Diamondback Terrapin Bycatch in Crab Traps

A review of BRD studies and their effectiveness

Please note that this StoryMap was originally created in early 2020. Terrapin bycatch regulations continue to change, and new studies are published each year. Though this StoryMap is occasionally updated, if you have new information that you feel should be added please contact the author.

Diamondback terrapins and blue crabs have been linked for much of their natural history. Found in a similar range along the Eastern and Gulf Coasts of North America, these species have frequently been found together - both in commercial fishing gear and in conservation planning efforts.

The Diamondback Terrapin (Malaclemys terrapin)

Living in brackish coastal waters in such far-flung places as Bermuda, the Florida Keys and Cape Cod, the diamondback terrapin flourishes in climates ranging from sub-tropical to temperate. Terrapins have endured several serious conservation threats over the past centuries. Originally captured and eaten by Native Americans, terrapins were extensively hunted from colonial times into the 1900's for dishes such as "Stewed Terrapin with Cream". Terrapin numbers began to noticeably shrink, and eating terrapins eventually fell out of fashion. However, today the adult terrapin is threatened by boat strikes and by accidental capture in crab traps.

The Blue Crab (Callinectes sapidus)

The blue crab has traditionally been a mainstay feature of East Coast fisheries. This "beautiful, savory, swimmer" (to take its Latin name literally) can be found in coastal waters from Canada to Argentina. An extreme population decrease in blue crabs along the East Coast in the 1990's led to more stringent management practices, including restrictions both on crab size and on the length of the crabbing season. Despite management efforts, blue crab populations unfortunately continue to decline in many estuaries.

An important period in modern diamondback terrapin research began in South Carolina in 1983, when James Bishop authored the first scientific paper documenting the bycatch of terrapins in crab traps.

4.5x12cm and 5x15 cm BRDs, and BRDs installed in a trap

Numerous other studies since then have identified the need for bycatch reduction devices (BRDs) in blue crab traps, which work to keep terrapins out of the wire mesh structures. Crab traps are the latest threat to terrapin conservation, though both terrapins and blue crabs have faced a variety of threats over the years.

Two different BRD designs. Scientists have found that red BRDs attract the most male crabs.

BRDs come in a variety of sizes depending on state regulations. Some BRDs are more effective at keeping out terrapins than others. Likewise, some BRDs have a greater effect on crab catch than others.

Oval BRDs are part of a new chapter in terrapin conservation. Read to the end of the Story Map for additional information!

So, why do we care about terrapins getting into crab traps? Terrapins must surface for air approximately every 45 minutes - 2 hours. Without BRDs, terrapins drown—sometimes in dramatic numbers—in crab traps.

Terrapins instinctively swim to the surface, instead of going back out through the trap entrance.

This trap contained 94 dead terrapins.

In 1989, scientists began studying terrapin bycatch reduction in crab traps, first in New Jersey and then eventually in almost every state where commercial crabbing and terrapins overlap.

The majority of these studies have shown that bycatch reduction devices (BRDs) dramatically reduce terrapin mortality in crab traps.

Additionally, many studies found that BRDs did not negatively impact crab catch, and in some studies actually increased crab catch.

Some studies, however, found that BRDs decreased crab catch.

Some states require BRD use in some situations. However, most BRD regulations are not enforced effectively.

Massachusetts preemptively banned commercial-style crab traps in their nascent blue crab fishery.

New York requires 5.1x15.3cm BRDs for all commercial and recreational traps fished in creeks, coves, rivers, tributaries, and near-shore harbors.

New Jersey requires 5.1x15.3cm BRDs for all commercial and recreational crabbing in bodies of water less than 150 feet wide at average low tide.

Delaware and Maryland require 4.5x12cm BRDs for all recreational traps.

Virginia provides a decreased fee for licensed, recreational 5-pot crabbers using BRDs.

North Carolina requires BRDs in specified conservation areas.

Florida requires 2x6 inch BRDs on all recreational traps.

In 2020, we conducted a review of 24 studies (with both published and unpublished data) on terrapin bycatch in 12 states, in order to examine variations in BRD size, construction and effectiveness. All of these studies compared crab catch and terrapin bycatch for traps fitted with BRDs versus control traps without BRDs.

1

New Jersey

Four BRD studies have been conducted in New Jersey since 1989, focusing on 5x10cm and 4.5x10cm wire BRDs. Wood (1997) published the results of his seminal research on BRD design, testing both 4.5x10 and 5x10 wire BRDs. All New Jersey studies to date have found 80%+ reductions in terrapin bycatch with BRDs. Some studies also found large increases in crab catch (from 10% to 49%).

2

Delaware

A study in Delaware that spanned multiple years (2001-2004) found varied bycatch reduction with wire BRDs. The most successful BRD size for excluding terrapins was 3.8x12cm, while the least successful size was 5x12cm. However, the most successful BRD size also reduced crab catch by 26%, while the least successful BRD showed no change in crab catch.

3

Maryland

Maryland BRD studies have shown mixed results. A 2000 study using wire BRDs reported only 50% terrapin bycatch reduction with a 5x10cm BRD, but an approximately 80% reduction with a 4.5x12cm BRD. A 2018 study showed approximately 80-85% reductions in terrapin bycatch with both 4.5x12cm and 7.3x5cm BRDs. Most studies also found no change in crab catch.

4

Virginia

Overall, studies in Virginia have shown very effective bycatch reduction when BRDs are used, with some studies reporting zero terrapins caught in traps with BRDs. Most BRDs used in Virginia studies have been either 5x15cm or 4.5x12cm, and all have been plastic. Some BRD sizes showed small or no change in crab catch. Studies using 4.5x12cm BRDs often showed large decreases in crab catch, but some of these studies did not use bait.

A 2021 study tested the effectiveness of oval BRDs, and preliminary results show a large reduction in terrapin mortality while maintaining crab catch.

5

North Carolina

Recent studies have shown effective reduction in terrapin bycatch with a variety of wire BRD sizes. However, the overall number of terrapins caught in these studies is low, and scientists have wondered how large the terrapin population in the state really is. Crab catch results from these studies were mixed: some studies showed no change, while others showed decreases of up to 29%.

6

South Carolina

Unfortunately, research in South Carolina suffers from the same problem as research in North Carolina: the relative absence of terrapins in bycatch. One BRD study in 2017 simply caught no terrapins at all. However, a 2009 lab study did show dramatic (up to 100%) reductions in terrapin bycatch with plastic BRDs in two sizes. BRD use did cause decreases in crab catch from 15% to 32%, according to the three studies that measured it. Additionally, Arendt et al. (2018) used mathematical modeling to find the ideal BRD size based on regional measurements of crabs and terrapins, settling on the BRD design shown at the beginning of this presentation.

7

Georgia

Georgia has only had one terrapin bycatch study, conducted in 2007. The results showed a dramatic reduction in terrapin bycatch with a 5x15 wire BRD, with a decrease in crab catch of 14%.

8

Florida

Florida has also only had one terrapin bycatch study, but with encouraging results. The comprehensive 2007 study using 4.5x12 wire BRDs showed a 90% reduction in terrapin bycatch, while maintaining crab catch numbers.

9

Alabama

With relatively little coastline, Alabama has only produced one terrapin bycatch study. That 2011 study showed a 90% decrease in terrapin bycatch with a 5x15 wire BRD, but overall terrapin numbers were low. The study also showed a large (30%) decrease in crab catch in traps with BRDs.

10

Mississippi

Only two studies have been conducted in Mississippi. Those studies caught no terrapins both in control and BRD groups, and neither showed a change in crab catch.

11

Louisiana

Only one study has been conducted in Louisiana, with no terrapins caught in either control or BRD traps. The study did show a 38% increase in crab catch in traps with BRDs.

12

Texas

Only one terrapin bycatch study has been conducted in Texas. That study caught only 2 terrapins overall, both in traps without BRDs. There was no change in crab catch in traps with BRDs.

We also analyzed the three most common BRD sizes studied, to compare how well they excluded terrapins, and how well they caught crabs.

Percent change in terrapin bycatch after BRD implementation, based on 24 studies. Many studies included multiple BRD sizes, for a total of 42 data points.

Histogram Bins

[-60,-50]: Upperman et al. 2014

[-50,-40]: Morris et al. 2011

[-30,-20]: Lukacovic et al. 2005, Powers et al. 2009, R. Lipcius, VIMS, pers. comm

[-20,-10]: Cole & Helser 2003, Morris et al. 2011

[-10,0]: Butler & Heinrich 2007, Jenkins and Roosenburg 2018, Roosenburg & Green 2000, Baxter 2014, Rook et al. 2010

[-20,-10]: Grant 1997

[0,+10]: Cole & Helser 2001, Roosenburg & Green 2000, Cuevas et al. 2000, Mazzarella 1994, Wood 1997

[+30,+40]: Guillory & Prejean 1998

[+40,+50]: Wood 1997

[-40,-30]: Coleman et al. 2011

[-20,-10]: Belcher et al. 2007, Chavez and Williard 2017, Grubbs et al. 2017

[-10,0]: Graham et al. 2011, Chavez 2014, Upperman et al. 2014

[0,+10]: Corso et al. 2017, Grubbs et al. 2017

Additional studies and sizes not included in histograms: Chavez 2014 (4x15cm), Chavez and Williard 2017 (3.8x15.2cm), Cole & Helser 2002 (3.8x12cm), Cole & Helser 2004 (5x12cm), Grant 1997 (4x12), Grubbs et al. 2017 (5.1-6.4x7.3cm), Hart & Crowder 2011 (4.5x16cm, 4x16cm, 5x16cm), Jenkins and Roosenburg (7.3x5cm), Wood 1997 (4.5x10cm)

Unfortunately, inconsistencies in published study designs can make it difficult to draw conclusions from the numerous field experiments conducted throughout the range of the terrapin’s overlap with the blue crab fishery. Studies differ not only in BRD size tested, but in type of crab bait used, distance of traps from the shore, and BRD construction material. Some studies also caught so few crabs or terrapins that it is difficult to determine if differences in crab or terrapin catch are the result of BRD use, relative to controls. For instance, studies for one trap size (5x10cm) yielded the best crab catch and reasonable terrapin exclusion, but all of those studies occurred before 2001. Were terrapin populations more or less abundant 20 years ago? Were crab populations more or less abundant? Have study designs changed significantly in the past 20 years? Similarly, 5x15cm BRDs only appear in field experiments from 2007 onward. Managers and scientists should use these data to inform BRD design and study design for their state and specific needs, with the above caveats noted.

Oval BRDs: a new chapter in terrapin conservation

In the summer of 2021, William & Mary researchers conducted a comprehensive field experiment comparing a new oval BRD design to the traditional rectangular design. Though the study results have not yet been published, oval BRDs were effective at excluding terrapins while still catching blue crabs, with no significant reduction in crab catch in most instances. They were also much easier to install than rectangular BRDs. You can view a video created on the study and BRD legislation in Virginia below.

Left: two oval BRD sizes (1.75 inch opening, top and 2 inch opening, bottom). Right: a oval BRD in a trap

Trapped: Diamondback terrapins, crab traps, and the struggle to keep them apart

Similarly, a recent North Carolina study conducted a lab test of several oval-shaped narrowed funnel openings and found that terrapin capture was reduced when compared to control openings (Wilson et al. 2022).

Final Conclusions

Results from research demonstrate that BRDs are incredibly important to reducing mortality of terrapins in crab traps. The effect of BRDs on crab catch, however, seems to vary by study, with some reporting decreases and others reporting increases. Especially for all recreational crabbing and for very nearshore commercial crabbing in terrapin habitat, the potential reduction in crab catch must be weighed against the conservation benefits to terrapin populations.

Questions? Contact Madeline at mbreinsel@wm.edu.

Photo Credits

Title video

Kory Angstadt, VIMS

Diamondback terrapin photo

David Malmquist, VIMS

Blue crab photo

David Malmquist, VIMS

First BRD photo

Randy Chambers, W&M

Second BRD photo

Madeline Reinsel, W&M

Trapped terrapins #1

Scott Belfit

Trapped terrapins #2

Andrew Grosse

New Jersey

Sujit Kumar.  View license .

Delaware

NASA

Maryland

NOAA

Virginia

NOAA

North Carolina

NOAA

South Carolina

Georgia

NOAA

Florida

NOAA

Alabama

NOAA

Mississippi

FEMA

Louisiana

NOAA

Texas

NOAA

Underwater video

David Stanhope, VIMS

Final video

Robert Galvin, W&M

Graphs

Madeline Reinsel, W&M

Terrapin photo

Michael Turns

The Diamondback Terrapin (Malaclemys terrapin)

The Blue Crab (Callinectes sapidus)

4.5x12cm and 5x15 cm BRDs, and BRDs installed in a trap

Two different BRD designs. Scientists have found that red BRDs attract the most male crabs.

Terrapins instinctively swim to the surface, instead of going back out through the trap entrance.

This trap contained 94 dead terrapins.

Percent change in terrapin bycatch after BRD implementation, based on 24 studies. Many studies included multiple BRD sizes, for a total of 42 data points.