The endangered Hawaiian monk seal, Neomonachus schauinslandi, population numbers ~ 1,400 animals. Since 1984, the Lalo, or French Frigate Shoals (FFS), subpopulation, which is located in the Northwestern Hawaiian Islands, has produced the highest number of pups annually (range 34-127) and currently accounts for ~20% of the yearly births.
This study reviewed reproductive and survival data collected at FFS for the 33 year period from 1984-2016 to identify trends in pupping site use and pup survival to weaning.
Births at FFS have declined to ~25% of the historic high in 1988 (127). Typically, pup survival to weaning is over 90%. However, birth to weaning survival at FFS is the lowest for all the subpopulations, primarily due to Galapagos shark (Carcharhinus galapagensis) predation on pups. This predation is unique to FFS and accounts for up to 25% of the annual pup losses since the late 1990s.
Bottom graph shows survival for the 5 other primary NWHI pupping islands (Laysan Island, Lisianski Island, Pearl and Hermes Reef, Midway Atoll, and Kure Atoll) in gray, compared to FFS in blue. Vertical gray line is year (1998) Whaleskate Island eroded away.
Pups were born on all 10 low-lying, sandy islets within this atoll, and these islets differ in physical attributes (such as, permanence, size, elevation, and protective reef) and threats to pup survival.
* 2010 measurements from Reynolds, M.H., Berkowitz, P., Courtot, K.N., and Krause, C.M., eds., 2012, Predicting sea-level rise vulnerability of terrestrial habitat and wildlife of the Northwestern Hawaiian Islands: U.S. Geological Survey Open-File Report 2012–1182, 139 p.
**1963 measurement from Amerson AB (1971) The natural history of French Frigate Shoals, Northwestern Hawaiian Islands. Atoll Res Bull 150.
These graphs compare the annual number of births and pup survival to weaning for the 6 primary FFS pupping islets (East, Trig, Whaleskate, Round, Gin, and Tern Islands). Graphs by islet are ordered by largest overall number of births to the lowest during the study period. The vertical gray line across the graphs is the year (1998) Whaleskate Island eroded away. The x-axes are not all the same scale.
Births were infrequent and at low levels (<4 pups/yr) at Little Gin, Shark, Mullet, and Disappearing Islands.
East Island was the primary pupping site (~40% of annual births) until 1992, when pupping levels declined with a simultaneous resurgence of threatened green turtle (Chelonia mydas) nesting activity on East Island.
Whaleskate Island became the primary pupping site in 1992 (~35% of annual births) until it completely eroded away in 1998. Pupping then shifted to nearby Trig Island and to a lesser extent to Gin Island.
Trig Island has been the primary pupping location since 1998 (~40% of annual births). Unfortunately, the pupping shift to Trig coincided with higher pup mortality, primarily due to Galapagos shark predation. To minimize pup losses, field teams move newly weaned pups from Trig and other islets of high predation risk to areas of lower shark activity (typically Tern Island).
This study highlights how invaluable long-term datasets are in identifying temporal trends. Overall, Hawaiian monk seals demonstrated plasticity in pup site use. The ability to use alternate pupping sites has enabled this endangered species to persist at FFS, despite exposure to increased threats to pup survival (e.g., shark predation, islands going awash, etc.). With rising sea levels, we expect a continued loss of quality pupping habitat at FFS.
We thank the truly dedicated National Marine Fisheries Service field teams, who lived for extended periods at the remote FFS field station, for their detailed observations which made this review possible. The U.S. Fish and Wildlife Service provided key logistical support. This work was conducted under the NMFS Marine Mammal Research Permit #s 413, 657, 778, 848-1335, 848-1695, 10137 & 16632.