¹Duke University, Nicholas School of the Environment and Earth Sciences Marine Laboratory, Beaufort, NC, USA
²US Geological Survey, Center for Coastal and Watershed Studies, St. Petersburg, FL, USA
³US Geological Survey, Leetown Science Center, Kearneysville, WV, USA

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Diamondback terrapins (Malaclemys terrapin) are distributed along the US coast from Massachusetts to Texas in brackish water habitats. This long-lived, sexually dimorphic turtle is subject to both terrestrial and aquatic threats that include bycatch in fishing gear and roadkill on highways adjacent to salt marsh habitat. Until now, no clear population definition had been established for this continuously-distributed species, so effective conservation efforts to mitigate population-level threats have not been realized.

photo by Ari Friedlaender [larger image]

To determine ecologically and evolutionarily relevant management units, we used molecular techniques (i.e., microsatellite DNA) to test the hypothesis that M. terrapin in the US comprise one single, homogeneous population. To assess the magnitude of the threat that actively-fished commercial crab pots pose for terrapins, we conducted experimental fishing studies with crab fishers in North Carolina and tested the ability of several bycatch reduction devices (BRDs) to exclude terrapins but retain valuable blue crabs. We also used elasticity analyses to rank different BRDs among several management options. Finally, to estimate adult survival rate, capture probability, and population size for the elusive mangrove terrapin, we conducted a mark-recapture study in the Big Sable Creek (BSC) complex of Everglades National Park, FL and analyzed individual encounter histories.

STUDY 1: Genetic Analysis: 1409 blood samples from 31 collection sites, 12 microsatellite loci (Figure 1)

Hypotheses tested:

H_o: M. terrapin from MA to TX comprise one homogeneous population.
H_o: Male and female M. terrapin disperse equally.

Figure 1. Map of collection sites for M. terrapin genetic analysis. [larger image]

STUDY 2: Tests of various Bycatch Reduction Devices (Figure 2)

A—The Problem [larger image] B—The Potential Solution [larger image] ©Scott Taylor Photography, Inc. C—Blue Crabs, Callinectes sapidus [larger image] Figure 2. Photos of (a) captured terrapins, (b) a BRD-outfitted crab pot, and (c) the target

STUDY 3: Mark-recapture of mangrove terrapins in Big Sable Creek, Everglades National Park (Figure 3)

A [larger image] B [larger image] C [larger image] Figure 3. Photos of (a) a typical capture site in a first-order stream within Big Sable Creek, (b) a mangrove terrapin being weighed, and (c) the effective dip-net method of capturing terrapins.

STUDY 1:

• M. terrapin exists as at least six distinct metapopulations or regional management units (MUs) throughout their range (Figure 4). These MUs do not coincide with previous morphologically-based subspecies designations.
• Additionally, microsatellite analysis elucidated that male-biased dispersal exists in M. terrapin.

Metapopulation = a network of semi-isolated populations with some level of regular or intermittent migration and gene flow among them, in which individual populations may go extinct but then can be recolonized from other populations -Meffe & Carroll (1997), Principles of Conservation Biology

A [larger image] B [larger image] Figure 4. (a) Neighbor-joining tree depicting chord distances and management units (ellipses), and (b) a rangewide map showing new genetically-determined management units (in pink), previous M. terrapin subspecies (in yellow, labeled A-G), and the range of the terrapin (stippled in blue).

STUDY 2:

• Terrapin interaction with blue crab fishery activity primarily occurs in the early spring (i.e., April and May), close (i.e., < 250 m) to shore.
• Terrapin bycatch in crab pots can be mitigated by using BRDs, although some BRDs still allow adult males to enter. Larger (i.e., 5.0 cm) BRDs did not significantly affect catch rates of crabs, but smaller (i.e., 4.5 and 4.0 cm) BRDs did (P<0.001) (Table 1).
• Elasticity analysis revealed that BRDs can positively affect population growth more so than other management alternatives that target adults rather than juveniles.

STUDY 3:

• A total of 300 terrapins have been marked during 5 sampling occasions (Nov. 2001-May 2003). Sex ratio is approximately 1:1, with primarily adults in the sampled population.
• Adult survival for terrapins in BSC is = 0.79 (95% CI 0.60-0.91), and it is constant across time and gender.
• Mean probability of capture was 0.41, and this parameter varied seasonally (winter = 0.28, summer = 0.53).
• The distribution of terrapins in BSC lies largely in first order tidal streams in certain creeks on the Cape Sable peninsula (Figure 5).

Figure 5. Location of study site in Everglades National Park. The site is located on the Cape Sable peninsula, southwest FL. [larger image]

This work was funded, in part by the US Geological Survey’s program on Global Climate Change and the Place-Based Science initiative. Funding was also provided by the Oak Foundation and NC Sea Grant. We thank Gary L. Hill, Cathy A. Langtimm, Thomas J. Smith III, Noah Silverman, Colleen Callahan, Shannon Julian, Lisa Eby, Paula Gillikin, Selina Heppell, Janet Gannon, Ari Friedlaender, Lesley Thorne, Dean Easton, Caroline Good, Tracy Ziegler, Mark Hooper, the Cahoon Family, Duke Marine Lab Grad students and staff, and the staff at Everglades National Park.

Related information:

Impacts of Hydrological Restoration on Three Estuarine Communities of the Southwest Florida Coast and on Associated Animal Inhabitants