Hawaiian Green Sea Turtles

Scientific name: Chelonia mydas

Hawaiian name: honu

Sea turtles are air-breathing reptiles that are well adapted for their life in the sea. Their body is streamlined to move easily through the water with little resistance. Their front flippers are used for swimming, pulling them through the water with powerful strokes, the back flippers act like rudders for steering. Like other reptiles, the sea turtle's skin is protected by tough scales, and like other turtles, fused bony plates form a protective shell (called the carapace on top, plastron on the bottom). There are seven species of sea turtles found in the world's warm oceans, and three of these have been reported from Hawaiian waters: the hawksbill, leatherback, and most common, the green sea turtle.

Green turtles are a long-lived, charismatic species that occur throughout the Hawaiian Archipelago. Six species of sea turtles occur in the Pacific, but only two sea turtle species, the green (honu) and Hawksbill (honu'ea), occur regularly in Hawaiian coastal waters. Sea turtles are part of the identity of the islands and hold a special place in the minds and hearts of the people of Hawaii. Nearly depleted during the 1960's, the green turtle population is on the road to recovery towards fulfilling their ecological role as part of the Hawaiian reef community. They are a true Hawaiian local, a genetically distinct stock that nests only in Hawaii.

Green sea turtles, once widespread throughout the Hawaiian Islands, are now recognized as a threatened species. Habitat destruction, human disturbance, and overharvesting of these slow-moving animals have led to a dramatic decline in their numbers. There is no current estimate of the overall Hawai`i population, instead the population is monitored by the number of nesting sea turtles at a particular site. Since protection, the number of nesting females has increased at approximately 5.7% per year, at French Frigate Shoals where 90% of the nesting in Hawaiian Islands occurs. French Frigate Shoals is in the remote Northwestern Hawaiian Islands and part of Papahānaumokuākea Marine National Monument. Many adult turtles live and feed around the main Hawaiian Islands, but make the migration of several hundred miles to the breeding grounds every two to four years.

Green turtles live in nearshore coastal habitats throughout Hawaii, with high fidelity (devotion) to specific reef, rock, bay, or lagoon feeding locations. Most of their time is spent at depths less than 100 feet but they can dive to depths of over 500 feet when migrating. During the breeding season males and females swim 500-800 miles from their feeding grounds in the main Hawaiian Islands to their nesting beaches, primarily at FrenchFrigate Shoals, in the Northwestern Hawaiian Islands. This journey takes approximately 30 days. The average swimming speed of a green turtle is about one mile per hour during migration; however, they have been known to reach speeds of 20 miles per hour in short bursts when fleeing a potential predator.

One reason why they may bask on the shore is to conserve energy and regulate their body temperature. Green turtles are reptiles, so they do not maintain a constant core body temperature; instead their body temperature is influenced by the temperature of their environment. Green turtles are known to bask only in the Galapagos Islands, Australia and Hawaii. Basking turtles provide excellent wildlife viewing opportunities. However, please give basking turtles space. Never try to push a sea turtle back into the ocean or pour water on it. Allow them a clear path and never block their access to land or sea.

It is impossible to tell juvenile female and male sea turtles apart by external observation. Once they are adults, males are distinguished from females by their significantly longer and thicker tails. Mature males also have a pronounced claw on each of their four flippers used to hold onto females during mating. The incubation temperature of a nest determines whether a sea turtle will be a male or female. Generally, higher nest temperatures produce more females and cooler temperatures yield more males.

Nesting

Green turtles are a long-lived, charismatic species that occur throughout the Hawaiian Archipelago. The following nesting information answers some of the most commonly asked questions about green turtles in Hawaii. Green turtles worldwide lay their eggs on tropical and subtropical beaches. Like all species of sea turtles, green turtles return to the nesting area where they were hatched to reproduce when they reach adulthood (maturity). In Hawaii, over 90% of green turtle nesting takes place at French Frigate Shoals in the Northwestern Hawaiian Islands. Limited nesting also occurs on other islands and atolls of the Northwestern Hawaiian Islands and in low, but increasing, numbers at some locations in the main Hawaiian Islands. One clutch (nest of eggs) laid by a Hawaiian green turtle may contain 60 to 120 eggs, with an average of 100 eggs. Incubation takes approximately 60 days and hatchlings emerge from their nest at night. On average, a female turtle will lay three to five nests in a season and each nest is laid approximately two weeks apart. She will migrate to nest every two to five years. We don't we see small, baby turtles in Hawaii, because once hatchlings (baby turtles) enter the ocean, they embark upon an oceanic offshore journey called the 'lost years' that can last up to a decade. No one knows exactly where they go or how they survive during this time. Eventually they return to Hawaii's nearshore reef habitats as small juveniles, about 14 inches (35 cm) in shell length. From that point forward they live, feed, grow, mature and migrate within the Hawaiian Archipelago for the remainder of their life. After mating at sea, female green turtles crawl ashore at night, dig a hole in the sandy beach, and lay about 100 leathery eggs (each about 2 inches/5 cm across). Each female may lay up to six clutches of eggs each breeding season. The young turtles hatch as a group after about two months, climb from their nest and rush to the sea. Young turtles are rarely seen during their first years of life, presumably they drift with the currents out on the open ocean, feeding on small animals that live close to the surface. When the turtles reach approximately 1½ to 2 feet (to 61 cm) in length (about 15 pounds/6.75 kg), their diet changes and they move into shallower water where they can graze on seaweeds. The life span of the green sea turtle is still unknown, but it appears that Hawaiian turtles grow very slowly and may be 20 to 35 years of age before they are sexually mature. Once mature, males can be distinguished by their longer, thicker tails and females by their higher, more domed shells. Adult turtles can reach shell lengths of 4 feet (1.2 m) and weight up to 400 pounds (180 kg). During the day, green turtles swim and feed on the reef, surfacing to breathe at roughly 15 minute intervals. Green sea turtles in most parts of the world only leave the ocean to lay their eggs on sandy beaches. In Hawaii, both females and males may crawl ashore to bask in the sun. This behavior may help them warm up their bodies and may protect them from their main predator, the tiger shark. At night, turtles are often found underwater, resting on the bottom or under ledges. With slower night-time metabolism, they only have to surface once every few hours. This species was in a steep decline as of the 1970s because of direct harvest of both turtles and eggs by humans. The population has grown steadily over the last thirty years after protection began in 1978. Greens are the most common species of sea turtle found in Hawaiian waters. Despite an overall declining trend globally, green turtle population growth rates are variable among nesting populations and regions. The Hawaiian green turtle population is actually increasing in abundance and has increased 53% over the last 25 years. Hawaiian green sea turtles are unique in spending their entire lives within the Hawaiian Island chain. All other populations of green turtles engage in international migrations that make it more difficult to protect them. The Hawaiian and other populations within U.S. waters are protected by the federal Endangered Species Act that forbids the harassing, harming, or killing turtles.