New Crab Trap Reduces Turtle Mortality By Almost 100 Percent, Study Finds

Ohio University | November 1997

ATHENS, Ohio — A new crab trap designed by Ohio University researchers has been shown to reduce the mortality rate of turtles accidentally caught in recreational traps by almost 100 percent while increasing the number of crabs caught.

Research suggests crab traps used throughout the southeast coastal region of the United States lead to the drowning of thousands of diamondback terrapins, a turtle that lives in the brackish waters found in coastal areas from Cape Cod, Mass., to Corpus Christi, Texas. Standard traps, designed to catch blue crabs, are only 2 feet high and are completely submerged. Turtles caught in the traps are unable to reach air, and can drown in as little as 45 minutes.

But the new traps are 6 feet tall, with 2 feet of the trap extending above the water surface, according to Willem Roosenburg, assistant professor of biological sciences at Ohio University and lead researcher on the project, which was funded in part by the Maryland Department of Natural Resources.

“Because blue crabs obtain oxygen directly from the water through gills, there is no need for standard crab pots (traps) to have access to air,” Roosenburg said. “But because terrapins are air-breathing reptiles, turtles caught in standard crab pots are vulnerable to drowning.”

In studies published in the October issue of Conservation Biology, the researchers examined terrapin populations in a small area of Chesapeake Bay in Maryland. They estimated that more than 2,000 turtles are caught each year in recreational crab pots, most drowning before they can be released. Even a conservative estimate of 25 percent mortality in crab pots suggests that the terrapin population cannot sustain the effect of the traps. And, since researchers suspect the mortality in crab pots could be as high as 100 percent, they say it’s possible the entire local terrapin population could be wiped out in three to four years.

The diamondback terrapin, named for its shell’s brightly colored, diamond-shaped markings, is the only North American turtle that thrives in the waters that separate fresh water from salt water. They make their home in the same areas occupied by blue crabs, and eat the same bait used in crab traps.

During the juvenile stage, male and female terrapins are equally susceptible to crab traps. But as adults, males — which are one-third the size of female adults — remain vulnerable to crab traps.

Roosenburg and other ecologists have documented declines in terrapin populations in recent years, and many researchers have suggested commercial and recreational crab pots are to blame. Reports of terrapin mortality from crab pots have come from North Carolina, New Jersey, South Carolina, Louisiana, Florida and Maryland, where Roosenburg has conducted his research.

Maryland law allows residents to set two crab traps in the water adjoining their property. A turtle caught in a crab trap when the water temperature is less than 50 degrees can survive for up to 24 hours with the aid of a slower metabolism brought on by the cooler water. But in the summer, when the water ranges between 75 and 80 degrees, turtles caught in the underwater traps can die in 45 minutes or less.

“We found 49 dead terrapins in one crab pot alone,” Roosenburg said. “I’ve been in places in Virginia where there’s a crab pot every 7 or 8 feet. You can imagine the cumulative effect of these crab pots in that area.”

In studies of their modified trap design, researchers caught about 1,000 turtles in the last two years, losing only one turtle. The turtles do well in these traps, Roosenburg said, because special holes in the traps allow turtles to surface for air during low and high tide.

And while the trap significantly reduced the mortality rate of turtles caught in the traps, it actually increased the number of crabs caught. During 11 days of analysis, researchers caught 284 crabs in the standard crab pots and 300 crabs using their new design.

The modified trap probably would cost about $20 more to produce than standard crab pots, although the cost could be less under mass production, Roosenburg said. The current cost of a recreational crab trap is about $16.

“We feel that the minimal increase in cost of the tall crab pots is trivial compared to the benefits of these traps for conserving terrapins,” Roosenburg said. “Simple modifications that entail a minimum effort at slightly greater than regular cost, such as our solution, can be an effective management and conservation tool.”

The researchers currently are working on methods to reduce terrapin mortality in the large-scale commercial use of crab traps, and hope to introduce similar turtle-friendly crabbing options soon.