Hawaiian Monk Seal Population - Northwestern Hawaiian Islands

The Northwestern Hawaiian Islands (NWHI), the geologically oldest and smallest islands of the Hawaiian Archipelago, extend approximately 2,000 kilometers (1,240 miles) from the island of Nihoa in the southeast to Kure Atoll in the northwest. The 8 primary islands and atolls include: Nihoa and Mokumanamana (Necker) Islands, French Frigate Shoals, Laysan and Lisianski Islands, Pearl and Hermes Reef, and Midway and Kure Atolls.

These volcanic islands, of once comparable elevation to Hawaii Island, have submerged and eroded over time. Many now lie a mere 2 meters (6.6 feet) above sea level. Most recently, several islets at French Frigate Shoals have shrunk or disappeared resulting in the elimination of some monk seal resting and pupping habitat. Currently, the NWHI provides 80 km (50 miles) of shoreline habitat to support the core population of Hawaiian monk seals (compared to 2,300 km (1,429 miles) in the main Hawaiian Islands). Projected global average sea level rise over the next century may further reduce this critical habitat. These extremely remote low-lying islands support ca. 80-90% of the monk seal population.

Historic utilization of the NWHI by Hawaiian monk seals is not fully known. Population numbers and distribution of the species throughout the Hawaiian Archipelago likely fluctuated over time in response to changes in island size and habitat availability. Following intense commercial and sustenance hunting in the 1800s (sealers, crews from wrecked vessels, guano (seabird excrement used for fertilizer) miners and feather poachers), the monk seal population was severely decimated. In the early 1900s, after a period of partial recovery, a large scale depletion of this species at some locations was likely caused by human disturbance from military or US Coast Guard activities. Further substantial decline of the species continued through the 1970s which led to the species listing as endangered under the US Endangered Species Act in 1976. Following a series of incremental protections, the NWHI were designated a national monument in 2006 and in 2007 was named the Papahānaumokuākea Marine National Monument. In July 2010, Papahānaumokuākea was named a World Heritage Site, becoming the only location in the United States to be listed for cultural and natural reasons.

Papahānaumokuākea Marine National Monument approximate boundary outlined.
Papahānaumokuākea Marine National Monument approximate boundary outlined. Click here to view the Hawaiian monk seal population at a glance.

The population sizes (including pups) for each of the 8 NWHI, including Nihoa and Mokumanamana (Necker) Islands, range from 50 to 250 animals each. Though the NWHI are demographically distinct, seals inhabiting the NWHI are not isolated from MHI seals. Some monk seals move between the subpopulations and genetic stock structure analysis suggest the entire population is comprised of a single stock.

Although there is a small degree of inter-island migration, each NWHI subpopulation is semi isolated with unique ecological pressures and conditions which affect monk seal survival. Survival rates among islands are variable though the overall body condition of seals across the NWHI suggests limited foraging success and food availability. Direct competition for prey resources with other seals and the abundance of other top predators in the NWHI, such as sharks and jacks (trevally or ulua), is likely contributing to limited foraging success. Additionally, shifts in ecosystem productivity, either from cyclical events and/or global climate change, may be contributing to food limitation. The relatively poor physical condition of the NWHI seals limits theirs ability to fight parasitic infections and recover from injury (shark bites, male aggression). Compromised physical conditions reduce reproductive rates and contribute to poor juvenile survival. French Frigate Shoals, formerly the largest reproductive site in the NWHI, is currently experiencing a severe decline due to a high rate of shark predation on nursing and newly weaned pups. For over a decade the loss of juvenile seals in this sub-population has been a major contributing factor in the species decline. However, in recent years there has been some improvement in juvenile survival at some sites, but overall loss of juveniles still occurs at too high a rate. Additionally, seals may become injured and die from entanglement in the large amount of marine debris that is washed ashore or ensnared on the reefs in the NWHI.

The continuing decline in the NWHI monk seal population is a primary concern. Ongoing efforts focus conservation resources to address factors detrimental to survival and which foster the recovery of the Hawaiian monk seal.

Further reading:

Antonelis GA, Baker JD, Johanos TC, Braun RC, Harting AL
2006. Hawaiian monk seal (Monachus schauinslandi): status and conservation issues. Atoll Research Bulletin 543: 75-101
Baker JD, Harting AL, Wurth TA, Johanos TC
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
Baker JD, Johanos TC, Wurth TA, Littnan CL
2013. Body growth in Hawaiian monk seals. Marine Mammal Science. DOI: 10.1111/mms.12035
Baker JD, Littnan CL, Johnston DW
2006. Potential effects of sea level rise on the terrestrial habitats of endangered and endemic megafauna in the Northwestern Hawaiian Islands. Endangered Species Research 4(1): 1-10. DOI: 10.3354/esr002021
Baker JD, Thompson PM
2007. Temporal and spatial variation in age-specific survival rates of a long-lived mammal, the Hawaiian monk seal. Proceedings of the Royal Society B: Biological Sciences 274(1608): 407-465. DOI: 10.1098/rspb.2006.3737
Carretta JV, Oleson E, Weller DW, Lang AR, Forney KA, Baker J, Hanson B, Martien K, Muto MM, Lowry MS, Barlow J, Lynch D, Carswell L, Brownell Jr. RL, Mattila DK, Hill MC
2013. U.S. Pacific marine mammal stock assessments: 2012. U.S. Dept. of Commerce, NOAA Technical Memorandum NOAA-TM-NMFS-SWFSC-504, 378 p
Gobush KS, Booth RK, Wasser SK
2014. Validation and application of noninvasive glucocorticoid and thyroid hormone measures in free-ranging Hawaiian monk seals. General and Comparative Endocrinology 195: 174-182. DOI: 10.1016/j.ygcen.2013.10.020
Harting AL, Baker JD, Johanos TC
2007. Reproductive patterns of the Hawaiian monk seal. Marine Mammal Science 23: 553-573. DOI: 10.1111/j.1748-7692.2007.00136.x