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Terrestrial Habitat Loss and the Long-Term Viability of the French Frigate Shoals Hawaiian Monk Seal Subpopulation

August 21, 2020

Hawaiian monk seal's terrestrial habitat loss leads to higher mortality rates for pups and effects seal population.

The endangered Hawaiian monk seal (Neomonachus schauinslandi) requires terrestrial habitat for critical life functions, such as birthing and nursing. Islands also provide monk seals with resting space that is both safe from shark attack and within commuting distance to their marine foraging habitat.

Terrestrial habitat in the Northwestern Hawaiian Islands (NWHI), where some 1,100 of the remaining 1,400 monk seals reside, is threatened by manifestations of global climate change, primarily sea-level rise. French Frigate Shoals currently hosts approximately 20% of the monk seals in the NWHI and for many decades accommodated the species’ single largest subpopulation.

The several sand islets used by seals throughout the atoll have been decreasing in size for at least several decades. Whaleskate Island was once the second largest in the atoll and for several years the site of the greatest number of monk seal pup births within French Frigate Shoals.

By 2000, Whaleskate Island had completely eroded away. Female monk seals subsequently began to give birth most commonly at Trig Island, the nearest to where Whaleskate Island had been. Trig Island had previously accounted for a small fraction of births, and it proved to be suboptimal habitat for rearing pups, as Galapagos sharks (Carcharhinus galapagensis) have been able to approach sufficiently close to the island to kill a large portion of the pups born there each year. In 2018, the two remaining primary birth sites, Trig and East Islands, washed away. Trig succumbed to progressive erosion in September 2018, and East Island was obliterated by Hurricane Walaka in October 2018.

Throughout the rest of the NWHI, pup survival from birth to weaning, a period of five to seven weeks, averages 95%. At French Frigate Shoals in 2018, only 57% of pups born survived to weaning. This high mortality of young pups was attributable to shark predation and drowning due to birth islands being inundated by storms or high tides.

Baker JD, Harting AL, Johanos TC, London JM, Barbieri MM, Littnan CL. 2020. Terrestrial habitat loss and the long-term viability of the French Frigate Shoals Hawaiian monk seal subpopulation. U.S. Dept. of Commerce, NOAA Technical Memorandum NOAA-TM-NMFS-PIFSC-107, 34 p.  https://doi.org/10.25923/76vx-ve75.

Last updated by Southeast Fisheries Science Center on 05/03/2022

Hawaiian Monk Seal