Terrapin population study underway in Coastal Georgia creeks to assess decline

On a recent June afternoon, three researchers explored the brackish, muddy waters of the Ogeechee River for a small Coastal Georgia treasure: the Eastern diamondback terrapin.

With nothing but time and the tide on their side, the three scientists traversed five different creeks over roughly four hours. However, they only found a handful of turtles. On another day, they found none.

That’s why this year’s population study is so important, said Georgia Department of Natural Resources Sea Turtle Coordinator, Mark Dodd. 

Heightened vulnerability

Terrapins, an indicator species at risk due to human encroachment, may be declining in areas where they have previously been abundant. By getting a statewide estimate, Dodd and his team may better understand which of Georgia’s creeks have plenty of terrapins and which have few to none. 

“We’re trying to hit essentially every (coastal) creek in the state, if possible, over the three month period during the summer,” said Georgia Department of Natural Resources Sea Turtle Coordinator Mark Dodd.

Terrapins exist near the top of the estuarine food chain, meaning that these marsh-dwelling creatures have a direct impact on managing the populations of snails, bivalves, small crabs and certain vegetation.

But they have long been exploited by humans. In the 19th century, they were harvested for turtle soup. Today, their biggest threats are roadway mortality and drowning after getting stuck in commercial and recreational crab pots.

Timing is critical for population monitoring as terrapin turtles are easier to spot during the months of April to July, when female terrapins leave the marsh to lay their eggs. Likewise, tide cycles impact whether terrapin turtles inhabit the high marsh, where they are difficult to spot, or if they are likely to be found in the water.

Highway shoulders near the marshes offer a higher ground for female terrapins to nest, making causeways a significant source of mortality for turtles.

“It’s roadways, particularly that have high speed limits, where people just don’t have the time to slow down to avoid a terrapin that might come on the road in front of them,” said Dodd.

The Georgia DNR intends to use the overall findings from the head-count population surveys to inform statewide management of diamondback terrapin turtles and propose techniques for preventing the species from becoming jeopardized by human interference. 

“What we need is a survey that allows us to look at how many significant populations we have statewide, how many places they’ve been extirpated or the populations have declined. So this is the next step in us trying to understand the population,” said Dodd.

Ongoing population research

Before beginning a head count, Dodd and his crew recorded variables that may determine the quantity of terrapins in the marshes. This includes the presence of crab pots, the average wind speed, salinity and cloud coverage. 

Using binoculars, the researchers conducted repeated scans of the creek, looking for the small terrapin heads that might pop up above the surface. The team waits one minute between scans. Dodd’s technicians took turns writing down the number of terrapins in each creek. 

The repeated head-count technique, developed by Danielle Bradke from the University of Georgia, allows researchers to get a relative abundance of the population size of terrapins in creeks across a large number of sites.

But there is more to the DNR terrapin project than just looking for turtles all summer. Data on population changes are used to diagnose the overall conditions of coastal marshes in Georgia. By monitoring a protected or non-game species, like the terrapin, researchers can gauge the impacts of environmental stressors and degradation.

“If you remove them from the system, the system essentially collapses,” said Dodd, “or at least abundance and diversity drops dramatically.”

Head counts may also save researchers time and resources than more intensive, capture-based studies.

“There’s a lot of different ways to sample populations to determine abundance,” said Dodd “but the problem with most of them is that they’re really labor intensive and so it takes a lot of people and time and energy just to get an abundance estimate for one creek. We’ve intensely studied a small number of creeks but it’s not necessarily representative of the statewide population abundance.”

The DNR researchers caution Coastal Georgians to pay attention to the seasonal flashing “Terrapin Crossing” signs near nesting areas, like the ones found on the Jekyll Island causeway and U.S. 80 toward Tybee Island.

Another cost-effective way to help terrapins is for crabbers to install a small excluder device on the entrance of crab pots, said researchers.  

For now, though, DNR is hoping that more data will help foster policy discussions to support terrapin populations.

“For once we can identify a problem,” said Dodd. “We want to try to do some active management before the population gets so low that we have to do this dramatic management that costs a lot of money. We want to be proactive with species.”