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2006/ Day 26
Hawaiian Green Sea Turtles
Patricia Greene, NOAA Teacher-at-Sea
A large green sea turtle basking in the sand. Hawaiian Islands National Wildlife Refuge, U.S. Fish and Wildlife Service, Department of Interior. Photo: Patricia Greene
Unlike the spinner dolphins, we have observed that Hawaiian green sea turtles
tend to be rather shy and elusive. We managed to catch just glimpses of
them from a distance. Based on past slaughters by man it
is no wonder these creatures would avoid contact with humans. From the late 1800s
until the 1970s these creatures were slaughtered and harvested. Finally,
in 1978 turtles were recognized under the U.S.
Endangered Species Act however, they are still harvested in many parts of the
luck changed on Southeast Island at Pearl and Hermes Atoll in the Hawaiian
Islands National Wildlife Refuge. While circumnavigating the island
we viewed a large adult green sea turtle pulled out and basking
on the beach. After lunch we viewed from a distance two more green sea turtles
basking and swimming in the surf. Researchers told us that they often
observe 10 to 20 turtles pulling out at sunset and sleeping on
the beach at night. It is more common in the Northwestern Hawaiian Islands
than the main Hawaiian Islands for green sea turtles to exhibit this basking
behavior. It is thought the behavior may be an adaptation
to the cooler waters (i.e., a mechanism of thermoregulation, or a predation
strategy due to the
high populations of tiger sharks).
Several adaptations make the green sea turtle well suited for life in the ocean. Special lachrymal glands
in the eye assist in the regulation of salt in the turtle’s body, preventing it from becoming dehydrated.
When sea turtles shed tears they are actually removing salt from their bodies. Sea turtles are capable of
storing large amounts of oxygen in their blood and muscle tissue, and their lungs are adapted for rapid
exchange of oxygen. Green sea turtles can stay under water up to five hours. Modified forelimbs give the
sea turtle an efficient forward power-stroke. Protective coloration in the form of counter shading and
blending gives the sea turtles camouflage. The underneath of the shell is cream color so the turtle blends
with the sky and water to anything looking up. The turtle also and has a dark top shell to camouflage it
from any predator looking down.
A curious Hawaiian green sea turtle approaches underwater at Puako in the main Hawaiian Islands. Photo: Claire Johnson/NOAA
Green Sea turtles reach sexual maturity at approximately 25 to 30 years and reach a weight in excess of
200 lbs. Green sea turtle’s breeding behaviors demonstrate great stamina. Pairs may remain coupled for 10
to 12 hours and both sexes have multiple partners throughout the mating season. Adult males can be
distinguished from females by their longer tails and curved claws on their flippers.
Green sea turtles are oviparous (lay eggs externally) in a sand pit on the beach. Nesting starts in May and
continues through August. Critical components of the nest site must include a lack of predators, a moist
substrate, suitable temperatures, and be located beyond the high tide mark. Typically the green sea turtle
will lay 75 to 150 eggs in a clutch at night, and lay multiple clutches during the breeding season.
Incubation takes 50 to 70 days depending upon ambient temperatures. Sex of the hatchlings is not
determined at the time of fertilization or conception (no sex chromosomes) but dependent upon the
temperature of the sand and individual position of the egg in the nest. This is called “TSD” or
Temperature-Dependent Sex Determination. The pivotal temperature for the green sea turtle is 28.26
degrees Celsius (82.9 degrees Fahrenheit). This is the temperature at which equal number of male and
female hatchlings will be produced. If the temperature falls below this number more males will be
produced; above, and more females will be hatched.
After the hatchling breaks out of the shell it must then reach the surface. Hatchlings demonstrate
“protocooperation” meaning they work together as a group for several days in a joint effort to reach
the surface. The hatchlings take turns digging and resting. Once they are near the surface the heat of
the day will immobilize them and they will not continue their escape until the evening temperatures
have cooled the sand. In this way they avoid heat stress and predators.
Now the hatchlings must find their way to ocean, avoiding the ghost crabs of the night. It is thought they
employ a variety of visual clues; a “wave compass” and perhaps a “magnetic compass” in their effort to
reach the ocean. Scientists believe the wave compass allows the hatchlings to get orientated directly
against the incoming waves. The magnetic compass refers to the magnetite found within the turtles’
brains that may align them with earth’s magnetic fields. Once they arrive at sea, they will dog paddle
to open water; hiding in algae, drift lines, or other floating debris.
During this pelagic stage they are carnivores and feed on plankton. They will remain at sea in this
hatchling/early juvenile stage for years, sleeping with their flippers folded over their back to diminish
their chance of becoming a morsel for some predator. This stage is a period of rapid growth; perhaps 8 to
10 cm the first year. The young turtles will re-appear in coastal waters where they will continue to grow,
graze on algae and become life-long herbivores. The green sea turtle has specialized microorganisms in the
hind gut that digest the cellulose in the plant material. It is possible that juveniles establish this
flora by practicing “scatophagy” or the ingestion of adult turtle feces.
Typically sea turtles thrive on sea grasses, seaweeds, and algae. Depending upon where they live their
diets may vary. For example green turtles of the Pacific Ocean are more dependent upon algae and seaweeds
than the sea turtles of the Atlantic Ocean that thrive on the sea grasses such as turtle grass. The diet
of the green sea turtles at Kure Atoll consist of an algae called Codium edule. In the main Hawaiian
Islands invasive alien species of algae in the marine ecosystems have displaced the native species of
algae that the turtles have traditionally fed upon. This has caused widespread damage to the coral reef
Another concern for the green sea turtle population has been the appearance of fibropapilloma tumors.
Tumors on the eyes, throat, lungs, kidneys, liver, and intestines have been documented. Scientists
believe a Herpes-type virus may be responsible. The disease is quite common in the main Hawaiian
Islands but relatively rare in the Northwestern Hawaiian Islands. These tumors may blind the turtle
or choke them depending on the location of the tumor. Fortunately, we did not observe any tumors
while we have been in the Northwestern Hawaiian Islands.
Three endangered Hawaiian green sea turtles bask on Southeast Island in the
Hawaiian Islands National Wildlife Refuge. Photo: Paulo Maurin
The Northwestern Hawaiian Islands are extremely important to the green sea turtles. It is one of the last
places where turtles are not affected by man’s desire for beach front property; no issues of coastal
development, domesticated predators, recreational activities, artificial lights, high speed boat traffic,
or general coastal degradation of the habitat. Over 90% of Hawaii’s green sea turtles return to nest at
the French Frigate Shoals. Turtles come from the far north end of the Northwestern Hawaiian Islands chain
(Kure, Pearl and Hermes, Midway Atolls) and from outreaches of the main Hawaiian Islands to lay their
eggs at French Frigate Shoals.
Special thanks to the Hawaiian Islands National Wildlife Refuge, United State 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
Gulko, David and Karen Eckert. 2004. Sea Turtles: An Ecological Guide. Mutual Publishing.
Gulko, David. 1998. Hawaiian Coral Reef Ecology. Mutual Publishing.
Rauzon, Mark K. 2001. Isle of Refuge: Wildlife and History of the Northwestern
Hawaiian Islands. University of Hawai`i Press.