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October 9, 2007

Multi-year research improves knowledge of sea turtle population

The eighth sampling season for a federally funded in-water sea turtle project designed to gain insight into the population trends, migration patterns and general health of sea turtles recently concluded off the coast of Charleston.

Since May 2000, biologists with the S.C. Department of Natural Resources (DNR) have been using modified shrimp nets to capture sea turtles and collect a suite of biological data on each turtle before tagging and releasing them a short while later. Between 2000 and 2003, trawling was conducted over an expansive area from Winyah Bay to St. Augustine, FL. 936 individual loggerhead sea turtles were captured, of which only 17 have been recaptured.

High catch and low-recapture rates for loggerhead sea turtles illustrated that further research would be needed to determine whether juvenile loggerheads encountered in coastal waters during the summer were resident or transient, and whether these animals returned to the same areas each summer. As such, the research focus shifted in the spring 2004 to a specific location offshore of the Charleston Harbor jetties to collect a suitable number of juvenile loggerheads for satellite tagging.

In 2007, twelve juvenile loggerhead turtles were tagged with satellite transmitters, bringing the total number of juvenile loggerheads satellite-tagged in this study to 36 since 2004. The transmitters enable researchers to monitor the turtles’ daily location and diving behavior for potentially up to 2.5 years; however, typical track durations are closer to 9 months due to the natural shedding of the outer layers on the turtle’s shell, which detaches the transmitter and wear and tear that can physically damage the transmitter. The longest track to date is around 16 months, and this turtle (named "David") is still actively transmitting signals.

In the four-year study of the distributional patterns of juvenile loggerheads, researchers made several important discoveries. First, despite the potential for loggerheads collected in near-shore coastalLoggerhead waters off SC in spring and summer to migrate long distances, more than two-thirds of the satellite-tagged turtles remained resident near Charleston through the fall.

Second, nearly all of the turtles which resided off of Charleston through the fall, and were able to be monitored during the winter, remained on the middle and outer continental shelf from southern NC to northern GA, surviving much colder water temperatures than previously thought possible for sea turtles. Changes in diving physiology were noted during exposure to cold winter water temperatures; instead of making numerous short dives (~30 min) each day, loggerheads made fewer and much longer (greater than 4 hours) dives followed by increased basking at the surface during the winter. Researchers are still able to track 7 of 12 juvenile loggerheads released this year, and hope to collect information for all of them this winter.

Monitor the turtles online.

A two-year study of the reproductive physiology and distributional patterns of adult male loggerheads collected near Cape Canaveral, FL also concluded this year. The spring mating aggregation of loggerhead sea turtles near Cape Canaveral is one of the largest in the world for this species, which enabled DNR researchers and collaborators to collect 38 different adult male loggerheads (of which 29 were satellite-tagged) in a short span between April 2006 and 2007. For comparison, only 31 of 936 loggerheads collected during the 2000-2003 regional survey were adult male loggerheads.

Reproductive data for nearly all adult male loggerheads collected indicated active mating with adult female loggerheads in the vicinity. However, satellite-telemetry data revealed that some of these reproductively-active turtles were migratory, while others were long-term residents. The migratory group remained close to shore during April and May before quickly moving away from Canaveral by the end of May, while the resident group shifted to the deep waters offshore of Canaveral at about the same time that the migratory animals also left the area. Migratory animals dispersed along the East Coast and into the Gulf of Mexico to a number of different areas ranging from NJ to the FL Keys and FL Panhandle, encompassing nearly the entire extent of nesting range for loggerhead sea turtles in North America.

Monitor the adult male loggerheads online.

Armed with new knowledge of the distributional patterns of both juvenile and adult loggerheads in the Southeast, the project will resume its regional sampling design during summers 2008 and 2009 to determine what, if any, changes have occurred in loggerhead catch rates and health in the past five years. Given incredibly slow growth rates of loggerheads, especially as they age, it’s important to repeat the regional sampling survey every few years, to provide a long-term monitoring tool for assessing recovery of sea turtle populations.

Data collected by DNR biologists and collaborators during 2000 through 2003 shows great potential for nesting trends to increase in the future, as the predominantly juvenile loggerheads found in our coastal waters mature and are able to participate in mating and/or nesting. However, because loggerhead growth slows considerably with age, it’s difficult to predict when exactly nesting in the region will increase. DNR biologist and principal investigator for the study Mike Arendt says, "We are hopeful that our data collection in 2008 and 2009 will substantiate that it’s still okay, five years later, for us to continue to tell the encouraging message of a brighter future for loggerhead sea turtles in the Southeast."

DNR protects and manages South Carolina’s natural resources by making wise and balanced decisions for the benefit of the state’s natural resources and its people.

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