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Sunday, April 7, 2013

G is for Green Turtle

I recently did a report on Green Turtles (Chelonia mydas), so you are about to receive a boatload of information regarding their life histories and their conservation.  A lot of the conservation issues also apply to the 6 other species of sea turtles out there- Loggerhead (Caretta caretta), Hawksbill (Eretmochelys imbricata), Kemp's Ridley (Lepidochelys kempii), Olive Ridley (Lepidochelys olivacea), Flatback (Natator depressa), and the Leatherback (Dermochelys coriacea).  Feel free to skim if you don't have the time to read.  Information on how you can help Sea Turtle Conservation can be found at the bottom.  Enjoy!


The green turtle (Chelonia mydas) is the largest of the hard-shelled sea turtles in the world.  Named after their fatty tissue of a greenish hue, this turtle is historically known for being a good source of meat.  This has led to its nicknames of soup turtle and edible turtle.  The turtles themselves eat large amounts of plants, the only sea turtle to do so.  Adult females weigh approximately 80-220kg with males slightly smaller.  Green turtles reach maturity in 20-50 years, the longest generation time of any sea turtle.   


Once mature, females reproduce every two to four years with one to seven nests laid in approximately two week intervals.  Females lay an average of 100 eggs per clutch and the process takes about 2-3 hours.  Females nest on warm tropical and subtropical beaches within 20 degrees of the equator.  Adults and juveniles live in tropical and warm temperate marine waters around the globe.  In the United States they can be found along the coasts of Virginia through Texas and San Diego south plus the Hawaiian Islands.   


 Green turtles in the Hawaiian Islands also display the unique behavior of basking.  This behavior warms them, protects them from predators, may speed up digestion and egg development, and dries out the shell which may help remove fungus and algae.  Across the globe green turtles can be found in coastal waters of 140 countries with nesting locations in 80 of them.


The green turtle is internationally recognized as endangered and has the possibility to go extinct in much of the world during this century.   The conservation history of this species is its most notable feature.  Turtles face danger from many areas including fishing, pollution, and harvesting.   The “leading cause of sea turtle mortality during the last 50 years has been their unintentional capture by three commercial fisheries: shrimp trawling, gill netting, and longline fishing.”

Shrimp trawling has long been the foremost cause for sea turtle deaths.  The process involves large nets dragged behind boats for over an hour at a time.  Turtles can only hold their breath for up to 45 minutes so if a turtle is caught in the net it is almost an immediate death sentence.  In 1978 the Turtle Exclusion Device (TED) was invented.  For only $300 USD a trap door is installed in the net which allows the turtle (an to escape the net.  The TEDs reduce turtle mortality by half as well as reduce unwanted fish catch by 50-70%.  Some fishermen believe that the device will reduce shrimp catch and so they deactivate it.  As beneficial as these devices are, their use is still not required or enforced in many countries. 


Gill netting involves the use of floating nets where turtles can become entangled and die.  Massive nets and open sea nets are now illegal.  However, coastal netting and illegal open sea netting is still in practice.  It is possible for this method to not kill turtles.  Nets need to be checked more often or smaller nets need to be used.  These methods are more costly to the fisherman and are unlikely to come into common practice.

The third fishing method that affects green turtles is longline fishing.  This involves the use of miles-long fishing line with hundreds of hooks to catch large, predatory fish.  Turtles are routinely caught instead, resulting in their drowning.  There are efforts to reduce turtle by-catch through the use of circular hooks which catch on the turtle’s beak instead of the stomach, different colored bait so the turtles are not interested, and changing the depth of the line or the way it is set.  Local populations of green turtles are most at threat of extinction from this method of fishing.


Fishing is not the only threat to these turtles.  They also face pollution dangers.  Approximately six percent of turtles washed ashore in the United States show signs of death by oil ingestion.  The oil congeals into tar and gathers in convergence zones.  These zones are also where nutrients concentrate and where hatchlings gain much of their food.  Other pollutants that green turtles face include plastic bags, balloons, Styrofoam pellets, plastic pieces, and rope.  All of these foreign objects may block turtle intestines.  In a case study involving a juvenile green turtle, the subject was picked up off of the coast of Florida exhibiting signs of severe illness.  After treatment, the turtle defecated a total of 74 foreign objects over the course of one month.  The turtle then returned to normal health.  This example shows that not only are turtles eating ocean-borne plastic, but that they are eating it in great abundance.  Eating ocean debris is not the only pollution danger to turtles, however.  They can also get entangled in old trash including plastic bags, old fishing nets, lobster pot lines, fishing lines, six-pack yolks, steel cable, burlap bags, and beach chairs.


The final major threat to green turtles is harvest.  Humans have long been exploiting green turtles as a food resource.  Even today people still eat thousands of turtles every year.  Egg collection still occurs at 45% of all nesting beaches worldwide, and adult female collection is around 27%.  Some locations in the Pacific islands still see between 50 and 100% of egg collection every year.  And in Southeast Asia over 100,000 juvenile turtles are harvested annually.  This overexploitation is in no way sustainable.  Many of these populations will go extinct without further action. 


In the United States, green turtle harvest stopped in the 1970s.  The results from this termination can be seen today.  George Balazs and Milani Chaloupka have been recording nesting female numbers in the French Frigate Shoals of Hawaii since 1973.  Due to their diligence, we can now see a trend in the rising numbers of green turtles present at the nesting beach.  When the surveys began in the seventies, there were approximately 100 female nesters.  Today that number hovers around 400-500 nesters.  The rise in population alongside the introduction of the Endangered Species Act suggests that it is a result of conservation  and is not simply multi-decade population fluctuations.


The results of Balazs’ studies helps bring hope for other sea turtle populations across the globe.  Both small and large populations of green turtles have seen growth in the past 30 years.  This suggests that the Allee effect, where the per capita reproductive output of small populations is low, does not have a major effect on green turtles. If this is true for all sea turtles, then enforced conservation efforts around the globe could lead to relatively quick results and healthier populations.


For more information on how you can help sea turtle conservation, including events, expeditions, and campaigns, please visit Sea Turtle Restoration Project.