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Egg-harvesting, Hatching, and Release: A Population Augmentation Tool for Bog Turtles in Tennessee

The bog turtle (Glyptemys muhlenbergii) was first discovered in Tennessee in May, 1986 from two wetland sites within one large valley in the northeastern tip of the state. These captures documented the last known state to be included in the 12-state range of this species in the eastern United States, and the fifth state within what has become known as the “Southern Population.”

Work in these five states has determined that the species is restricted to spring-fed wetlands, called Mountain Bogs, in the Blue Ridge and upper Piedmont regions, and North Carolina serves as the stronghold for this species in the south. Bog turtles appear to be largely peripheral in the other states, and searching to date has not identified the occurrence of additional populations in Tennessee. The discovery of bog turtles in Tennessee began what has become known as The Tennessee Bog Turtle Program, an effort combining 11 primary components involving both in-situ and ex-situ priorities and a partnership between the Knoxville Zoological Gardens, The Nature Conservancy of Tennessee, Tennessee Wildlife Resources Agency, and numerous private landowners.

Radio-telemetry within the study valley documented the presence of two additional, satellite wetlands with turtles adjacent (by one-half mile each) to the historic sites in 2002 and 2003, and turtles are known to have moved between these sites. Based on mark-recapture studies, a total of 89 turtles are currently thought to comprise the marked turtle population of this valley meta-population. Population estimates from 1986 to1996 provided numbers of 46 and 39 respectively for the two historic sites, but turtles are known to have changed sites, and eight deaths of mostly telemetry turtles have been documented since 2001.

At the two historic sites, sub-adult turtles were found, albeit infrequently, but those found at 3-6 years of age were not only located with some regularity, but they appeared to be the only turtles in their respective age classes residing in these sites. At one site, the last two juveniles were found in 1995 and 1997, and at the other site, a six year old male was found in 1994. In addition, since 2001, predators eliminated 12 known turtle nests in these two sites. In some cases, shredded eggshells indicated a mammalian predator, but many times the entire clutches simply disappeared from the nest cavity without visual disturbance of the nesting area. The predator in these cases is thought to be a snake, mole, or perhaps even a bird, but no traces of shell or egg-slime was apparent. The eggs were just gone.

Based on this and the lack of apparent reproductive recruitment in the two historic turtle sites, the component of Egg-harvesting, Hatching, and Release was initiated in 2006. In addition to those turtles radio-tracked season-long, all other females found in each spring were tracked short-term to determine reproductive status. Reproductive success varied in each study season, for example at one site, ten of eleven females produced eggs in 2006, while in 2008, only three of eight laid eggs. From both sites, of eleven females tracked in all three seasons, four produced clutches in each of the three years, two in two years, one in one of the three seasons, and only one turtle did not lay eggs. In the three-year period for 21 turtles, 47 female tracking periods were documented, and of these, 30 clutches of eggs (64%) were produced. Of these, 18 clutches (51 eggs) were harvested and incubated in a secured and remote situation within the study valley.

Prior to the 2006 egg-laying period, a simple outdoor facility was constructed which would serve as a holding station for the incubation of the eggs. It became readily apparent that despite best efforts, egg-laying was missed, nests could not be found, or if found the identity of the female could not be accurately determined. Currently, this 2.4 x 6.0 m area contains two oval 220-liter Rubbermaid tubs for female nesting, and once eggs are laid, the clutches are transferred to one of two 176-liter incubation tubs which correspond to respective sites. Two additional tubs are available for nesting as needed. The tubs are mostly sunken into the ground and overflow plumbing is installed. Substrate is comprised of sphagnum peat in which a matrix of small branches are placed which support plugs of natural nesting vegetation such as sphagnum moss and small sedges. Each tub has a domed lid of 3.2 cm mesh or chicken wire and each is hinged and latched. The tubs are surrounded by portable electric livestock fencing of six-inch metal mesh, and the entire area, since it also contains a garden, is backed up with additional electric fencing. During the hottest periods, fresh water is flushed through each tub daily, and the substrate is lightly misted in mornings when rainfall is sparse.

Based on the prevailing weather in mountainous terrain, incubation periods have been highly variable (67-105 days). Some degree of infertility has routinely been documented in the field throughout this study, and of the harvested eggs, 16 (32%) were infertile. Of the 35 fertile eggs, six had died at some point during incubation, and 29 successfully hatched. For identification purposes, hatchlings were marked with a Queen Marker (bee dot) which was glued with epoxy to one costal scute. Release took place within one week after hatching at respective sites. In July 2007, one 2006-hatched juvenile with the bee dot still firmly attached, was found and represented the first new sub-adult found at either historic site in over 10 years.

Based on the degree of habitat destruction documented over the past 30-plus years in the south, plus the fact that minimally 85% of all viable turtle habitat is found on privately-owned land, meaningful conservation programs for bog turtles remain difficult and tenuous. Population augmentation remains highly controversial but it seems that some techniques such as the one described herein may provide a safe, reliable, relatively inexpensive, and potentially successful conservation “tool” for the enhancement of some declining turtle populations.

– Bern Tryon, Knoxville Zoo

The TSA provided support to the habitat enhancement component of the Tennessee Bog Turtle Project in 2009. Both Bern Tryon and the Knoxville Zoo are long time supporters of the TSA.