Terrapin Populations Research Articles

Effects of roads and crabbing pressures on diamondback terrapin populations in coastal Georgia

AbstractHuman activities, including the harvesting of natural resources and land development, place substantial pressure on wildlife. The diamondback terrapin (Malaclemys terrapin) is a small, estuarine species of emydid turtle in decline and at risk due to a suite of human activities. Vehicle‐induced mortality from increasing coastal traffic and bycatch mortality in crab pots have been recognized as 2 of the primary conservation concerns for terrapins. We used mark‐recapture estimates of terrapin density and sex ratio from repeated seining samples of 29 randomly stratified selected tidal creeks to evaluate the current relationships between road and crabbing pressures and the abundance, sex ratio, and size distribution of terrapin populations along the Georgia coast. We obtained 2005 captures of 1,547 individual terrapins among 29 tidal creeks sampled. Population density estimates ranged from 0 to 1,040 terrapins/km among tidal creeks with a median density of 65 terrapins/km. Among all sites, terrapin density declined with increasing crabbing activity within the creek, but was not related to proximity to roads. Sex ratios did not vary significantly with crabbing activity or proximity to roads; however, we found a significantly larger proportion of smaller‐sized terrapins in creeks with no crabbing activity. Although roads may have significant localized effects on terrapin populations, we found no measurable association between proximity to roads and current variation in terrapin density along the Georgia coast. However, we did find that terrapin density and the proportion of smaller sized individuals within the population were negatively associated with crabbing activities. Bycatch from commercial and recreational activities threaten many species. We add to a growing body of research showing crabbing activities are affecting diamondback terrapin populations across much of the species' range. States committed to the conservation of terrapins and coastal species should focus on reducing bycatch risk; for example by regulating soak times and locations, requiring the use of bycatch reduction devices, and removing abandoned or lost crab pots from coastal habitats. © 2011 The Wildlife Society.

Sex-biased dispersal and natal philopatry in the diamondback terrapin, Malaclemys terrapin

Nesting ecology and population studies indicate that diamondback terrapins (Malaclemys terrapin) exhibit nest site fidelity and high habitat fidelity. However, genetic studies indicate high levels of gene flow. Because dispersal affects the genetics and population dynamics of a species, we used six highly polymorphic microsatellite markers to investigate sex-biased dispersal and natal philopatry of M. terrapin in Barnegat Bay, NJ. We compared results of spatial autocorrelation analysis, assignment methods and Wright's F(ST) estimators to a mark-recapture analysis. Mark-recapture analysis over a 4-year period indicated that most individuals have relatively small home ranges (<2 km), with mature females displaying greater home ranges than males. Goodness of fit analysis of our mark-recapture study indicated that some juvenile males were likely transient individuals moving through our study location. Mean assignment indices and first-generation migrant tests indicated that mature males were more prone to disperse than mature females, but first-generation migrant tests indicated that per capita there are more female than male dispersers. Thus, the relative importance of males and females on gene flow in terrapin populations may change in relation to population sex ratios. Spatial autocorrelation analysis indicated that mature females exhibited natal philopatry to nesting beaches, but first-generation migrant tests indicated that a small number of females failed to nest on natal beaches. Finally, we discuss the important conservation implications of male-biased dispersal and natal philopatry in the diamondback terrapin.

Predator recognition of native but not invasive turtle predators by naïve anuran tadpoles

The impact of alien predators on prey populations is well known by conservation biologists, but little attention has been paid to the negative effects that the introduction of exotic predators may have on native predators through competition for food. In the Iberian Peninsula, the red-eared slider, Trachemys scripta elegans, and other exotic freshwater turtles have been introduced, competing and displacing the native endangered terrapins (the European pond turtle, Emys orbicularis, and the Spanish terrapin, Mauremys leprosa). Although the nature of competitive interactions remains unclear, direct competition for food is likely to occur. Both native and invasive freshwater turtles are common predators of amphibian tadpoles. Naive amphibian tadpoles are known to recognize and respond to local predators with no prior experience, but tadpoles might not recognize new predatory species, since they have not shared a long evolutionary history with them. We examined the ability of four species of Iberian anuran tadpoles to recognize and respond to chemical cues from invasive and native freshwater predatory turtles. Three of the four tadpole species tested reduced their swimming activity when cues from native turtles were present in water, but not when cues belonged to exotic turtles. This inability of tadpole prey to respond innately to chemicals from introduced predatory turtles might be one of the causes that explains the displacement of native populations of Iberian terrapins by invasive exotic turtles, and may help clarify why alien predators sometimes prosper better in new habitats than locally adapted predators.

Diamondback Terrapin Mortality in Crab Pots in a Georgia Tidal Marsh

Abstract Recreational and commercial crab pots are considered major threats contributing to recent declines in diamondback terrapin populations. In a single Georgia (USA) tidal marsh, 133 diamondback terrapin (Malaclemys terrapin centrata) carcasses were observed in abandoned crab pots, consisting of more than double the remaining estimated population. We suggest that the potential for just a few neglected or abandoned crab pots to significantly deplete a terrapin population makes enforcement of explicit soak laws and implementation of a derelict crab-pot removal program a priority for conservation of diamondback terrapins in Georgia.

Development of a rapid-assessment technique for diamondback terrapin (Malaclemys terrapin) populations using head-count surveys

Although diamondback terrapins appear to be declining throughout much of their geographic range, more information is required to evaluate population trends. Unfortunately, sampling terrapin populations is both time and labor intensive. We initiated studies to examine the efficacy of using head counts in tidal creeks as a rapid-assessment technique for monitoring terrapin populations. From 2005 to 2007, we conducted headcount surveys in conjunction with regular aquatic sampling. Head-count surveys consisted of recording the number of terrapins we observed from a boat going up (run 1) and down (run 2) tidal creeks. These surveys were conducted before aquatic sampling (i.e., low tide) as well as other times (e.g., high tide). We found the strongest positive relationship between the number of terrapins observed in run 1 combined with run 2 and the number of terrapins captured (R 2 = 0.538). We examined the effects of variables such as day of year, time of day, cloud cover, and creek location on the number of heads seen. Such models will allow effective monitoring of terrapin population trends and improve implementation of appropriate conservation measures. We recommend a refinement of head-count surveys and the involvement of citizen scientists to aid in the establishment of a range-wide monitoring program that will greatly increase survey effort while saving time and money. © Koninklijke Brill NV, Leiden, 2009

Ecological effects of major injuries in diamondback terrapins: implications for conservation and management

Abstract Many turtle species frequently suffer major injuries due to attempted predation or anthropogenic factors. Diamondback terrapins (Malaclemys terrapin) are one species known to be affected by anthropogenic activity, but little is known about the causes of injuries. In declining diamondback terrapin populations, learning more about causes and results of injuries can be helpful in developing sound management plans. Patterns of limb loss and major shell injuries were examined in a population of terrapins studied for 24 years at Kiawah Island, South Carolina to infer the causes and effects of injuries and possible predators on terrapins. The rate of shell injuries increased temporally, possibly as a result of increased watercraft activity. Because no differences in rates of limb loss were found between males and females, limb loss probably results from aquatic encounters (i.e. limb loss does not appear to be the result of terrestrial predation during nesting). Furthermore, males experienced reduced body condition when injured, and terrapins with a major injury had lower survivorship than uninjured terrapins. Therefore, in addition to reducing sources of mortality and protecting nesting habitat, measures to protect terrapins from watercraft activity may increase the survivorship of adult terrapins. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Crab trapping causes population decline and demographic changes in diamondback terrapins over two decades

Diamondback terrapins ( Malaclemys terrapin) are thought to be declining throughout their range. Although many factors have been proposed to contribute to terrapin declines, including increased predation of nests and adults, habitat loss and degradation, road mortality, commercial harvest for food, and mortality as bycatch in crab traps, few studies have provided evidence linking these agents to population declines. Because male and small female terrapins are most susceptible to mortality in crab traps, population declines should coincide with shifts in the age and size distributions of the population and a shift to a more female-biased sex ratio. We used twenty-one years of mark-recapture data (>2800 captures of 1399 individuals) from a declining diamondback terrapin population in South Carolina to test the prediction that the decline is the result of mortality in crab traps. Since the 1980s, the modal size of both male and female terrapins has increased substantially and the proportion that are females is higher than in earlier samples. Additionally, the population now contains more old and fewer young individuals than before. The changes in demography and sex ratio we observed suggest that this terrapin population has declined as a result of selective mortality of smaller individuals in crab traps. The use of bycatch-reduction devices on crab traps may help prevent terrapins from entering the traps, but current models are too large to prevent mortality of males and many females in this population. Future research should focus on design and testing of effective bycatch-reduction devices for specific regions and other methods to prevent terrapin mortality in crab traps.

Population genetics of the diamondback terrapin (Malaclemys terrapin)

We examined the population genetic structure of the diamondback terrapins (Malaclemys terrapin), within and among estuaries. Based on mark-recapture studies, these estuarine turtles have high site fidelity that is likely to make them vulnerable to local extinctions. We tested if observed site fidelity of adults would be reflected in intraestuarine population genetic structure of six highly polymorphic microsatellite loci (five tetranucleotide and one dinucleotide). No evidence was found for population structuring within the Charleston estuary nor among three different estuaries in South Carolina. We then examined four other terrapin populations from North Carolina to New York, as well as from the Florida Keys and from Texas. With increasing geographical distance, genetic differentiation increased from South Carolina through New York, but overall values were low. The dinucleotide locus contributed significantly more to the genetic differentiation of some population comparisons than any of the other loci. Interestingly, terrapins from South Carolina to New York were much more genetically similar to those from Texas (rho = 0.154) than to those from Florida (rho = 0.357). We attribute this pattern to extensive translocations of terrapins during the early 20th century to replenish diminished populations and to provide turtle farms with stocks. Terrapins collected in Texas were especially sought for shipment to the northeastern US because of their larger size. Our study indicates no population structure within or among adjacent estuaries. Thus, the mark-recapture information from adult and subadult feeding locations is a poor predictor of population genetic structure. Additionally, it appears that past human activities may have drastically altered the genetics of current populations. Finally, our data suggest that translocation of eggs or head starting of terrapins within estuaries or among adjacent estuaries is acceptable from a genetic standpoint.

Nesting Ecology and Predation of Diamondback Terrapins, Malaclemys terrapin, at Gateway National Recreation Area, New York

We studied Diamondback Terrapins, Malaclemys terrapin, at Gateway National Recreation Area, New York. We found evidence of nesting terrapins at three locations within the Recreation Area and focused our research on the islands of Jamaica Bay Wildlife Refuge. Female terrapins nested from early June through early August and oviposited at least two clutches per year. Mean clutch size was 10.9 eggs. Nesting activity increased with daily high temperature and high tide. The greatest number of terrapin nests was found in shrubland, dune, and mixed-grassland habitats, but nest densities were higher on a human-made sandy trail and on sandy beaches. We estimate that approximately 2053 nests were oviposited on the largest island in the refuge in 1999. In 1998 and 1999, we counted 1319 and 1840 depredated nests, respectively, throughout the refuge. Raccoons were introduced into Jamaica Bay Wildlife Refuge approximately 20 yr prior to this study. Raccoons depredated 92.2% of nests monitored on the largest island during the 1999 nesting season. We also found the carcasses of adult female terrapins that apparently were killed by raccoons as they came on land to nest. This terrapin population may be undergoing demographic changes as a result of the introduction of raccoons.

Demography and viability analyses of a diamondback terrapin population

The diamondback terrapin, Malaclemys terrapin, is a long-lived species with special management requirements but quantitative analyses to support management are lacking. I analyzed mark–recapture data and constructed an age-classified matrix population model to determine the status and viability of the only known diamondback terrapin population in Rhode Island. Female diamondback terrapins were captured, marked, and recaptured while nesting during 1990–2001. Population growth rate (λ) was 1.034 (95% confidence interval = 1.012–1.056). For the preceding 5 years, however, abundance had been stable at about 188 breeding females. Adult apparent survival was high but declined slightly by 0.14% per year from 0.959 in 1990 to 0.944 in 2000. Recruitment of breeding females also decreased during the study period; therefore, survival was increasingly a greater component of population growth rate. Juvenile survival was 0.565 at λ = 1.034 and 0.446 at λ = 1. Both retrospective (mark–recapture) and prospective (matrix population model) analyses showed a greater influence of survival versus reproduction on population growth. Population- model projections showed that capping nests to improve reproductive success could increase population growth rate, but the magnitude of increase was positively related to pre-reproductive survival, therefore negating nest capping as a remedy for declining populations or poor survival. Extinction attributable to demographic stochasticity is unlikely.

Estimates of adult survival and migration for diamondback terrapins: conservation insight from local extirpation within a metapopulation

As a case study we analyze a recent extirpation within a metapopulation of diamondback terrapins (Malaclemys terrapin) in a South Carolina salt marsh. Survival and migration probabilities were estimated by means of a multistratum approach in Cormack–Jolly–Seber mark–recapture models to obtain transition probabilities among four tidal creeks adjoining a river. Terrapins showed high fidelity to each subpopulation and the low migration rates were insufficient for recolonizing a creek that had suffered extirpation. Annual survival rates for adult female terrapins averaged 0.84 across all creeks, ranging from 0.748 to 0.971. Survival rates were converted to instantaneous mortality rates (M) for the purpose of calculating the mean life-span as 1/M. When re-expressed in terms of mean life-span, the results confirmed that the average female terrapin did not survive to its estimated age at maturity, as might be predicted if additional mortality of females in crab pots was implicated. The results emphasize that incidental mortality of terrapins in crab pots is a conservation concern. Simple modifications of crab-pot design, such as an entrance reducer, can successfully mitigate this threat to the survival of terrapin populations.

Estimates of adult survival and migration for diamondback terrapins: conservation insight from local extirpation within a metapopulation

As a case study we analyze a recent extirpation within a metapopulation of diamondback terrapins (Malaclemys terrapin) in a South Carolina salt marsh. Survival and migration probabilities were estimated by means of a multistratum approach in Cormack–Jolly–Seber mark–recapture models to obtain transition probabilities among four tidal creeks adjoining a river. Terrapins showed high fidelity to each subpopulation and the low migration rates were insufficient for recolonizing a creek that had suffered extirpation. Annual survival rates for adult female terrapins averaged 0.84 across all creeks, ranging from 0.748 to 0.971. Survival rates were converted to instantaneous mortality rates (M) for the purpose of calculating the mean life-span as 1/M. When re-expressed in terms of mean life-span, the results confirmed that the average female terrapin did not survive to its estimated age at maturity, as might be predicted if additional mortality of females in crab pots was implicated. The results emphasize that incidental mortality of terrapins in crab pots is a conservation concern. Simple modifications of crab-pot design, such as an entrance reducer, can successfully mitigate this threat to the survival of terrapin populations.

Diamondback Terrapin (Malaclemys terrapin) Mortality in Crab Pots

The entrapment of diamondback terrapins ( Malaclemys terrapin) in crab pots frequently results in drowning and death of the trapped turtles. We determined the rate of capture, size, sex, and age of terrapins captured, and the potential impact crab pot mortality has on local terrapin populations. We estimated terrapin capture rates of 0.17 terrapins/pot/day in shallow water areas of Chesapeake Bay (Maryland, USA). The sex ratio of terrapins caught in crab pots was 3:2 male biased because female terrapins become too large to enter crab pots by the time they reach 8 years of age. Males, however, remain vulnerable to entrapment throughout their life. Our estimates of capture rates and local population size suggest that 15–78% of a local population may be captured in a single year. As a consequence, crab pots may be the major reason terrapins are extirpated in coastal, shallow water areas with heavy crab pot fisheries. Additionally, the selective removal of males may also contribute to female‐biased sex ratios observed in this diamondback terrapin population. We developed and tested a modified crab pot that increases terrapin survival and does not reduce the number of crabs caught. Our modified crab pot maintained permanent access to air and prevented the drowning of terrapins. Additionally, our modified crab pot caught more crabs than standard commercial crab pots, suggesting that the modified crab pot could be a viable alternative to standard traps that result in terrapin mortality.

Molecular and Population Genetic Aspects of Mitochondrial DNA Variability in the Diamondback Terrapin, Malaclemys terrapin

Diamondback terrapins (Malaclemys terrapin) occupy brackish waters along North America's Atlantic and Gulf coasts. Despite nearly continuous distribution, terrapin populations exhibit extensive geographic variation, with seven subspecies recognized. To assess population-genetic structure in Malaclemys, we used 18 restriction enzymes to assay mitochondrial DNA (mtDNA) genotypes in 53 terrapins collected from Massachusetts to western Louisiana. MtDNA size polymorphism and heteroplasmy were observed, attributable to variation in copy number of a 75-bp tandem repeat. In terms of restriction sites, mtDNA genotypic diversity (G = 0.582) and divergence levels (p < 0.004) were exceptionally low. Only one restriction site polymorphism appeared geographically informative, clearly distinguishing populations north versus south of Florida's Cape Canaveral region. Nonetheless, the probable zoogeographic significance of this single site change is underscored by its (1) perfect concordance with the distribution of a key morphological character and (2) striking agreement with phylogeographic patterns observed for mtDNA profiles of several other coastal marine species. The possible isolation of Atlantic and Gulf terrapin populations during late-Pleistocene glacial maxima conceivably accounts for the observed patterns of mtDNA (and morphological) variation.

Incidental Capture of Diamondback Terrapin by Crab Pots

Two-hundred eighty-one diamondback terrapins (195♂, 86♀) were captured in baited and unbaited crab pots during premolt crab capture studies in South Carolina. Sampling was conducted four consecutive days each week from mid April to mid November, 1979, in the Ashley River estuary, and daily from April to July, 1980 and 1981, in the Wando River estuary. Fifty-five percent of the total were captured in April, and 32% in May. Males outnumbered females 2.3∶1, but this ratio may reflect crab pot entrance bias which restricted entry of large females. Median plastron length of females was 121 mm (range 76–175) and that for males 100 mm. (range 79–128); only two male terrapins equalled or exceeded the minimum legal size limit of 127 mm. Few captured terrapins were drowned when crab pots were checked daily, and it is estimated that capture mortality amounted to 10%. Terrapin catch per baited crab pot averaged 0.16 daily in April and May. Incidental terrapin capture by 743 commercial crabbers (fiscal year 1982 numbers) in South Carolina during April and May was estimated at 2,853 daily, and mortality was estimated at 285. These estimates are exclusive of mortalities resulting from lost pots. The impact of crabbing activities is believed to pose no threat to the terrapin population at current levels of commercial terrapin harvest. Because female terrapins mature at the minimum legal size, no segment of the egg laying population is protected except seasonally. If direct harvest of terrapins ever approaches that of the early twentieth century, then it is recommended that legal size limits be changed to protect a segment of the mature female population.

Occurrence and Effects of Barnacle Infestations on Diamondback Terrapins (Malaclemys terrapin)

The occurrence and effects of barnacle infestations on two populations of diamondback terrapins were studied from 1977 to 1979 at the Merritt Island National Wildlife Refuge, Brevard Co., Florida. Three species of barnacles were found on Malaclemys: Balanus eburneus, Chelonibia manati and C. testudinaria. Of 125 turtles examined, 76% were infested by barnacles. Barnacles settled on the carapace of turtles significantly more frequently than on the plastron. Infestation rates differed sharply between the two populations. No differences in fouling rates were found between the sexes. The major effects of barnacles on Malaclemys were interference with nesting activity and shell erosion. In a few cases, such erosion was severe enough to cause fatal injuries to turtles. It is suggested that barnacles infesting turtles be considered as parasites, rather than commensals.

An Ecological Study of a Natural Population of Diamondback Terrapins (Malaclemys t. terrapin) in a Delaware Salt Marsh

A two-year study of a population of the northernediamondback terrapin (Malaclemys t. terrapin) was undertaken in a salt marsh in Delaware. Population size estimates based on markrelease-recapture data indicate an early season mean population size of 1655 individuals during June of 1975, declining to a mean of 378 by August. Captures per unit effort declined during this same period, supporting this trend. The decline is a result of later season dispersal. Data from 1976 indicate that this pattern may have been repeated, and that the population had experienced no significant new recruitment between years. Size class frequency data reveal a lack of juveniles in the sampled population, which could not readily be attributed to sampling techniques. The estimated size of the terrapin population from this study indicates that terrapins may be important, though frequently overlooked, components of the marsh food web.

A Louisiana Terrapin Population (Malaclemys)

TURTLES of the genus Malaclemys occur in isolated populations along the Louisiana coast. Reports of fishermen indicate that in some localities they are exceptionally abundant and are often taken in nets. The demand for these turtles as food is now limited and they are not usually brought to market. The difficulty of collecting them without special equipment has discouraged their study by herpetologists. The investigations of Hay, Hildebrand and others have contributed much to our knowledge of the life history of captive specimens but little information is available on natural populations. Too, most of the data reported on captives were derived from a study of a hybrid stock of two or three subspecies; Malaclemys terrapin terrapin (Schoepf) and Malaclemys terrapin centrata (Latreille), and of these two with Malaclemys terrapin littoralis Hay. It is difficult to relate the information so accrued to the existing populations. Two subspecies are reported to occur on the gulf coast from the Florida panhandle to southern Texas, Malaclemys terrapin pileata (Wied) and Malaclemys terrapin littoralis Hay. The ranges of neither of these is adequately defined. The two subspecies are reportedly different in these respects (Hay, 1904; Carr, 1946):