Encouraging Results for Turtle-Saving Crab Trap Devices
Crab traps don’t just trap crabs.
The stinking smell of chicken necks and fish scraps attracts all manner of marine animals, including an unusual marsh turtle called the diamondback terrapin. These beautifully patterned reptiles breath air, which means they often drown when they become trapped in crab pots.
Biologists and crabbers along the East Coast have sought solutions to the terrapin problem for years. One of the most promising options developed has been the bycatch reduction device, or BRD (pronounced “bird”), a small plastic cutout that’s intended to block unwelcome visitors such as terrapins from entering crab trap entrances. Researchers between Texas and New Jersey have tested nearly two dozen BRD designs in recent decades, with mixed results -- primarily because many designs intended to exclude terrapins have been found to exclude large blue crabs as well.
“The perception of conflicting results has precluded most states from feeling confident about requiring these devices on crab traps,” said Dr. Michael Arendt, a biologist with the South Carolina Department of Natural Resources (SCDNR).
To determine how useful a tool BRDs might be for terrapin conservation in South Carolina, Arendt led a team that modeled results for all existing BRDs and field-tested a new one of their own design. Their work was recently published in the North American Journal of Fisheries Management – and the findings are encouraging.
The results obtained by the SCDNR researchers and across published studies from other states show that all BRDs essentially work as predicted for blue crabs and appear to exclude even more terrapins than predicted. The new, SCDNR-designed BRD excluded the highest percentage of terrapins.
“What sets our study apart is that we developed a simple scoring system to evaluate the potential for any BRD size to exclude terrapins or allow crabs to enter traps based on detailed body measurements of actual terrapins and blue crabs,” Arendt said.
The team took detailed measurements of 736 legal-sized blue crabs and 985 diamondback terrapins caught in the Ashley and Stono Rivers between 2013 and 2017. They crunched the numbers on average size and shape of the animals, which allowed them to predict how >8,100 potential BRD sizes would each perform in terms of allowing legal-sized crabs to enter while excluding terrapins.
By recording footage of terrapins and crabs as they entered crab trap holes, the team also noted something important: terrapins tend to enter traps head-first, whereas crabs typically scuttle in sideways. This distinction made a difference in their own design – unlike previously tested BRDs, the SCDNR design includes a rounded top and bottom to accommodate the body shape of blue crabs as they enter traps.
Since 2016, the SCDNR researchers have also distributed the new BRD design to nearly 100 recreational and more than a dozen commercial crabbers for real-world evaluation across the South Carolina coastal plain.
“Results from our crabber-generated data sets have been mixed, which suggests the need to continue to collect animal measurements across the state to make sure that Charleston crabs are proportionally the same size as, say Beaufort or Georgetown crabs,” says Arendt. Individual crab behavior may play a role as well; in recordings, the team saw instances of crabs that could have fit through the BRDs avoid them entirely, as well as very large crabs that unexpectedly find a way to squeeze through the BRDs.
Although more work is still needed, the recently published SCDNR study shows the potential of properly sized BRDs to exclude terrapin bycatch without compromising crab catch. These findings don’t just apply to South Carolina, Arendt says – diamondback terrapins are listed as species of high conservation concern in every coastal state from Texas to New Jersey.
We Need Your Help!
As SCDNR’s biologists continue to refine and test BRD designs, there are several ways you can get involved to help protect diamondback terrapins in South Carolina.
Do you live on or near the water and regularly go crabbing? We’re still looking for recreational and commercial crabbers to help test these BRDs and report back on their observations. Contact Michael Arendt at firstname.lastname@example.org or 843-953-9097 for more information.
We urge anyone who encounters a diamondback terrapin to report their sighting online here.
Spring marks the only time of year diamondback terrapins come ashore, as females leave the salt marsh to find safe, dry places to lay their eggs. Unfortunately, this often brings terrapins onto roads and into other dangerous locations. We welcome all sightings of terrapins, whether dead or alive. Injured terrapins may be referred to your closest wildlife rehabilitator.
For the full study: Michael Arendt, Jeffrey Schwenter, Julie Dingle, Christopher Evans, Ellen Waldrop, Brooke Czwartacki, Amy Fowler, and David Whitaker (2018). A “BRD” in the Hand Worthy of Four in the Trap: Validation of Optimal Bycatch Reduction Device (BRD) Size to Maximize Blue Crab Callinectes sapidus Entry and Diamondback Terrapin Malaclemys terrapin Exclusion Through Theoretical Modeling and Application. North American Journal of Fisheries Management. DOI: 10.1002/nafm.10045.