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Approximately 230,000 (15%) of the 1.8 million known species worldwide are marine species, and there may be up to ten times as many that remain to be discovered. Marine species face increasing risks of extinction especially from changing climate, habitat destruction and industrialization of the ocean (McCauley et al. 2015) which also entails ocean acidification, overexploitation, pollution, and increased competition from alien species. 

The International Union for the Conservation of Nature and Natural Resources (IUCN) began assessing Risk of Extinction levels in 1973. Species assessments are carried out by Specialist Groups, comprised of scientists, who participate in workshops where conservation actions for each species are evaluated according to data on population trends, distribution, life history, past and existing threats, taxonomy, and ecology. These factors determine if a species meets the criteria to be listed within a threatened category under the IUCN Red List of Threatened Species.

The IUCN Red List Categories include eight levels of extinction risk:

•      Extinct (EX)

•      Extinct in the Wild (EW)

•      Critically Endangered (CR)

•      Endangered (EN)

•      Vulnerable (VU)

•      Near Threatened (NT)

•      Least Concern (LC)

•      Data Deficient (DD)

The IUCN determines whether or not a species qualifies for a particular threat category by deciding if it meets the threshold for that category in one of five criteria (A-E). Critically Endangered species face an extremely high risk of extinction in the wild. The other threatened categories – Endangered and Vulnerable – refer to species that are either at a very high, or high, risk of extinction in the wild.

Specifics regarding criteria for all threatened categories can be found on the IUCN Red List website.

In 2007, only about 1,500 of the 41,500 plants and animals assessed under the IUCN Red List Criteria were marine species. To address this gap, IUCN, Conservation International and Old Dominion University initiated the Global Marine Species Assessment (GMSA) in 2006, with the goal of completing IUCN Red List assessments for 20,000 marine species by 2012.  As of 2015, GMSA has assessed over 13,000, falling short of the original goal, but increasing the percentage of marine species on the Red List from less than 1% in 2001 to 13%.  

Working with IUCN Specialist Groups, GMSA has completed, among others, assessments of all the world’s known species of sharks and rays, groupers, and reef-building corals (Polidoro et al. 2008). However, several species in these groups are of unknown status due to lack of information and many taxonomic groups still await assessment.

How Was It Measured?

The Ocean Health Index utilizes data for all marine species categorized in the most recent IUCN Red List. The main taxonomic groups included in the list are: habitat forming corals, mangroves, seagrasses, sea snakes, marine mammals, seabirds, marine turtles, angelfish, butterflyfish, groupers, wrasses, parrotfish, hagfish, and tuna and billfishes, but new species are added steadily. 

The average extinction risk is calculated as the weighted sum of the International Union for Conservation of Nature’s (IUCN) assessments of species. Weights used are based upon the level of extinction risk following Butchart et al. (2007): EX (extinct) = 0.0, CR (critically endangered) = 0.2, EN (endangered) = 0.5, VU (vulnerable) = 0.7, NT (not threatened) = 0.9, and LC (least concern) = 0.99.

For the status of the Species subgoal of the Biodiversity goal, the average risk status of all species assessed was used. The status of the Iconic Species subgoal of Sense of Place was calculated as the average extinction risk of those species identified as iconic.

The average risk status was also used as a proxy for ecological resilience in the Carbon Storage goal, Coastal Protection goal, Fisheries sub-goal of Food Provision, Iconic Species sub-goal of Sense of Place, Natural Products goal, Habitat Diversity sub-goal of Biodiversity and Artisanal Fishing Opportunities goal. Because the Natural Products and Artisanal Fishing Opportunities goals are mostly linked to coastal systems that are easily accessible from the shore, only the species found within 3 nautical miles of the coastline were used in their calculation.

More species will be assessed in the future, but it will require a significant amount of time to complete assessments for every relevant group, and additional time will be necessary to assemble information on newly discovered species.

Get More Information

The Convention on Biological Diversity (CBD)

CBD provides a strategic plan for the conservation of biological diversity and the sustainable use of its component factors on local, national, and international scales. 


Carpenter, K. E. et al. (2008). One-third of reef-building corals face elevated extinction risk from climate change and local impacts. Science 321, 560–563.

McCauley, D.J., M.L. Pinsky, S. R. Palumbi et al. (2015). Marinedefaunation: animal loss in the global ocean. Science 347(6219). DOI:10.1126/science.1255641. 16 January 2015

Mora, C., D.P. Tittensor, S. Adl, A.G.B. Simpson and B. Worm. (2011).  How many species are there on Earth and in the ocean? PLoS Biol 9, e1001127.

Polidoro, B.A., S.R. Livingstone, K.E. Carpenter, B.  Hutchinson, R.B.  Mast, N. Pilcher, Y. Sadovy de Mitcheson, S. Valenti. (2008). Status of the world’s marine species, in: J.-C. Vié, C., Hilton-Taylor, and S.N. Stuart (eds.). The 2008 Review of The IUCN Red List of Threatened Species. IUCN. Gland, Switzerland.

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