Hawaiian Monk Seal Population at a Glance
The Hawaiian monk seal, one of the rarest pinniped (seal and sea lion) species on Earth, was listed as Endangered under the US Endangered Species Act (ESA) in 1976.
The majority of the population (ca. 80-90%) is distributed throughout the 8 remote islands and atolls of the Northwestern Hawaiian Islands (NWHI) within the Papahānaumokuākea Marine National Monument (PMNM), (Nihoa Island, Mokumanamana (Necker Island), French Frigate Shoals, Laysan Island, Lisianski Island, Pearl and Hermes Reef, Midway Atoll, and Kure Atoll). Each island contains a semi-isolated subpopulation of seals that face unique ecological pressures and conditions. While fishing pressure and human activity have been minimized within this vast marine environment, the population as a whole continues to decline.
The NWHI population has declined 3.3% annually over the past decade from 2003-2012. However, this decline has recently slowed, in part due to NOAA Fisheries recovery efforts. From 2003 to 2007 the rate of decline was nearly 5% per year; whereas from 2008-2012 the decrease was only 0.6% per year. Though there has been some improvement in juvenile survival at some sites, the loss of juveniles overall still occurs at too high a rate.
Conversely, the smaller main Hawaiian Islands (MHI) population has been steadily increasing despite high levels of human and fishing activity. Of note, the MHI provide over 2,300 km (1,429 miles) of shoreline habitat, which is nearly 30% larger than the NWHI.
The demographics of the monk seal populations in the NWHI and MHI are strikingly different. Seals residing in the NWHI have lower survival rates, reproductive success, and poorer overall body condition compared to those of the MHI. A variety of physical and environmental factors can account for these differences, and necessitate specific conservation and recovery strategies for each area.
To learn more about the 2 main monk seal subpopulations, click the following links:
- Hawaiian Monk Seal Northwestern Hawaiian Islands Subpopulation
- Hawaiian Monk Seal Main Hawaiian Islands Subpopulation
Hawaiian Monk Seal Population Size by Island
The sizes for each of the 8 NWHI subpopulations range from 50 to 250 animals. Overall, the best total population estimate for 2012 was 1,153 seals (1,015 in the NWHI and 138 in the MHI). However, the MHI population is probably closer to 200 seals as there is limited coverage at some of the remote and more inaccessible MHI locations. The highest numbers of seals in the MHI are sighted at the northwestern end (Niʻihau, Kauaʻi, Oʻahu, and Molokaʻi) with far fewer animals observed at the southern end (Maui, Lānaʻi, Kahoʻolawe and Hawaiʻi Island). Even though there is sparse data for Nihoa and Mokumanamana (Necker) Islands and for some MHI locations, evidence suggests these populations are increasing and together account for about 25% of the total population.
|Location||Minimum 2012 Population Size (including pups)1|
|Pearl and Hermes Atoll||160|
|French Frigate Shoals||177|
|Mokumanamana (Necker) Island||50|
|Main Hawaiian Islands||138|
2010. Dramatic shifts in Hawaiian monk seal distribution predicted from divergent regional trends. Marine Mammal Science. DOI: 10.1111/j.1748-7692.2010.00395.x
2013. U.S. Pacific marine mammal stock assessments: 2012. U.S. Dept. of Commerce, NOAA Technical Memorandum NOAA-TM-NMFS-SWFSC-504, 378 p
Monk Seal Range and Movements
Monk seals range throughout the entire Hawaiian Archipelago, a distance of 1,549 miles (2, 500 kilometers). Births have been documented on every Hawaiian island except for Lānaʻi (in the MHI). A few monk seals have been sighted outside of the Hawaiiian chain at Johnston Atoll, and a birth was documented there in 1969.
Overall, the majority of seals tend to remain at their natal (birth) island for life; however, movement rates are higher among islands in close proximity. Accordingly, the MHI has the highest movement rates, and within the NWHI, the greatest movement occurs at the western end of the chain among Kure and Midway Atolls and Pearl and Hermes Reef. Movement also occurs between the NWHI and MHI. The genetic stock structure analysis of the Hawaiian monk seal revealed that the entire population is comprised of a single stock because of the seal movements between islands.
2014. Range-wide movement patterns of Hawaiian monk seals Marine Mammal Science 30(3): 1165-1174. DOI: 10.1111/mms.12084