The Hawaiian Green Turtle (Chelonia mydas)
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Aikane, the quintessential Hawaiian green turtle, settling into her favourite resting spot at the Turtle House, Honokowai. 57K JPEG |
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Aikane, the quintessential Hawaiian green turtle, settling into her favourite resting spot at the Turtle House, Honokowai. 57K JPEG |
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The following is based on information from the Interim Recovery Plan for Hawaiian Sea Turtles, National Marine Fisheries Service, 1992. Some biological information has been extracted from Synopsis of Biological Data on the Green Turtle in the Hawaiian Islands, G. H. Balazs, U.S. Department of Commerce, National Oceaonographic and Atmosphere Administration, National Marine Fisheries Service, NOAA-TM-NMFS-SWFC-7, 1980.
The green turtle is listed as endangered by the International Union for the Conservation of Nature. In 1978, the Hawaiian population of the green turtle was listed as threatened under the U.S. Endangered Species Act of 1973.
Green turtles were a source of food, tools, and ornamentation for early Hawaiians. With the arrival of western culture, however, the level of exploitation of this resource increased dramatically. Large numbers of green turtles were harvested throughout the Hawaiian Islands through the nineteenth and early twentieth centuries. In 1974, the State of Hawaii finally passed a regulation providing some protection, but this was virtually ignored until 1978, when the Hawaiian green turtle was placed on the list of threatened species.
In other parts of the world, green turtles face a serious threat from the destruction and loss of nesting sites. Fortunately, over 90% of nesting activity for the Hawaiian green turtle population occurs at the French Frigate Shoals, inside a National Wildlife Refuge System administered by the U.S. Fish and Wildlife Service. This, combined with its threatened status under the U.S. Endangered Species Act, has created an environment in which the Hawaiian green turtle should prosper. Unfortunately, the Hawaian green still faces severe threats, most notably fibropapilloma tumors and degradation of foraging habitat. Current Hawaiian green turtle population levels are still thought to be below pre-western contact, and probably pre-World War II levels as well. In 1992, the estimate of mature female green turtles associated with the French Frigate Shoals was set at roughly 750.
The green sea turtle is the largest hard-shelled sea turtle. Adults of this species commonly reach 100 cm in carapace length and 150 kg in mass. The average size of a female nesting at the French Frigate Shoals was reported in 1980 to be 92 cm straight carapace length, with an average body mass of 115 kg. This is somewhat smaller than her Atlantic cousin.
Hatchling green turtles weigh about 25 g (about a handful of Smarties) and have a carapace about 50 mm long. Hatchlings are black on top and white underneath. The plastron of the Hawaiian green turtle becomes orange or yellowish orange, and the carapace remains predominantly black with various shades of olive and yellowish gold forming swirls and irregular patterns on their shells. A small number of adult females (perhaps 3%) possess predominantly brown carapaces, with patterns of yellow, gold, and reddish brown.
Growth rates of pelagic-stage Hawaiian green turtles (during their Lost Years) have not been measured under natural conditions, since contact with them is extremely rare. Growth rates of juveniles, sub-adults, and adult turtles measured by George Balazs at seven resident sites in the Hawaiian Archipelago revealed substantial variation. Along the Kau coast of the island of Hawaii, annual growth was approximately 4.50 cm to 6.25 cm. At the French Frigate Shoals, annual growth ranged from approximately .25 cm to 1.50 cm. These differences are probably a function of food availability and quality.
Based on growth rate measurements, George Balazs estimates the age of the Hawaiian green turtle at sexual maturity can range from 11 to 59 years. (Yes, 59 years!)
Hawaiian green turtles occupy three habitat types:
Females deposit egg clutches on beaches in the French Frigate Shoals, digging a deep nest cavity above the high water line. Eggs incubate for approximately 65 days before hatching. Hatchlings leave the beach and apparently move into convergence zones in the open ocean where they spend an undetermined length of time. Hawaiian green turtles reach a carapace length of approximately 35 cm, about 10 cm larger than juveniles of other green turtle populations, before leaving the pelagic habitat and entering benthic feeding grounds.
In Hawaii, there is a wide variety of feeding habitat. For example, Balazs reports observations of feeding behaviour along the Kau coastline of the island of Hawaii. This is a lava coast with few protective reefs, and most foraging occurs under turbulent conditions. He also reports observations of foraging at Bellows Beach, Oahu, which has a sandy bottom extending 25 to 100 m from shore and subtidal reefs protecting the coastline from heavy surf.
One interesting behaviour of the Hawaiian green turtle is its fondness for crawling ashore at isolated sites in order to bask. Basking is rare among marine turtles, and has been observed in only a few populations in the Pacific. Hawaiian green turtles bask, but this behaviour seems to be limited to the Northwestern Hawaiian Islands. It is thought that they do this for thermoregulation (they like to warm up in the sun), resting (they like to sleep in the sun), and perhaps for protection from tiger sharks (they don't like to be eaten). Sounds just like people to us.
Scientists assume that post-hatchling, pelagic-stage green turtles are omnivorous, but there are no data on diet from this age class. Our personal experience with juvenile Hawaiian greens that appear to have just arrived inshore supports this theory. Our first turtle encounter resulted when a juvenile swam up to us and attempted to take some of the cuttlefish we were using to attract eels for photographs. We have since observed juveniles eating sponges and seaweed, as well as jellyfish.
Scientists do know that once green turtles shift to benthic feeding grounds, they are almost exclusively herbivores. They feed on both seagrasses and algae. In Hawaii, there is only one form of seagrass, Halophila hawaiiana, but this angiosperm is not common in Hawaii and is not a major food source. George Balazs identified nine species of algae (out of approximately 400 in the Islands) as the principle foods of the Hawaiian green turtle.
Balazs also notes that occasionally, juvenile and subadult turtles feed on certain drifting invertebrates: Physalia, Velella, and Janthina. In 1995, we observed turtles of all sizes enthusiastically feeding upon a large mass of an unidentified drifting invertebrates. While Hawaiian greens are primarily vegetarians, it is clear that they aren't fanatics about it.
Female green turtles emerge at night to deposit eggs, the process taking an average of two hours. Up to seven clutches are deposited at 12 to 14 day intervals, but the average is probably two or three clutches. Accurate counts of the number of clutches per season are difficult to get. The average clutch size is 100-110 eggs.
It is uncommon for females to produce clutches in successive years. Usually 2, 3, 4 or more years intervene between breeding seasons. Mating occurs in the water off the nesting beaches. Little is known about the reproductive biology of males, but evidence is accumulating that males migrate to the nesting beach every year.
One interesting discovery in recent years is that incubation temperatures determine the sex of hatchling turtles. In 1985, Standora and Spotila reported this effect on green turtles. Eggs incubated below a pivotal temperature--which might vary among populations--produce primarily males, and eggs incubated above this temperature produce primarily females.
The hatching success of undisturbed nests is typically high. The Hawaiian green turtle enjoys the benefits of a protected and isolated nesting habitat and low levels of predation. Unlike many nesting areas throughout the world, there are no nest-raiding predators (not even humans) in the French Frigate Shoals. Ghost crabs prey upon hatchlings, but estimates of losses to crabs do not exceed 5%. Unlike other nesting beaches throughout the world, hatchlings are not greeted by predatory birds, and the loss to carnivorous fishes does not appear to be significant. Hatchlings in the French Frigate Shoals do not suffer from human interference either; there are no distracting lights from developments and no destructive beach activities.
The navigation feats of the green turtle are well known, but poorly understood. We know that hatchlings and adult females on the nesting beach orient toward the ocean using light cues. For a long time, no one knew what cues were employed in pelagic movements, in movements among foraging grounds, or in migrations between foraging grounds and nesting beaches. Recently published work, however, has suggested that the earth's magnetic field plays a role in these feats.
Hawaiian green turtles nest in the isolated French Frigate Shoals, but forage throughout the entire Hawaiian Archipelago, a range of 2450 km. Tagging has indicated that most Hawaiian green turtles seem to settle at a specific foraging ground and leave only to reproduce. Our observations at Honokowai strongly support this theory, since we have seen the same core group of animals every year since 1989.
It has been generally accepted, but not proven, that green turtles return to nest on the beach where they were born. Green turtles do exhibit strong site-fidelity in successive nesting seasons.
The Hawaiian green turtle is fortunate that it shares only some of the same threats that menace all marine turtles, as described in Threats To Marine Turtles. The Hawaiian green does not face most of the common threats to turtle nests and nesting beaches. Some of the common threats are still present: possible ingestion of marine debris and now, bad law. The most serious threats to the Hawaiian green turtle, however, are the fibropapilloma tumor and the rapid deterioration of its foraging sites.
Hawaiian green sea turtles are listed as threatened.
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More about Hawaiian green turtles (from the US NMFS) | ||||
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About Marine Turtles | ||||
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