Food Limitation

An emaciated yearling in the Northwestern Hawaiian Islands.
An emaciated yearling in the Northwestern Hawaiian Islands.

Limited prey availability is a central factor in the continued decline of the Northwestern Hawaiian Islands (NWHI) Hawaiian monk seal subpopulation which resides in the protected Papahānaumokuākea Marine National Monument. The ongoing lack of foraging success has negatively affected body condition, growth and age of maturation, and of particular concern, survival rates of juvenile seals. The relatively poor physical condition limits the seal's ability to fight parasitic infections and recover from injury (shark bites, male aggression). Fewer young animals, particularly females, mean fewer reproductive individuals for the future and likely a continued decline in population growth. In contrast, the main Hawaiian Islands (MHI) population is steadily increasing with higher juvenile survival rates and most seals in a robust body condition, indicating greater food availability or foraging success.

To understand movements and dive patterns in various habitat types utilized by foraging seals, various types of instruments are used, often multiple instrument types attached simultaneously, that record seal location (using satellites or cell phone towers), dive patterns (time depth recorders), and underwater behavior (using National Geographic Crittercams - seal born video cameras and 3-axis accelerometers which measure the seal’s body orientation and movement).

In the NWHI, foraging dives are generally to depths of 150 meters (492 feet) or less; however, some dive over 3 times this depth. Although the depth of foraging dives and distance a seal travels from a haul out location may vary by sex and age, foraging effort differs substantially between the NWHI seals and MHI seals. In the MHI, recorded foraging depths are substantially shallower and in shorter distance from haul out sites than monk seal foraging effort in the NWHI. These foraging patterns, correlated with growth rates and body condition, suggest differences in prey abundance among sites. Additionally, the higher abundance of seals and top predators, such as sharks and jacks (trevally or ulua) in the NWHI may also lead to direct intra- and inter-species competition for prey resources. In the NWHI, limited food resources may be limiting population recovery versus MHI seals where robust body condition and increasing population growth suggest sufficient prey resources.

As Hawaiian monk seals overturn rocks on the seafloor in their hunt for prey, fish follow the endangered mammals and steal their food (video contains no audio). Video by National Geographic CRITTERCAM.

Enhancement activities undertaken to mitigate food limitation include translocation of seals to other islands or atolls with improved foraging conditions and captive rehabilitation of malnourished animals.

Further Reading:

Diet Related to Seal Body Condition

Baker JD
2008. Variation in the relationship between offspring size and survival provides insight into causes of mortality in Hawaiian monk seals. Endangered Species Research 5(1):55-64. DOI: 10.3354/esr00122
Baker JD, Johanos TC, Wurth TA, Littnan CL
2013. Body growth in Hawaiian monk seals. Marine Mammal Science. DOI: 10.1111/mms.12035
Cahoon MK, Littnan CL, Longenecker K, Carpenter JR
2013. Dietary comparison of two Hawaiian monk seal populations: the role of diet as a driver of divergent population trends. Endang Species Res 20: 137–146. DOI: 10.3354/esr00491
Parrish FA, Marshall GJ, Buhleier B, Antonelis GA
2008. Foraging interaction between monk seals and large predatory fish in the Northwestern Hawaiian Islands. Endangered Species Research 4(3):299-308. DOI: 10.3354/esr00090

Translocation to Improve Survival

Health Screening and Quarantine Protocols for HMS translocation between Subpopulations
Baker JD, Becker BL, Wurth TA, Johanos TC, Littnan CL, Henderson JR
2011. Translocation as a tool for conservation of the Hawaiian monk seal. Biological Conservation, 144(11), 2692-2701. DOI: 10.1016/j.biocon.2011.07.030
Baker JD, Harting AL, Littnan CL
2013. A two-stage translocation strategy for improving juvenile survival of Hawaiian monk seals. Endangered Species Research 21: 33-44. DOI: 10.3354/esr00506

Rehabilitation to Improve Survival

Gilmartin W, Sloan AC, Harting AL, Johanos TC, Baker JD, Breese M, Ragen TJ
2011. Rehabilitation and relocation of young Hawaiian monk seals (Monachus schauinslandi). Aquatic Mammals 37(3): 332-341. DOI: 10.1578/am.37.3.2011.332
Norris TA, Littnan CL, Gulland FMD
2011. Evaluation of the captive care and post-release behavior and survival of seven juvenile female Hawaiian monk seals (Monachus schauinslandi). Aquatic Mammals 37(3): 342-353. DOI: 10.1578/am.37.3.2011.342
Schofield TD, Levine G, Gulland FMD, Littnan CL, Colitz CMH
2011. Short Note: The first successful hand-rearing of a neonate Hawaiian monk seal (Monachus schauinslandi) and post-release management challenges. Aquatic Mammals 37(3): 354-359. DOI: 10.1578/am.37.3.2011.354