Spinner Dolphins, Stenella longirostris, in Northwest Hawaiian
Atolls: Site Fidelity and Long-Range Movement

Rickards, Susan H.(1), Karczmarski, Leszek (2), Vanderlip, Cynthia (1)

(1) Oceanic Society
First Author Address:
68-1815 Lina Poepoe Street, Waikoloa, Hawai'i 96738, United States

(2) Marine Mammal Research Program, Texas A & M University at Galveston

Population and social structure of spinner dolphins, Stenella longirostris, inhabiting Midway Atoll National Wildlife Refuge, northwest Hawaiian Archipelago, were investigated by means of vessel-based photo-identification surveys from 1998 to 2001. Opportunistic sightings in Kure Atoll (94 km northwest of Midway) took place in October 1998 and January 2001. Approximately 322 spinner dolphins were found to frequent Midway Atoll.

These animals formed two distinct groups, consisting of 261 (145 individually identifiable) and 61 (46 individually identifiable) animals. Photo-identification analyses indicated that both groups represented stable and apparently socially discrete units. However, the smaller of the two was not recorded at Midway Atoll prior to February 1999, and was thereafter frequently sighted at Midway. Furthermore, 16 members of this group were photographed in Kure Atoll in October 1998. In contrast, there was 100% re-sighting of the larger dolphin group at Midway between 1998 and 1999, indicating a high level of site fidelity.

The stable and highly regular re-sighting pattern of these animals changed, however, in early 2000, with 48 of the previously frequently sighted and photographically identified members of the larger group no longer sighted at Midway after May 2000. The total number of "missing" dolphins has been estimated at 90. Interestingly, two of these "missing" animals, both females, were seen in January 2001 within 1 km of Kure Atoll. These findings represent the first documentation of inter-atoll movement of spinner dolphins in the northwest Hawaiian Archipelago.
They may also indicate an interesting phenomenon of "competitive displacement" of previously highly resident animals. Causes of this possible phenomenon remain unknown, but could include a limited carrying capacity of the atolls. Further research is currently underway.