2001: A population study of diamondback terrapins in Queens Creek.
Adam Spiller (Biology)
Introduction
The diamondback terrapin, Malaclemys terrapin, is one of the only turtles that is specifically adapted to living in brackish water conditions. Historically, diamondback terrapins suffered mass exploitation during the 1800s and the early 1900s as a result of their harvest for turtle soup. Today these turtle populations face a number of ecological challenges, including habitat destruction, death by drowning in crab pots, low recruitment because of nest predation, and unregulated harvesting for foreign and domestic markets. Environmental conditions necessary to support healthy populations of terrapins have degraded to the point that in many states their populations are so low that they are classified as "threatened" or "at risk" species.
Aside from ongoing studies of diamondback terrapins populations from the Patuxent River, no studies have been completed in other river systems of the Chesapeake Bay. This research project serves as the only current study of any population of diamondback terrapins in Virginia.
Methods
Terrapin sampling was conducted by canoe on 26 dates between 4 June and 2 August. Two small marsh creeks coming off of Queens Creek, in York County, served as the research site for the study of these two populations. These creeks are located between the New Quarter Park dock and the mouth of Queens Creek, a tributary of the York River.
For terrapin collection, a three-layered trammel net, of 10 inch, 2 inch, and 10 inch mesh size, was stretched across the creek. The net was checked from canoe every 45 minutes, with all by-catch and turtles untangled to prevent any unnecessary stress. Trapped turtles were placed in the canoe and all of the by-catch was returned to the creek.
Upon collection, each turtle was weighed in a pillowcase attached to a hanging scale, its carapace was measured for length and width using a pair of calipers, the number of growth rings on the carapace was counted, any identifying features were recorded, female terrapins were palpated to determine if they were gravid, and finally a code based on the turtle's corresponding binary number was filed into the right, outer scutes of the turtle.
Results
In Creek 1, a total of 37 terrapins were caught and marked, and six of these were recaptured. In Creek 2, 28 diamondback terrapins were marked, and of these only two were recaptured over the course of the study. The size and age of the terrapins varied greatly. Males were identified by their small size, usually weighing below 450g, and possessing long thick tails as compared to female terrapins. The mean weight of the captured males was 337g with a range of 125g to 450g. This male mean weight is substantially less than the female mean of 1119g. The mean length of female turtles was 187mm with a range of 154mm to 217mm; the mean male length was 128mm, based on a range of 90mm to 141mm. Age estimates for males ranged from 3-8 years; for females, 5-12 years.
Female terrapins gain weight at a faster rate than males, and continue to gain weight as they age, unlike males which have a low rate of weight gain. Another sexually dimorphic characteristic is the fact that the ratio between weight and carapace length is much greater for females than for males. A similarity in growth patterns of male and female terrapins is the relationship between carapace length and width. This is at the same rate of growth for both sexes.
There was some type of physical damage evident on 13 of the 65 turtles that were marked. The damage varied from a missing foot to broken off scutes. None of this damage appeared fresh, it all seemed well-healed. Another observation was that only two turtles showed any type of growth anomalies, both of these had 13 scutes on each side instead of the usual 12.
I found no recaptures between creeks, even though the creeks are relatively close, less than a quarter mile apart, and easily accessible from the main body of Queens Creek. Terrapin populations tend to stay extremely localized.
Informal fecal observations of the terrapins that were captured and defecated in the boat revealed a diet of fiddler crabs, blue crabs, periwinkle snails, and ribbed mussels. These are among the typical dietary patterns of terrapins, which have also been found to include marine annelids, clams, captured or scavenged fish, and plant material (Tucker et al. 1995).
Discussion
Roosenburg (2000) determined that currently the largest threat to diamondback terrapins is crab pots, especially recreational crab pots. Commercial crab pots tend to be set in deeper waters, while the smaller, juvenile and male terrapins tend to stay in more shallow waters, closer to shore (Roosenburg et al., 1999). My data suggests that for the Queens Creek populations, male terrapins suffer from increased mortality due to drowning in crab pots, as opposed to the females who "escape" crab pots by growing large.
Diamondback terrapins exhibit sexual dimorphism. Even when their ages overlap, females weigh noticeably more than males. Another comparison is illustrated by observations in the field and has been calculated by Tucker et al. (1995) showing that females have larger heads in comparison to males. Male head width stops increasing at about age 5 while the heads of females continue to grow with age even after shell growth stops. This means that females can forage for larger food sources, which may be one reason for the differences in growth between males and females.
This project serves as an important resource to the future study of diamondback terrapins. The potential impact of terrapin drowning in crab pots could be researched through a survey of local crabbers. This would also be beneficial for policy issues having to do with the management of the crabbing industry. For example, the results of this study could be used to argue for a bycatch reduction device mandatory on Virginia commercial crab pots, as is the law in other states. The future of terrapin research in this area is quite promising considering the current void of information on Virginia populations of diamondback terrapins.
Acknowledgments
This project was supported by a Howard Hughes Medical Institute undergraduate biological sciences education grant to the College of William and Mary. I am grateful to Dr. Randy Chambers for his advice with this study. This study would not have been completed without the help of A. Deer, R. Dave, A. Holinka, K. Murphy, S. Riggen, M. Turns, and M. Whitfield.
Literature Cited
Tucker, A.D., FitzSimmons, N.N., and Gibbons, J.W. 1995. Resource partitioning by the estuarine turtle Malaclemys terrapin: trophic, spatial, and temporal foraging constraints. Herpetologica 51(2):167-181.
Roosenburg, W.M., Haley, K.L., and McGuire, S. 1999. Habitat selection and movements of diamondback terrapins, Malaclemys terrapin, in a Maryland Estuary. Chelonian Conservation and Biology 3(3):425-429.
Roosenburg, W.M. and Green, J.P. 2000. Impact of a bycatch reduction device on diamondback terrapin and blue crab capture in crab pots. Ecological Applications, 10(3):882-889.
