Dive Dry with Dr. Bill

#301: Green Turtles... in Hot Water

The guy who didn't really discover the Americas, Christopher Columbus, made four voyages to the New World... which wasn't "new" to the Arawaks and other native Caribbean tribes already living there. One thing Columbus did do was to document the incredible numbers of turtles seen on these expeditions. It is written that at times his fleet had to stop and wait for hours to allow the migrating turtles to pass by. A crew member commented that the turtles were so thick he thought he could walk across their backs all the way to shore, about a mile away. Columbus' accounts give us an early "baseline" measure of how productive the Caribbean was at the point of western contact.

The native peoples referred to the region as Turtle Island. Researchers estimate that 99% of the sea turtle population witnessed by Columbus has disappeared since his arrival. How? Because many were captured, killed and eaten as turtle soup... which became a delicacy in some European countries. As an ecological side note, on Columbus' second voyage in 1493 he brought the first eight pigs to the New World from the Canary Islands. Prior to that they had only been found in Europe, so Columbus' is indirectly responsible for an ecological disaster that has even affected our own island's ecology. I could also comment on his crew's disastrous impacts on the native peoples they encountered, but that's one for the history books.

My personal interest in turtles began during a dive on the Great Barrier Reef in 2001. I hovered a mere foot or two from the head of one as it munched on green algae. I have dived with green, loggerhead, hawksbill and other turtles in Australia, Florida, Belize and other locations. However, I never really expected to see one here in our waters. Then, last summer on a dive off the King Neptune at an undisclosed location, I was returning to the boat and saw a turtle swim off in the distance. It was too far away for my video camera to capture any detail, but Andy Crawford and Adam Wucherpfennig, two of the divers on board, managed to get several good still images of it.

Green sea turtles are usually found in tropical and subtropical waters throughout the world's oceans. Although they prefer these warmer waters, they have been sighted from Alaska to Chile on the west coast of North and south America. However, they are rarely seen north of San Diego. There are distinct populations in different regions of the world, including the Atlantic and Pacific Oceans.

Adult green turtles may reach five feet in length. They average about 440 pounds with the largest recorded weighing 871 pounds. Their shells are lighter and more streamlined than those of land and freshwater turtles. The upper portion of their shell is known as the carapace and has a black, gray, green, brown or yellow color. A "line" of five central plate-like structures known as scutes are flanked on either side by four lateral scutes. The underside of the shell, called the plastron, is yellowish white. Although they are the largest of the hard-shelled sea turtles, they have a comparatively small head. Unlike turtles such as the hawksbill, the snout is short and has no hook. Mature individuals have just a single claw on their feet. Males have a long, thick tail whereas females have a short, stumpy one.

The juveniles eat small invertebrates. However mature green turtles have a strictly vegetarian diet and are referred to as obligate herbivores. This is unique among sea turtles. Other species have a wider range in their diets. Greens feed on sea grasses and seaweeds, especially green algae. Turtles digest this plant material in a manner similar to terrestrial cows, by incorporating bacteria in their digestive tracts. They spend most of their time in shallow coastal waters where this food is abundant. The vegetarian diet is responsible for the green fat deposits under the carapace which give this species its color... and its name. There are often well defined feeding areas which the turtles frequent. Only humans and large cartilaginous fish like tiger sharks eat the older individuals.

Nesting areas are very specific. In the Pacific, reproduction occurs in Mexico, the Hawaiian Islands, the south Pacific, SE Asia and northern Australia. Mature adults may swim about 1,500 miles to reach their breeding areas. In Hawaii these turtles become sexually mature at 10 to 50 years, with an average age of 25. Females may mate only every 2-4 years while males mate every year. Copulation occurs in the water, and the female controls who she mates with. Afterwards, the female hauls out on the beach above the high tide zone, then digs a hole with her flippers in which to deposit the 100-200 eggs depending on her age. The males remain in the water.

The eggs hatch in 45 to 75 days depending on temperature. The young hatchlings break through the eggs with a temporary structure known as an egg tooth and begin digging out of the nest hole. If the hatchlings near the surface sense the temperature is too warm, they stop digging out until it is cool and they emerge under the cover of darkness. The small young begin a mad dash to the sea, but many are quickly picked off by crabs, mammals and sea birds.

Swimming speeds have been reported at up to 35 mph. When actively swimming or feeding these turtles may submerge for 4-5 minutes, then surface for 1-3 seconds to breathe. Their lungs are adapted to allow a rapid exchange of gasses and are capable of almost fully evacuated the air inside to ensure optimum intake of fresh air. Green sea turtles may exit the sea and haul out on deserted sandy beaches. It is believed they do this to avoid predation by sharks, and to increase their body temperature in the sun. The turtle tracks observed on remote Catalina beaches may reflect this behavior rather than nesting as some have assumed.

Green sea turtles are listed as endangered by the World Conservation Union, and are protected by many countries. This status was largely triggered by the harvest of the adults for food, including turtle soup, which decimated the once abundant numbers observed by Columbus. In Moslem countries turtle meat is considered "unclean," but the eggs are harvested in large numbers and are considered to be delicacies. The nets on shrimp trawlers used to kill many adult turtles. However, Turtle Exclusion Devices (TED's) are now required on them. Turtles are also captured in other types of fishing tackle. Habitat loss, both for nesting and for feeding, due to coastal development by humans is another factor. Although these turtles are herbivores, apparently some ingest plastic trash like other sea turtles. These plastics reside in the stomach undigested and can release toxic chemicals. They may also block the digestive tract causing the turtle to starve to death.

© 2008 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

The green turtle in the distance as seen in my video camera, and close-up shots
taken by Andy Crawford and Adam Wucherpfennig.

This document maintained by Dr. Bill Bushing.
Material and images © 2008 Star Thrower Educational Multimedia