Investigations of Hawaiian Monk Seal....
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GRANT NUMBER: NA66FD0058           NMFS NUMBER: 95-SWR-068

REPORT TITLE: Investigations of Hawaiian Monk Seal, Monachus schauinslandi, Pelagic Habitat Use: Range and Diving Behavior

AUTHOR: Kyler Abernathy, Donald B. Siniff, Ph.D.

PUBLISH DATE: November 4, 1998

AVAILABLE FROM:  National Marine Fisheries Service, Southwest Region, 501 West Ocean Boulevard, Room 4200, Long Beach, CA 90802-4213.   TELEPHONE:  (562) 980-4033


Addressing the apparently food related decline in the population of Hawaiian monk seals at French Frigate Shoals in the Northwestern Hawaiian Islands has been hampered by an almost complete lack of information about monk seal foraging ecology. This study investigated movement and diving behavior of adult seals at FFS using satellite-linked, time-depth recorders. Many seals of both sexes that left the atoll spent time at various shallow banks 30 - 200 km away. Males and females traveled to equally distant banks, but there was some indication that a lower percentage of females depart the atoll. There was a slight, but inconclusive difference in distance traveled by adult and subadult seals. Comparison of dive depth and duration histogram data indicated a lower proportion of dives below 80m by females. Distinct modes of diving activity were seen in the 40-80m, 110-180m, and >300 ranges. There were day versus night shifts in diving activity that likely correspond to prey availability. There appeared to be three rough classifications of movement and diving behavior: (1) Mid-range divers that frequented the banks, (2) Shallow divers that remained in the vicinity of FFS, and (3) Deep divers that were active along a ridge to the east of the atoll. Hawaiian monk seals at French Frigate Shoals frequently exceed the current critical habitat boundaries.

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