Shark Predation

Two mom and pup pairs wary of nearby Galapagos shark.
Two mom and pup pairs wary of nearby Galapagos shark.
Nursing pup with healing shark bite.
Nursing pup with healing shark bite.

At French Frigate Shoals (FFS), located in the Northwestern Hawaiian Islands (NWHI), pup survival has been lower than any other NWHI location for more than a decade. The primary cause of mortality of nursing and recently weaned pups at FFS has been attributed to Galapagos shark (Carcharhinus galapagensis) predation in nearshore waters. It is estimated that up to 30% of all pups at FFS are lost annually to shark predation. Galapagos sharks are found throughout the Hawaiian Archipelago, yet shark predation on young seal pups at other subpopulations is exceedingly rare. It is hypothesized that a relatively small group of sharks are involved in this source of monk seal mortality, penetrating the shallow, nearshore waters where young pups reside.

Historically, the small islands within the atoll of FFS were home to the largest subpopulation of Hawaiian monk seals. This population actually increased from the early 1960s through the late 1980s. However, from 1989-2010, the population decreased by over 75%, largely driven by high juvenile mortality. The demographic structure of the FFS population is now skewed, with relatively few reproductive age individuals leading to concerns that this subpopulation is likely to further collapse.

Beginning in the late 1990s dedicated effort was directed to characterize and mitigate shark predation. Intensive observation (direct and via a remote camera system) of both Galapagos shark and monk seal mother and pup behavior led to the identification of high shark use areas, frequency and time of day of shark activity, and identification of individual sharks likely responsible for seal mortality. In addition, Carl Meyer, PhD. from the Hawaii Institute of Marine Biology (HIMB) tagged Galapagos sharks for a more detailed analysis of their movement behavior.

Mitigation efforts which began in 1997 include translocation of weaned pups from areas with high Galapagos shark predation risk to relatively safer islets within FFS and have been ongoing. Nursing pups are at the greatest risk of shark predation, but cannot be moved until they wean as they need to stay with their mothers to nurse. Thus, while translocation of weaned pups to low shark use areas within FFS may alleviate predation pressure, alternative methods of protection from shark predation are required to protect nursing pups.

In 2008 and 2009, visual deterrents and electromagnetic and auditory instruments were deployed to deter shark predation. While such non-lethal deterrents are preferable to shark removal, their effectiveness was inconclusive; however other deterrents may be tested as they become available.

Lethal removal of a limited number of Galapagos sharks from the FFS atoll has the potential to alleviate this source of seal mortality. The Galapagos shark is a top predator at this atoll and the removal of a limited number of sharks is not decided upon without intensive evaluation. The removal of a select, limited number of sharks known to prey on endangered Hawaiian monk seal pups, also a valuable part of the system, will likely maintain the overall biodiversity of the atoll.

Further Reading:

Dale JJ, Meyer CG, Clark CE
2011. The ecology of coral reef top predators in the Papahānaumokuākea Marine National Monument. Journal of Marine Biology. 2011:1-14. DOI: 10.1155/2011/725602
Dale JJ, Stankus AM, Burns MS, Meyer CG
2011. The Shark Assemblage at French Frigate Shoals Atoll, Hawaiʻi: Species Composition, Abundance and Habitat Use. PLoS ONE 6(2): e16962. DOI: 10.1371/journal.pone.0016962
Gobush KS
2010. Shark Predation on Hawaiian Monk Seals: Workshop II and Post-Workshop Developments, November 5-6, 2008. U. S. Dep. Commer., NOAA Tech. Memo. NMFS-PIFSC-21
Gobush KS, Farry SC
2012. Non-lethal efforts to deter shark predation of Hawaiian monk seal pups. Aquatic Conservation: Marine and Freshwater Ecosystems. Early View. DOI: 10.1002/aqc.2272
Harting AL
2010. Shark Predation on Hawaiian Monk Seals Workshop Honolulu, Hawaii January 8-9, 2008. National Oceanic Atmospheric Agency Administrative Report H-10-02C 36 p.
Lowe CG, Wetherbee BM, Meyer CG
2006. Using acoustic telemetry monitoring techniques to quantify movement patterns and site fidelity of sharks and giant trevally around French Frigate Shoals and Midway Atoll. Atoll Res Bull 543:281–303.
Meyer CG, Papastamatiou YP, Holland KN
2010. A multiple instrument approach to quantifying the movement patterns and habitat use of Tiger (Galeocerdo cuvier) and Galapagos sharks (Carcharhinus galapagensis) at French Frigate Shoals, Hawaii. Marine Biology. 157:1857–1868. DOI: 10.1007/s00227-010-1457-x