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expeditions/June-July
2006/ Day 24
Spinner Dolphins in the Northwestern Hawaiian Islands
Patricia Greene, NOAA Teacher-at-Sea
Spinner dolphins in the lagoon around Green Island at Kure Atoll,
State Wildlife Refuge. Photo: Patricia Greene
The first creatures we experienced at Kure Atoll were the spinner dolphins. These creatures delight in
playing in the wake of our bow; doing somersaults, spins, and jumps; crisscrossing fearlessly in front
of our boat, then losing interest when we slow down. Scientists are not sure what make spinner dolphins
exhibit this type of behavior.
We observed mothers with calves at their side; the babies easily keeping up and enjoying the sport as much
as the adults. During the day the dolphins are relatively inactive and take group naps but at night they
leave the atoll to forage and feed.
Interestingly, scientists have observed that spinners in the Northwestern Hawaiian Islands have a different
social structure than those around the main Hawaiian Islands. In the Northwestern Hawaiian Islands,
dolphins demonstrate group cohesion and typically stay together in the same group socializing in the
lagoons or when they feed offshore. In the main islands a spinner dolphin may join a different feeding
group every night; scientists have dubbed this behavior “fission-fusion,” since groups form and split
repeatedly.
We observed a large group of spinner dolphins at Kure; approximately 70, although they swam so rapidly
they were difficult to count. Other pods or groups have been identified at Pearl and Hermes and Midway.
Typically, crossover between these groups in the Northwestern Hawaiian Islands is rare.
Recently we visited Southeast Island at Pearl and Hermes and interviewed NOAA Fisheries monk seal
researchers; Hugh Finn, Jessie Lopez, and Kennedy Renland regarding their spinner dolphin research.
Basically, the dolphin research is done at the same time as they do the atoll counts for the monk seals;
approximately every third day if the weather cooperates. If winds exceed 15 knots, safety becomes a
concern and researchers will not go out in the small boats. During an atoll count day the researchers
leave camp at 9:00 am and return at approximately 4:00 pm. For safety reasons, only two researchers go
out in the boat at a time; one person remains on shore and monitors the radio in case assistance is needed.
During an atoll count they will visit North Island, Seal Kittery, Grass Island, and various sand spits to
assess the population.
Cynthia Vanderlip and her team conduct spinner dolphin surveys in the
lagoon around Green Island at Kure Atoll. Photo: Dena Deck
Dolphin surveys involve taking digital photographs of as many dolphins as possible. This year the Pearl
and Hermes Atoll researchers have taken approximately 2,000 photographs to date. These digital images will
be forwarded to Dr. Lezek Karczmarski at Texas A&M; and fed into a database for his research. Individual
dolphins usually have distinctive cuts, scars or marks that help identify them. Researchers also take
small biopsy samples from the dolphins. This is accomplished by using a crossbow type instrument with
a dart that removes a tiny piece tissue from the skin.
Dolphins at Pearl and Hermes Atoll usually travel in groups or pods of 50 to 60. Mothers with calves are
often seen at this time of year. Researchers explained that the age of the calf can often be estimated by
the existence of “fetal folds.” The female dolphin has a 12-month gestation period and while inside the
mother the calf develops creases in its body. These “fetal folds” will exist until the calf is two or
three months of age.
Majestic Hawaiian spinner dolphins in the clear lagoon waters of
Kure Atoll, State Wildlife Refuge. Photo: James Watt
During a dolphin survey, researchers record the start and end times, initial and final GPS coordinates,
swell, water depth, water temperature, and bottom type. They assess the numbers and ages of any calves
observed and record the numbers of juveniles and adults. Total number of digital images taken and any
ID ratio is also recorded.
Spinner dolphins have a wide range; found in tropical waters, subtropical, the Pacific, Atlantic, and
Indian Oceans. They feed on mesopelagic fish, squid, and shrimp. The females reach sexual maturity at
7 to 10 years, and give birth to a single calf every other year. Calves are weaned at seven months.
Spinner dolphins may have a life span exceeding 20 years.
The Northwestern Hawaiian Islands spinner dolphin pods have a habitat relatively free from typical human
interference. Threats to dolphins in more populated areas include collisions with vessels, entanglement
in fishing nets and other marine debris, and acoustic disturbances. In the Northwestern Hawaiian Islands
few of these threats exist. Spinner dolphins are protected under the Marine Mammal Protection Act. Exact
population numbers worldwide are unknown.
Special thanks to the Hawaiian Islands National Wildlife Refuge, United States Fish and Wildlife Service, Department of Interior for access to Southeast Island and an opportunity to spend a day with the NOAA Fisheries biologists to learn more about the spinner dolphin research they conduct during their field season.
References
Rauzon,
Mark J. 2001. Isle of Refuge: Wildlife and History of the Northwestern Hawaiian
Islands. University of Hawai`i Press.
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