Platichthys flesus Vääna-Jõesuu in Estonia. Photo by Tiit HuntWe strive to include all marine species known to occur in the region from the selected taxonomic groups. However, there are limitations to the data, due mainly to incomplete knowledge of marine species. The following details should be noted in particular as limitations of the data and any analytical outputs.


Data Deficient Species

Globally, nearly 25% of all marine species published on the IUCN Red List are assessed as Data Deficient; of the nearly 5,000 marine bony fishes, 17% are assessed as Data Deficient. At the regional level, the percentage assessed as Data Deficient varies from about 21% in the Persian Gulf and European marine waters to only 9% in the Gulf of Mexico.

Missing Species

On average, 100-150 new fishes are described each year, and the rate of discovery in other taxa can be substantially higher. As additional work is completed, species that have not been included in our global and regional initiatives will be found. We intend to include all known marine species with each update, but this may not always possible, due to time and funding constraints and access to the literature.

Incorrect Ranges

The best available data were utilized to develop the distribution maps of species’ native ranges, which were then vetted by the foremost taxonomic and regional experts. Marine species are mapped to general habitat and depth preferences, with buffers utilized as necessary for visualization and standardization purposes. For example, all nearshore, coastal species (i.e., with minimum depths less than 200 m) are mapped according to the 200 m isobath or 100 km from the coast. This represents some species better than others; for example, it will exaggerate the distribution of species typically found from 0-10 m depth. However, these general maps can be refined as desired to more accurately reflect the true species distribution.


The relative importance of threatening processes to marine species is difficult to determine, and can be open to interpretation. In these assessments, we coded all threats that were thought to have an important impact on the species, but did not attempt to determine the relative importance of each threat. For example, many species are experiencing both substantial habitat degradation and high levels of exploitation (e.g., many large-bodied, reef-associated species).