduce in a specific place outside the reserve. This 'spillover' of adult and juvenile fishes and invertebrates can contribute to marine populations living in fished waters outside reserves, where they can benefit fisheries.
The graph to the left shows how far different fish species (which had been tagged inside a marine reserve) traveled from the reserve - in some cases up to hundreds of miles.
Case studies show how scientists have documented spillover from marine reserves in the United States, The Bahamas, and the Mediterranean Sea.
Movements of young (eggs and larvae)
Larval dispersal
When fishes and invertebrates reproduce, they typically release huge numbers of tiny young into the open ocean. The young can stay in the water for days or months, potentially traveling far from their origin. Depending on how far they travel, some young produced in a marine reserve may remain inside it, while others may settle far away. Through this export of young, animals or plants in marine reserves can help replenish populations in outside waters. Scientists use genetic data, life-cycle information, computer models, and advanced tagging techniques to learn how many young are exported from marine reserves and where they go.
See a case study in the Bahamas where scientists used computer models to learn whether ocean currents in and around reserves may result in export of larvae from marine reserves.
Read more about all of our case studies from marine reserves around the world and lessons learned for better designing reserves and reserve networks.


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.

Subscribe to our newsletter