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#06-160 June 19, 2006

Multi-year research improves knowledge of sea turtle population

The seventh 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. During the first four seasons of the project, trawling was conducted over an expansive area from Winyah Bay to St. Augustine, Fla., and 936 individual loggerhead sea turtles were captured, of which only 13 have since been recaptured.

In addition to the high catch rates of turtles, low-recapture rates also illustrated that further researchLoggerhead turtle 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. The research focus shifted in spring 2004 to a specific location off of Charleston to collect a suitable number of juvenile loggerheads for satellite tagging.

This year, six juvenile loggerhead turtles were outfitted with satellite transmitters, which are designed to track the turtle’s location and diving behavior for up to one year. Since 2004, 24 juvenile loggerhead turtles have been collected through a series of trawls in the Charleston Harbor shipping entrance channel and satellite tagged.

Tracks showing the patterns for all of these animals can be viewed online at: http://www.seaturtle.org/tracking/index.shtml?project_id=27&dyn=1149534840.

With few exceptions, loggerheads tagged off Charleston between mid-spring and late summer have remained resident off of the coast until late fall, though these turtles sometimes moved further offshore than where the researchers would have encountered them during their earlier regional trawl survey. Most surprising, however, is that most of these animals also spend the winter and early spring off the South Carolina and  Georgia coast, in much colder water than previously reported, and predictably return to Charleston during April.

According to DNR biologist Mike Arendt, who manages the data for the project, “It’s pretty wild to watch these turtles spend the winter at different locations, sometimes hundreds of miles apart, only to converge back within a few days of each other and to the same general area where they were caught the previous spring.”

In April 2006, a third dimension to the project was added, a study of the distributional patterns of elusive adult male loggerhead turtles, which comprised only 31 of the 936 loggerheads that were collected during the 2000-2003 regional survey. Because of difficulties gaining access to adult male loggerhead turtles, only a handful have ever been monitored with satellite telemetry, and thus, very little is known about their behavior and distribution patterns. Given the past successes of this research project, the team was granted permission to target adult male loggerheads at the Cape Canaveral, Fla., shipping entrance channel, a known hotspot for adult males during the spring mating season.

Though it is still early in the monitoring cycle of the adult males from Canaveral, already the researchers are intrigued by what they are finding. Of nine adult males which were satellite tagged in mid-April, seven continue to be tracked daily, a major success story considering the vulnerability of the satellite transmitters to physical damage by competitor male turtles during the breeding season. Of the seven adult males currently being tracked, four have recently traveled north from Cape Canaveral, as far as the Outer Banks of North Carolina, while three have continued to be very resident in the vicinity of Cape Canaveral.

Next April, biologists will return to Cape Canaveral to repeat their work in order to increase the total number of animals studied as well as to determine if the same patterns persist in multiple years.

The researchers will also increase efforts to address a fourth area of uncertainty for sea turtle biologists: the origins and distributional patterns of sick and emaciated sea turtles, also referred to as “Barnacle Bills.” A theory regarding these emaciated turtles is that they become cold-stunned during exposure to cold winter waters, presumably from more northern areas, and then passively drift into the southern region until they are so weak that they strand on area beaches. Limited evidence crediting this theory was obtained in summer 2005, when a rehabilitated loggerhead caught by the DNR research team was released with a satellite transmitter following three months of care at the South Carolina Aquarium, and the animal immediately returned to North Carolina waters. This migratory behavior was completely different than patterns observed for any of the other 17 juvenile loggerheads collected from the same area.

Over the past six years these researchers and their collaborators have contributed greatly to the understanding of sea turtle life history in the Southeast. As DNR biologist Julia Byrd explains, “Through this program’s collaborative studies not only were catch rates analyzed, but baseline health parameters, contaminant levels, sex ratios and the genetic composition of the turtles in the waters of this region have also been investigated. The results from these studies will have a large influence on future research and monitoring of sea turtles throughout these regions.”
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