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May 20, 2009

Increased in-water sea turtle catch rates suggest management efforts paying off

Increased in-water catch rates of immature loggerheads between 2000 and 2008 suggest that management efforts such as nest protection and the required use of Turtle Excluder Devices (TED’s) in commercial shrimp trawl nets may be paying off.  These findings, as well as other aspects of this research, are detailed in an extensive five-year report submitted to National Oceanic and Atmospheric Administration Fisheries in April.  This encouraging news coincides with the S.C. Department of Natural Resources (DNR) In-Water Turtle Trawl Survey preparations to commence its 10th consecutive year of monitoring beginning the day after Memorial Day.

DNR randomly sampled more than 550 stations annually off the South Carolina, Georgia and northern Florida coasts during the summers of 2000-2003 and 2008 using trawl nets with large mesh webbing without TED's. Tows were less than 30 minutes to minimize the risk of turtles drowning. 

Although sea turtles were not captured in 75 percent of the tows, several noteworthy trends were evident for the 25 percent of tows that did capture sea turtles.

"Increased catch rates in 2008 for the size representing mature animals corresponded with a strong nesting year from North Carolina to Florida," says DNR marine biologist Mike Arendt. "But it's also based on in-water collection of relatively few larger turtles capable of breeding." 

In contrast, increased catch rates for immature loggerheads was based on collection of a large number of sea turtles.  Given the small size of these turtles in the early 1990’s, they would’ve also been among the first loggerhead sea turtles to benefit from use of TED’s off the southeastern coast.

Increased loggerhead catch rates may represent either increased abundance or increased concentration of turtles in the sampling area.  Loggerhead catch rates in 2008 were 1.5 times greater than in 2000 and notably higher than catch rates reported between the late 1950’s and early 1990’s; however, it is unknown to what extent differences in sampling design among these different studies affected catch rates.  However, where differences in sampling design do not preclude comparisons, such as in the Charleston shipping channel, catch rates have increased significantly since 1990.
 
Although a longer time series of standardized in-water surveys is needed to detect true in-water population trends, the results that have been accumulated to date are encouraging for the future recovery of loggerhead sea turtles in the southeastern United States.

DNR sea turtle reports are available online and the in-water report submitted to National Oceanic and Atmospheric Administration (NOAA) Fisheries in April is available online (PDF file).  Find out more about DNR’s Marine Turtle Conservation Program.

DNR has studied sea turtles in their marine and terrestrial habitats since the 1970’s, with annual in-water studies conducted since 2000 in addition to the annual monitoring of sea turtle nests and strandings since 1980.

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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