Comparison of northern diamondback terrapin 

(Malaclemys terrapin terrapin) hatching success among 

variably oiled nesting sites along the Patuxent River 

following the Chalk Point Oil Spill of April 7, 2000 

By 

Roger C. Wood, Ph.D. 

and 

L. Stanton Hales, Jr., Ph.D. 

Final Report 

December 7, 2001 
 

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INTRODUCTION

On April 7, 2000, approximately 126,000 gallons of a mixture of #2 and #6 fuel oil were 

released from a break in a pipeline providing fuel to the Chalk Point generating station. At the time of 

the oil spill the pipeline was owned by the Potomac Electric Power Company (PEPCO) and operated by 

Support Terminal Services Operating Partnership, LLP (ST Services). The spill initially leaked into 

Swanson Creek and the surrounding tidal wetlands. On April 8, high winds, rain and tides spread the oil 

downstream approximately 27 linear kilometers along the Patuxent River and into several tributaries, 

including Indian Creek, Trent Hall Creek, Washington Creek, and Cremona Creek, fouling many more 

kilometers of shoreline. 

 One of many resident species potentially affected by the oil spill in the Patuxent River and its 

tributary creeks is the northern diamondback terrapin, Malaclemys terrapin terrrapin. 

Female terrapins are known to nest on sandy beaches along the shores of the spill zone. Thus, not only 

were adult terrapins potentially exposed to and affected by the oil spill, but also eggs laid under the 

surface of the nesting beaches were potentially exposed and affected. 

Therefore, we were asked to assess the impact of the oil spill upon terrapin reproductive success 

along the Patuxent River downstream from Swanson Creek. We were asked to undertake our research at 

the beginning of September, nearly five months after the April 7th spill. Thus we first became actively 

involved with this project during the second month of the hatching season (well after the nesting season 

had ended). 

Since 1987, various aspects of the life history, ecology, and behavior of diamondback terrapins 

in the Patuxent River have been the subject of long-term studies (see below) by Dr. Willem Roosenburg 

and his many colleagues. Some of the shoreline that was oiled included the primary study site for much 

of this research, an eight-kilometer stretch along the western bank of the river from Long Point, just 

downstream from Benedict, to Marsh Point. Oiling of the shoreline within this area ranged from heavy 

to light. Thus, some of those studies (see below) were helpful in our developing a plan of investigation. 

Some characteristics of the Patuxent terrapin population reported in the scientific literature are 

summarized in Table 1. Egg incubation time varies between 56 and 110 days, with hatching typically 

commencing in early August and continuing into November, depending on the weather (Roosenburg, 
 

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1992). Roosenburg (1994) has reported nesting densities ranging from 240 to 1125 nests ha-1. Nest 

predation rates were highest (up to 95 % of all nests) at sites with the highest nesting density; thus, 

beaches with low nest densities may produce a larger number of hatchlings than beaches with high nest 

densities (Roosenburg, 1994). 

OBJECTIVE 

 The objective of our investigation is to determine the hatching success of terrapins at nesting 

beaches that were variously oiled (ranging from heavily to lightly) compared to beaches that were not 

oiled. Data from this study are intended to provide a basis for assessing the magnitude and extent of 

injury to the year 2000 hatchling cohort within the spill zone. Results will be used to assist in 

determining the level of compensation necessary to replace terrapin losses resulting from the spill. 

METHODS 

 We designed a methodology to compensate for the start of our investigation in September, after 

the entire nesting season and a portion of the hatching season had passed. The methodology consisted of 

the following components: (1) a preliminary survey to look for evidence of nesting activity in Swanson 

Creek, and along creeks and both banks of the Patuxent River downstream from the mouth of Swanson 

Creek; (2) selection of suitable sites for monitoring of continuing hatching activity, and the construction 

of standardized exclosures at each of these sites; (3) monitoring of those sites throughout the fall to 

collect terrapins, and obtain other size and developmental information from collected hatchlings; and (4) 

excavation of selected exclosures at the end of the fall hatching season to look for hatchlings or nests 

overwintering underground, as well as to identify the locations of nests from which hatchlings had 

already emerged. Although not equivalent to being present at the start of the nesting season, all of this 

information gives us a reasonably accurate picture of nesting at these sites. 

Though terrapins lay their eggs in nests in the supratidal zone (above mean high water), 

there are several potential mechanisms by which nesting success at oiled beaches might be affected by 

the oil spill. First, females nesting at oiled sites presumably lived and fed in areas impacted by the spill. 

Thus, any reduction in the available prey base in their feeding areas could possibly reduce the number or 
 

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quality of eggs that females produce. Even without any prey-base effect, oil is a potential stressor of 

their physiology and might affect reproduction. Finally, though nests were above the high tide mark, 

high spring and storm tides inundate most nests in the study area (Roosenburg, 1992). Thus, nests in the 

supratidal zone at those sites could also potentially experience effects of any residual oil. 

Site Selection and Designation of Oil Category 

The initial three days (beginning Sunday, September 3, 2000) of field work were dedicated to 

surveying potential nesting sites along the banks of Swanson Creek and downstream from its mouth 

along the shoreline of the Patuxent River for several kilometers. Some 22 nesting sites had previously 

been identified by Mountford (2000) within the area affected by the oil spill. We visited the majority of 

these, including two beaches (Burton’s Beach and Marsh Point) upon which Roosenburg (1992) had 

focused much of his previous field research. 

At each beach we visited we looked for the remains of terrapin egg shells on the surface. This 

shell material is evidence of nest predation, which in turn means that one or more female terrapins had 

previously dug a nest and deposited eggs at that site. Samples of egg shells were collected at many of 

these localities. 

Based on this preliminary survey, and with the aid of a map provided by ENTRIX documenting 

the severity of shoreline oiling, we selected nine study sites (Table 2 and Figure 1). Three of these were 

characterized on our map as heavily-oiled nesting beaches; three of these represented moderately- to 

lightly-oiled nesting beaches; and finally, three were unoiled nesting beaches. We divided these nine 

beaches into three categories: 1) heavily-oiled sites, 2) moderately- to lightly-oiled sites, and 3) unoiled, 

control sites. 

Several months after our field work was initiated, the oiling assessment of one site originally 

designated as heavily oiled (Playground) was changed to very lightly oiled (ENTRIX, 2000). To keep 

equal sample sizes within oiling categories for nested analyses of variance (Zar, 1984), we subsequently 

regrouped the sites into new categories: 1) heavily- to moderately-oiled sites, 2) lightly- to very-lightly 

oiled, and 3) unoiled, control sites (see Table 2). This regrouping essentially enabled us to maintain our 

original study goals and experimental design. 
 

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It was our original expectation that two nesting localities included in our study would be 

Burrton’s Beach and Marsh Point, where terrapin nesting was previously studied (Roosenburg, 1992). 

After visits to these sites, it was apparent that neither was suitable for inclusion in this study. Dense 

vegetation present at Burton’s Beach made it an unsuitable site for exclosures. Successional changes and 

other observations (e. g., frequent tidal inundation, dense vegetation, lack of nest shell material 

compared to other sites) at Marsh Point suggested that this beach was no longer the highly productive 

nesting site that it once was. 

Exclosure Design and Construction 

At each of the nine selected nesting beaches we constructed two approximately 50-square meter 

exclosures that would enable us to detect any terrapin hatching activity within them. An exclosure is 

designed to retain hatchlings within it until they can be censused, while at the same time keeping out 

potential hatchling predators, such as raccoons, foxes and birds. The number, size and shape of these 

exclosures varied slightly from one study site to the next because of varying terrain and vegetation 

cover, but the total area enclosed at each of our study sites was as close to 100 m2

 as beach topography 

would allow (Table 2). 

The total area of the exclosures at each site was based on reported nest densities ranging from 

240 to 1125 nests ha-1 (Roosenburg, 1994). Thus, at the low density (240 nests ha-1= 0.024 nests m-2), 

our 50-m2

 exclosures should be sufficiently large to yield one nest. At the high nest density (=0.113 

nests m-2), the exclosures would be predicted to produce about six nests. Thus, these exclosures were 

considered suitable to recognize significant impacts to terrapin hatchlings. 

A typical exclosure is shown in Figure 2. The perimeter fencing (1.5 cm-square plastic mesh) 

was buried 10 to 15 cm into the substrate. This was intended to discourage predators outside the 

exclosure from burrowing underneath to get in, and it also prevented hatchlings that have emerged from 

their nests from escaping. (During the course of our field work, we did not find any evidence of attempts 

to burrow underneath the buried fencing.) Plastic fencing (2.5 cm-square mesh) was stretched across the 

top of the exclosure to deter predators from above. 
 

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Construction of the last of our exclosures was completed on September 21st. Thereafter, we 

monitored the exclosures regularly (almost daily) through early November. We concluded our exclosure 

monitoring activities on November 10th, one week after no new hatchlings were found. 

Exclosure Monitoring, Data Collection and Statistical Comparison 

Within each exclosure, the vegetation was identified using two regional keys (Duncan and 

Duncan 1987, Silberhorn 1999). Vegetation was also mapped (using tape measures) onto graph paper to 

determine the extent to which the exclosures were vegetated. We recorded ground temperatures inside 

and outside the exclosures. The purpose of this was to determine whether the plastic mesh cloth 

stretched over the exclosures was either 1) warming the exclosure by absorbing solar radiation or 2) 

shading the ground inside the fencing enough to lower its temperature significantly compared to the 

ground temperatures outside the fencing. This was important to determine, because 1) we did not wish 

to alter the rate of development of hatchlings compared to other, non-exclosed areas and 2) terrapins are 

characterized by temperature-dependent sex determination. Eggs deposited in nests that are dug in sunny 

areas develop at relatively warm temperatures, resulting in relatively short incubation times and female 

hatchlings. Eggs deposited in nests that are dug in shaded areas develop at relatively low temperatures 

and produce male hatchlings. 

Not surprisingly, ground temperatures varied from day to day over the time that we monitored 

our sites (Appendix 1). However, our measurements of ground temperatures inside (mean = 13.1°C, 

standard error = 0.37°C, N = 138) and outside (mean =13.2°C, standard error = 0.38°C, N =138) 

exclosures did not significantly differ (t = 0.08, p> 0.50). This lack of a significant difference suggests 

that the exclosures should not have any effect on the development of any nests located within them due 

to temperature effects. 

Except for the first seven hatchlings that were recovered prior to availability of needed 

equipment, every hatchling that we discovered was examined in a standardized manner (Herlands et al. 

1993). For those first seven hatchlings, only carapace and plastron lengths were obtained. For all other 

terrapins, four shell dimensions were measured to the nearest 0.1 mm with a pair of calipers: carapace 

length = midline length of the carapace (top part of the shell); carapace width = maximum width of the 
 

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carapace; plastron length = midline length of the plastron (bottom part of the shell); and maximum depth 

= depth of the shell from the most dorsal margin of the carapace to the most ventral margin of the 

plastron). Weight was determined to the nearest 0.1 g on an Ohaus portable scale. In addition, scute 

anomalies were recorded (scutes are the enlarged scales that cover terrapin shells). 

All of these parameters were of interest as potential indicators of the effects of oil as a 

developmental stressor. For example, if hatchlings from heavily-oiled beaches had low weights and/or a 

high frequency of scute abnormalities compared to hatchlings from unoiled beaches, then one might 

reasonably suspect that these differences were attributable to detrimental effects of oiled substrate on 

embryonic development. 

Because terrapins were not obtained from exclosures at all sites in each oiling category (and 

thus, there were no measurements to include in any statistical comparison), one-way analysis of variance 

was used to compare size measurements among oiling categories and sites (Zar, 1984). Contingency 

table analyses employing log-likelihood ratios were used to compare the frequency of anomalies among 

oiling categories (Zar, 1984). 

Excavation of Selected Exclosures 

In typical terrapin populations, not all hatchlings emerge in the fall. Some hatchlings remain 

underground throughout the winter and do not dig their way to the surface until the following spring. 

Anything that slows development of hatchlings, such as the relatively cool weather that characterized 

the summer of 2000 or potential effects of the oil spill, might 1) increase the percentage of spring 

emergers (hatchlings that overwinter in the ground and emerge the following spring) or 2) sufficiently 

slow or interrupt development to (directly or indirectly) cause increased mortality. In southern New 

Jersey, the only place where attempts to quantify the spring emergence of terrapin hatchlings have been 

made, approximately 25% of the hatchlings remain underground throughout the winter and emerge the 

following spring (R. C. Wood, unpublished data). 

In order to supplement the data from our previous field work, we undertook a second type of 

investigation in December, 2000. This involved excavation of exclosures at both oiled and unoiled sites 

to document the number of overwintering hatchlings, unhatched eggs, and opened eggshells 
 

8

(representing successfully hatched terrapins) beneath the surface to a depth of approximately 15 to 20 

cm below the ground. Because the upper part of a typical terrapin nest chamber is no more than eight to 

ten cm below the surface, this enabled us to locate the positions of all nest chambers within each 

exclosure that we excavated. 

Each excavated exclosure had the potential to provide the following information: 

(1) the density of nests as indicated by concentrations of eggshell (from successfully hatched 

nests) found underground, or clusters of hatchlings that have liberated themselves from their eggs but 

remained underground, or nests of unhatched eggs, either viable or not; 

(2) the number of eggs in each nest (here defined as nest size; however, nest size in this study is 

not being determined at the time of nesting as in previous studies in this area [Roosenburg, 1992]); and 

(3) the condition of eggs in each nest. Viable eggs are chalk-white in color, non-viable eggs are 

buff-colored, and open-shell remnants underground represent successfully hatched terrapins. Inviable 

eggs were taken to our laboratory and dissected to determine the developmental stage at which death 

occurred. Eggs believed to be viable were given to personnel of the Maryland Department of Natural 

Resources for incubation and possible rearing. 

This information, in effect, would be nearly equivalent to our having been present during the 

nesting season and marking every nest within each exclosure. Combining this information with other 

data previously accumulated should afford a fairly complete evaluation of nesting success at 

representative sites during the summer of 2000 along the banks of the Patuxent River downstream from 

the power plant. 

This phase of our field work commenced on December 7th and continued through December 

22nd. Snow, frozen ground, and bitter cold weather prevented us from excavating nests from all 

exclosures; nonetheless, all remaining exclosures were removed from all sites by December 22nd. This 

was reportedly one of the coldest Decembers on record according to the National Weather Service 

(Associated Press article in Atlantic City Press, January 6, 2001). 

One-way analysis of variance (Zar, 1984) was used to compare nest sizes among oiling 

categories and sites, and to compare nest sizes from this study with nest sizes reported by Roosenburg 

(1992) for five previous years. Scheffe’s multiple contrast was used to compare data from this study 
 

9

with previous studies (Zar, 1984). Contingency table analyses employing log-likelihood ratios were used 

to compare the different condition of embryos (hatched, viable, and dead) among oiling categories (Zar, 

1984). Inter- and intra-nest variability were not considered in those contingency table analyses. 

RESULTS 

Hatching Study 

 Over the course of nearly 9 weeks (from September 10th to November 9th, 2000), we recovered 

63 terrapin hatchlings from seven of the nine sites that we monitored (Table 3). No terrapins were 

recovered from the Trent Hall beach (a lightly-oiled site) or Prison Point (a control site). 

It should be emphasized that the absence of hatchlings at certain sites does not mean that there 

was no nesting activity at these places. To the contrary, our exclosure sites were selected in part because 

we had found evidence of recent nesting activity (i. e., moderate to considerable amounts of fresh egg 

shell remains littering the ground) at all of them during our preliminary survey. Based on our knowledge 

of terrapin nesting habits both along the banks of the Patuxent River and elsewhere, we suspect that the 

relatively large numbers of hatchlings at Caney Creek and Jack Bay indicate that these were sites where 

1) adult female terrapins double-clutched (i. e., laid a second set of eggs) later in the nesting season or 2) 

predation was lower. Such eggs would have been likely to produce hatchlings later than eggs laid earlier 

in the nesting season. Presumably, hatching at sites where we did not find terrapin hatchlings was 

completed prior to the initiation of our study; alternatively, nests at those sites may have experienced 

high predation rates. 

In terms of numbers of hatchlings found, there was considerable variability among sites 

irrespective of oiling category. The majority of hatchlings came from only two sites, one heavily oiled 

(Caney Creek; N=26) and one unoiled, control (Jack Bay; N=19). Nested (or hierarchical) analysis of 

variance showed no significant difference in numbers of hatchlings (adjusted for minor differences in 

areas of the exclosures) at heavily oiled, moderately to lightly oiled, and control sites (F[2, 6]=1.13, 

P>0.05). In addition, there were significant differences in the numbers of hatchlings among sites within 

oiling categories (F[6, 9]=1.59, P>0.05).

 Analysis of variance indicated that there were no significant differences in any size 

measurements of hatchlings among oiling categories. Carapace length (F[2, 61]= 2.98, P=0.06), plastron 
 

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length (F[2, 61]= 2.68, P=0.08), maximum width (F[2,56]=1.02, P=0.40), maximum height (F[2, 54]=0.69, 

P=0.51), and weight (F[2, 54]=0.28, P=0.76) did not differ among oiling categories (Table 5). Carapace 

length and plastron length, hatchling measurements that exhibited marginal levels of significance 

(P=0.06 and P=0.08), exhibited an unexpected trend: average lengths of hatchlings at lightly oiled sites 

(CL= 3.09 cm, PL=2.68 cm) > average lengths at heavily oiled sites (CL=3.02 cm, PL=2.64 cm) > 

average lengths at unoiled, control sites (CL=2.96 cm, PL=2.55 cm). Trends in those data may have 

been affected by site-specific factors on development. Size measurements varied considerably across 

sites, but only carapace length and maximum height differed significantly between sites within the same 

oiling categories (see Table 5). 

Significant differences in some developmental features among oiling categories were infrequent 

and generally inexplicable, and potentially due to a number of site-specific environmental variables (e. 

g., temperature). The occurrence of an egg tooth on hatchlings did not differ among oiling categories 

(G[2]=3.54, P=0.19), whereas the occurrence of a yolk sac differed inexplicably among heavily oiled 

(42%), lightly oiled (11%), and unoiled, control categories (69%; G[2]=8.67, P=0.02*). The number of 

axial scutes (G[2]=2.92, P=0.24) and inguinal scutes (G[2]=3.40, P=0.20) did not differ among oiling 

categories (see Table 6). The only scute anomalies observed to differ among oiling categories were split 

cervical scutes (G[2]=6.84, P=0.03*), which occurred in 32% of hatchlings at heavily oiled sites, 11% of 

hatchlings at lightly oiled sites, and 5% of hatchlings at control sites. Split cervical scutes occur 

occasionally in terrapins in southern New Jersey that we have had under observation for many years (R. 

C. Wood, unpublished data). No grotesquely deformed specimens were encountered throughout the 

course of our study. 

Nest Excavation Study 

Two heavily oiled sites (Caney Creek and Long Point) and two control sites (Jack Bay and 

Sheridan Point) were excavated. Inclement weather prohibited excavation of any of the other sites. A 

total of 68 different “nests” containing more than 539 eggs and/or hatchlings in different stages of 
 

11

viability and development were excavated (Table 7, Appendix 2). In some cases these groups 

undoubtedly represent complete nests; in other cases, it seemed clear that the group was not a complete 

nest (i. e., a “partial” nest, such as finding a single hatchling). For these reasons, information from nests 

containing one or two eggs and/or hatchlings were excluded from some analyses or the analyses were 

performed on data sets that included and excluded those “partial” nests. 

Including all data, nest densities ranged from one per 100 m2

 at Sheridan Point to 26 per 100 m2 

at Jack Bay. Both of these were non-oiled control sites. Nest densities were relatively high at the two 

heavily oiled sites (approximately 22 per 100 m2 at Caney Creek and 23 per 100 m2 at Long Point). If 

data from “partial” nests are excluded, nest densities ranged from one per 100 m2 at Sheridan Point to 23 

per 100 m2 at Jack Bay; nest densities at the two heavily oiled sites ranged from 16 to 17 per 100 m2 at 

Long Point and Caney Creek respectively. Irrespective of the data used, nest density at three of the four 

sites that we excavated far exceeded the maximum nest density (approximately six nests per 100 m2) 

previously reported for terrapins in the Patuxent River (Roosenburg, 1994). 

The oil contamination and subsequent cleanup did not appear to result in localized effects on 

terrapin nest density within the spill area. Irrespective of the inclusion or exclusion of data from partial 

nests, nest density did not differ between oiled (mean=22.5 vs. 16.5 nests per 100 m2) and unoiled 

(mean=13.5 vs. 12 nests per 100 m2) exclosures (ANOVA, F[1, 2]=0.52, P>0.50 for all data; F[1, 2]=0.16, 

P>0.50 without partial nest data. Similarly, nest density did not differ among sites within oiling levels 

(ANOVA, F[2, 4]=0.53, P>0.50 for all data; F[1, 2]=0.63, P>0.50 without partial nest data). 

 As with nest density, the oil contamination and subsequent cleanup did not appear to result in 

localized effects on terrapin nest size within the spill area. Nest sizes in oiled (mean= 9.2 eggs nest-1, 

SE=0.74) and unoiled, control exclosures (mean = 8.6 eggs nest-1, SE=0.83 eggs) did not differ 

(ANOVA, F(1,59)=0.22, P=0.64). Similarly, nest size did not differ among the four excavated sites 

(ANOVA, F(4,59)=0.66, P=0.62), where average nest sizes ranged from a low of 6 eggs nest-1 at Sheridan 

Point to a high of 10.1 eggs nest-1 at Caney Creek. 

 However, even with data from partial nests not included in the analyses, nest sizes from this 

stretch of the Patuxent River in 2000 (overall mean=8.92, S.E.=0.55, N=60) are significantly smaller 

than the nest sizes reported by Roosenburg (1992) for terrapins in the Patuxent River from 1987 to 1991 
 

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(overall mean= 12.90, S.E. 0.13, N=504; ANOVA, F [5,559]=19.65, P<0.01; Scheffe’s S for 2001 vs. all 

other years =9.62, P<0.01). Our nest size estimates for 2000 did not use all observations from our 

excavations, that is, presumably partial nests consisting of only one or two terrapins and/or eggs were 

not included in these comparisons. 

It is unknown whether nests included in this analysis were complete (because nest size was not 

determined at nesting, but several months later) and whether the low nest size is directly attributable to 

effects of the oil contamination; however, these low nest sizes concern us for several reasons. First, 

selective removal of very small nests (nest size < 3) from our analyses of nest size would not change the 

significance of the differences we observed. Second, we do not believe that the low nest sizes in all 

exclosures are due to partial predation of nests. Previous studies reported considerable variability in 

predation (Roosenburg, 1992) but average nest sizes were low at all excavated sites. We also do not 

believe that the low nest sizes were due to nests being partially washed away. With the exception of one 

nest at Caney Creek (embryo and egg N=8) excavated only 4 to 5 cm below the substrate surface, all 

nests at the four sites were excavated at typical depths (at least 10 to 12 cm) and did not exhibit any 

signs of having been partially washed away (e. g., some eggs visible at the substrate surface). Finally, if 

oil contamination affected adult female terrapins prior to egg-laying, then one would not necessarily 

expect diminished nest size to occur only at nesting sites that were oiled. 

Unfortunately no studies have ever been done on the effects of oil on terrapins (Rattner et al., 

2000). Very little information, in fact, is available about the effects of oil on any kind of turtles. Studies 

of this nature to date have focused on marine turtles (Alam and Brim, 2000; Fritts and McGehee, 1982; 

Geraci and St. Aubin, 1987), which are not closely related to diamondback terrapins (and are therefore 

not necessarily good models for the effects of oil on terrapins). However, those studies indicate that 

oiling of nests can result in embryonic abnormalities and mortality. 

Significant differences in the condition of eggs excavated from nests in exclosures at oiled and 

unoiled (control) sites could be a consequence of oil contamination. Consistent with our previous 

comparison of hatchlings collected in the different exclosures, the frequency of eggs that had apparently 

hatched did not differ between oiled (177 of 311 eggs) and unoiled, control areas (132 of 228 eggs, 
 

13

G[1]=0.05, P>0.50.). However, the frequency of potential spring emergers (viable eggs that would 

presumably result in spring emergers) differed significantly between oiled (50 of 311 eggs) and unoiled 

sites (63 of 228 eggs, G[1]=10.51, P<0.001). This significant difference in the number of spring emergers 

was consistent with the significantly higher numbers of dead embryos at the oiled sites (84 of 311 eggs) 

compared to unoiled, control sites (33 of 228, G [1]= 10.51, P<0.001). 

At least 18 inviable eggs from several different nests at one Caney Creek exclosure (#2) are of 

particular interest. These nests contained the skeletons of late-stage embryos inside well-formed eggs 

that did not hatch. Based upon our extensive experience of rearing thousands of embryos in our 

laboratory hatching program over the past 12 years, mortality of late-stage embryos is rare (R. C. Wood, 

unpublished data). 

The cause of mortality of these embryos is unknown and may be due to many factors other than 

the effects of the embryos’ exposure to residual oil present in sediment after the cleanup. It is possible 

that the embryos might have died as a result of being inundated too frequently by tidal waters. Or, 

conceivably, the embryos might have been killed by excessive thermal stress if their nest chamber 

overheated. This could happen, for example, if wind erosion removed several inches of sand from above 

the nest chambers within which the eggs were laid. However, the summer of 2000 was cool, and there 

was no evidence of erosion at those exclosures. 

 Finally, as part of the effort to assess the impact of the oil spill on terrapin reproductive success 

in the year 2000, it is helpful to keep in mind some of the important population characteristics of these 

turtles. They have long adult lives during which females nest every year, sometimes multiple times, for 

periods of time that may exceed two decades. There is naturally high predation upon their nests and 

undoubtedly upon hatchlings too, although this latter probability has not been sufficiently documented. 

Thus, a typical terrapin population consists of a high proportion of adults and a relatively small number 

of juveniles. There is a very low probability that hatchlings from any annual cohort, even under the best 

of circumstances, will survive to become adults. 
 

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SUMMARY 

Oiling of terrapin nesting sites potentially has a number of impacts upon terrapins. Based on 

comparisons of hatching and hatchlings between variously oiled and unoiled sites, and comparison of 

egg and nest information from the excavation of terrapin nests at heavily oiled and unoiled sites, we 

conclude the following about the impacts of the oil spill upon the diamondback terrapin population(s) in 

the Patuxent River. 

(1) Terrapin nest density on oiled and unoiled beaches downriver from the Chalk Point 

generating station did not differ during the 2000 nesting season. 

(2) The hatching of terrapins in fall (as reflected by number of hatchlings recovered over a twomonth period at all study sites and our assessment of fall hatching from excavations at selected sites) did 

not differ between oiled and unoiled nesting beaches downriver from the Chalk Point generating station. 

(3) Fall hatchlings recovered from variously oiled and unoiled sites were comparable in 

size and weight. Although some minor differences in developmental features of hatchlings at oiled and 

unoiled sites were observed, no consistent or noteworthy developmental abnormalities were observed. 

(4) Our best estimates of nest sizes across oiled and unoiled sites did not differ. On average, 

however, nest sizes in 2000 were significantly lower than the average nest reported by Roosenburg 

(1994) for the years 1987-1991 in that same stretch of the Patuxent River. If real, those differences 

probably do not indicate an effect of oil contamination at nest sites, but more likely represent some 

widespread factor affecting adult female terrapins that may or may not be related to oil contamination. 

(5) Comparison of the condition of eggs recovered at selected oiled and unoiled sites revealed 

 a significantly higher frequency of dead embryos and a lower frequency of presumed spring-emergers 

at oiled sites compared to unoiled sites. The cause of death of those embryos is not known and may not 

necessarily be attributed to the oil spill, but the observed mortality is of concern for two reasons. (A) 

Spring emergers would experience the potential effects of any residual oil contamination for a protracted 
 

15

period compared to fall hatchlings, and (B) based upon our extensive observations of mortality in 

laboratory rearing studies, the high observed mortality of terrapin embryos in late stages of development 

at oiled sites is unusual. 

In summary, results of our study suggest that the oil spill may have contributed to a 

reduction in nest size throughout the area of study and may have increased the mortality of presumed 

spring emergers at oiled sites. Based on the life history characteristics of this species, we believe these 

potential impacts to terrapin reproduction during 2000 are probably minor. We found no evidence of a 

major, catastrophic impact of the oil spill on terrapin reproduction in the Patuxent River. 

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Alam, S. K., M. S Brim. 2000. Organochlorine. PCB, PAH and metal concentrations in eggs of 

loggerhead sea turtles (Caretta caretta) from Northwest Florida, USA. Journal of Environmental 

Science and Health: Part B, Pesticides, Food Contaminants and Agricultural Waste, 35(6), pp. 705-724. 

Duncan,W. and M. 1987. Smithsonian Guide to Seaside Plants of the Gulf and Atlantic Coasts. 

Smithsonian Press, Washington, D. C. 409 pp. 

ENTRIX Inc. 2000. Swanson Creek incident summary of SCAT activities and data management. 

Fritts, T. H. and M. A. McGehee. 1982. Effects of petroleum on the development and survival of marine 

turtle embryos. U. S. Fish and Wildlife Service, Biological Services Program, FWS/OBS-82/37: 41 pp. 

Geraci, J. R., and D. J. St.Aubin. 1987. Effects of offshore oil and gas development on marine mammals 

and turtles. In Long Term Environmental Effects of Oil and Gas Development. Boesch, D. F. and, N. N. 

Rabalais (eds.). London, Elsevier Applied Science, pp. 587-617. 

Herlands, R. L., S. Schnackenberg, and R. C. Wood. 1993. Life after death: conservation of northern 

diamondback terrapins in New Jersey. Proceedings, 49th Annual Northeast Fish and Wildlife 

Conference, pg. 135. 

Jeyasuria, P., W. M. Roosenburg and, A. R. Place. 1994. Role of P-450 aromatase in sex determination 

of the diamondback terrapin, Malaclemys terrapin. The Journal of Experimental Zoology 270, pp. 95-

111. 

Mountford, K. 2000. Diamondback terrapin survey note. Unpublished. US Environmental Protection 

Agency, Chesapeake Bay Program, Note and map. 

Rattner, B.A., N. H. Golden, J. L. Pearson, J. B. Cohen, L. J. Garrett, M. A. Ottinger and, J. B. Erwin. 

2000. Biological and ecotoxicological characteristics of terrestrial vertebrate species residing in 
 

16

estuaries: diamondback terrapin. U. S. Geological Survey. www.pwrc.usgs.gov/bioeco/terrapin.httm. 3 

pp. 

Roosenburg, W. M. 1990. Chesapeake diamondback terrapin investigations. Chesapeake Research 

Consortium, Publication Number 133. 

Roosenburg, W. M. 1991. The diamondback terrapin: population dynamics, habitat requirements, and 

opportunities for conservation. Chesapeake Research Consortium, Publication Number 137, pp. 227-

234. 

Roosenburg, W. M. 1992. Life history consequences of nest site choice by the daiamondback terrapin, 

Malaclemys terrapin. Doctoral dissertation, University of Pennsylvania, 206 pp. 

Roosenburg, W. M. 1994. Nesting habitat requirements of the diamondback terrapin: a geographic 

comparison. Wetland Journal 6(2), pp. 8-11. 

Roosenburg, W. M. 1996. Maternal condition and nest site choice: an alternative for the maintenance of 

environmental sex determination? American Zoologist 36, pp. 157-168. 

Roosenburg, W. M. and, A. E. Dunham. 1997. Allocation of reproductive output: egg- and clutch-size 

variation in the diamondback terrapin. Copeia 1997(2), pp. 290-297. 

Roosenburg, W. M. and, K. C. Kelley. 1996. The effect of egg size and incubation temperature on 

growth in the turtle, Malaclemys terrapin. Journal of Herpetology, 30( 2), pp. 198- 204. 

Roosenburg, W. M. and, A. R Place. 1994. Nest predation and hatchling sex ratio in the diamondback 

terrapin: implications for management and conservation. Chesapeake Research Consortium, Publication 

Number 149. pp. 65-70. 

Silberhorn, G. 1999. Common plants of the Mid-Atlantic Coast: A Field Guide, Revised Ed., JHU Press, 

294 p 

Zar, J. H. 1984. Biostatistical analysis, 2nd ed. Prentice Hall, Englewood Cliffs, New Jersey, 716 pp. 
TABLES, FIGURES AND APPENDICES
 

 

ii

Table 1. Summary of published information about the terrapin population in the Patuxent River. All 

information is taken from Roosenburg, 1990, 1991, 1992, 1994, 1996; Roosenburg and Place 1994; and 

Jeyasuria et al. 1994. 

 

Estimated size of population within study area: between 1700 and 2900 adults 

Nesting season: roughly from the beginning of June through the end of July 

Number of eggs per nest (a nest = all the eggs laid in a single nest): 

 7 to 22 (mean = 13) 

Number of nestes per female per nesting season: 1 to 3 

Length of egg incubation: minimum number of days = 56; shaded nests take longer (80-110 days); some 

hatchlings are reported to overwinter in their nests 

Number of hatchlings produced per successful nest: 3.4 to 10.75 

Known nest predators: 

 raccoons (responsible for 59 to 70 % of all nest predation); 

 foxes (5 to 9 %); 

 other mammal species (1 to 3 %); 

 unknown: (20 % or more) 

Effect of natural predation: 55 to 95 % of all nests destroyed 

 
 

 

iii

 

Table 2. Dimension (m x m), area (m2), and vegetated area (area in m2) and % vegetation (%Veg) of the 

terrapin exclosures at each site for each oiling category. NR indicates non-rectangular shape. 1 indicates that 

assignment of the site to this oiling category was changed due to modification of the oiling category of another 

site. 2 indicates the original oiling level was changed from heavy to very light, which changed the grouping of 

sites in the oiled categories. 

 

Oiling category & site Dimension Area Vegetated Area % Veg 

Heavily (h) to moderately (m) oiled sites

 Caney Creek 1(h) 2.5 X 20.0 50.0 1.6 3.2 

 Caney Creek 2(h) NR 37.8 1.9 5.0 

Long Point 1 (h) NR 75.0 18.8 25.0 

Long Point 2 (h) 5.0 X 10.0 50.0 37.5 75.0 

Hoyer Farm 11

 (m) 5.0 X 10.0 50.0 34.9 69.8 

Hoyer Farm 21 (m) 2.5 X 20.0 50.0 37.5 75.0 

Lightly (l) to very lightly (vl) oiled sites

Grandfather's Pond 1 (lt) 5.0 X 10.0 50.0 37.5 75.0 

Grandfather's Pond 2 (lt) 5.0 X 10.0 50.0 37.5 75.0 

Trent Hall 1 (lt) 2.5 X 20.0 50.0 37.5 75.0 

Trent Hall 2 (lt) 2.5 X 20.0 50.0 37.5 75.0 

Playground 12 (vl) 5.0 X 10.0 50.0 35.6 71.3 

Playground 22 (vl) 5.0 X 10.0 50.0 32.8 65.6 

Non-oiled, control (c) sites

Prison Point 1 (c) 7.5 X 7.5 56.3 28.1 50.0 

Prison Point 2 (c) 7.5 X 6.0 45.0 22.5 50.0 

Sheridan Point 1 (c) 5.0 X 10.0 50.0 26.8 53.6 

Sheridan Point 2 (c) 5.0 X 10.0 50.0 36.8 73.5 

 

Jack Bay 1 (c) 5.0 X10.0 50.0 21.2 42.3 

Jack Bay 2 (c) NR 28.8 8.6 29.9 

 
 

 

iv

 

Table 3. Numbers of hatchlings of diamondback terrapins (n) collected from each exclosure during the nesting 

study at each site for each oiling category. Summary statistics (mean ± 2 standard errors, and the total size [N] 

of the catch) for each oiling category are also provided. Other abbreviations as in Table 2. 

 

Heavily (h) to moderately (m) oiled sites (5.5± 5.7, N=33) n

 Caney Creek 1 (h) 7 

 Caney Creek 2 (h) 19 

Long Point 1 (h) 0 

Long Point 2 (h) 2 

Hoyer Farm 11 (m) 2 

Hoyer Farm 21 (m) 3 

Lightly (lt) to very lightly (vl) oiled sites (1.5± 1.8, N=9)

Grandfather's Pond 1 (lt) 0 

Grandfather's Pond 2 (lt) 2 

Trent Hall 1 (lt) 0 

Trent Hall 2 (lt) 0 

Playground 12 (vl) 2 

Playground 22 (vl) 5 

Non-oiled, control (c) sites (3.5±6.2, N=21)

Prison Point 1 (c) 0 

Prison Point 2 (c) 0 

Sheridan Point 1 (c) 1 

Sheridan Point 2 (c) 1 

 

Jack Bay 1 (c) 19 

Jack Bay 2 (c) 0 

 
 

 

v

Table 4. Collection data (terrapin ID #, date, oil category [Oil Cat], Site, exclosure number [Excl]) and 

measurements (carapace length [CL], plastron length [PL], maximum width [Wth], maximum height [Ht], and 

weight [Wt]) of terrapins collected in each exclosure. Measurements in cm unless noted. 

# Date Oil Cat Site Excl CL PL Wth Ht Wt (g)

1 9/10/00 Control Jack Bay 1 3.00 2.56 

2 9/10/00 Control Sheridan Point 2 3.14 2.73 

3 9/12/00 Control Jack Bay 1 2.89 2.40 

4 9/13/00 Heavy Caney Creek 1 3.00 2.80 

5 9/13/00 Heavy Caney Creek 1 3.00 2.80 

6 9/16/00 Control Jack Bay 1 3.26 

7 9/19/00 Control Jack Bay 1 2.97 2.61 2.71 

8 9/21/00 Heavy Hoyer Farm 1 2.95 2.44 2.39 1.40 5.90 

9 9/21/00 Heavy Hoyer Farm 2 3.12 2.69 2.83 1.63 7.40 

10 9/22/00 Control Jack Bay 1 2.79 2.35 2.43 1.51 5.40 

11 9/22/00 Heavy Hoyer Farm 2 2.97 2.52 2.57 1.45 7.50 

12 9/22/00 Heavy Hoyer Farm 2 3.08 2.79 2.59 1.54 7.60 

13 9/22/00 Heavy Long Point 2 2.52 2.21 2.30 1.30 4.00 

14 9/22/00 Heavy Caney Creek 2 2.97 2.71 2.64 1.55 7.50 

15 9/22/00 Heavy Caney Creek 2 3.08 2.73 2.60 1.59 7.60 

16 9/22/00 Heavy Caney Creek 2 2.95 2.35 2.66 1.63 6.10 

17 9/22/00 Heavy Caney Creek 2 2.91 2.62 2.47 1.63 6.20 

18 9/22/00 Heavy Caney Creek 2 3.04 2.64 2.58 1.64 7.60 

19 9/22/00 Heavy Caney Creek 2 3.02 2.60 2.49 1.78 7.60 

20 9/22/00 Heavy Caney Creek 2 2.97 2.65 2.73 1.74 7.80 

21 9/25/00 Heavy Caney Creek 2 3.06 2.69 2.67 1.61 7.80 

22 9/25/00 Heavy Caney Creek 2 3.21 2.73 2.58 1.66 8.50 

23 9/25/00 Heavy Caney Creek 2 3.08 2.58 2.51 1.58 7.80 

24 9/25/00 Heavy Caney Creek 2 2.99 2.59 2.42 1.56 7.00 

25 9/25/00 Heavy Caney Creek 2 3.16 2.85 2.65 1.67 8.60 

26 9/25/00 Heavy Caney Creek 2 3.12 2.81 2.49 1.66 8.00 

27 9/25/00 Heavy Caney Creek 2 3.23 2.94 2.69 1.68 9.30 

28 9/25/00 Light Grndfthr Pnd 2 3.31 2.86 2.40 1.66 7.80 

29 9/28/00 Light Playground 1 2.83 2.48 2.60 1.49 5.90 

30 9/28/00 Light Playground 2 2.86 2.49 2.37 1.48 5.90 

31 9/29/00 Control Jack Bay 1 2.94 2.42 2.63 1.56 7.00 

32 9/30/00 Heavy Caney Creek 2 2.99 2.62 2.64 1.51 4.70 

33 9/30/00 Light Grndfthr Pnd 2 3.24 2.75 2.81 1.58 7.40 

34 10/5/00 Control Jack Bay 1 2.89 2.53 2.53 1.69 6.80 

35 10/6/00 Control Jack Bay 1 2.97 2.66 2.60 1.79 7.80 

36 10/12/00 Heavy Long Point 2 2.93 2.50 2.50 1.51 5.70 

37 10/13/00 Control Jack Bay 1 2.95 2.48 2.48 1.68 6.40 

38 10/18/00 Control Sheridan Point 1 3.04 2.56 2.68 1.49 6.30 

39 10/19/00 Light Playground 2 3.08 2.63 2.68 1.54 7.10 

40 10/19/00 Light Playground 2 2.98 2.63 2.46 1.58 7.30 

41 10/19/00 Heavy Caney Creek 2 2.91 2.30 2.30 1.42 5.90 
 

 

vi

Table 4 continued. 

# Date Oil Cat Site Excl CL PL Wth Ht Wt (g)

42 10/20/00 Heavy Caney Creek 2 2.97 2.54 2.49 1.46 7.00 

43 10/24/00 Control Jack Bay 1 2.88 2.50 2.61 1.54 7.20 

44 10/24/00 Control Jack Bay 1 2.91 2.50 2.45 1.53 6.90 

45 10/24/00 Control Jack Bay 1 2.68 2.23 2.16 1.41 4.50 

46 10/24/00 Control Jack Bay 1 3.03 2.71 2.49 1.60 7.40 

47 10/24/00 Control Jack Bay 1 3.00 2.63 2.47 1.64 7.50 

48 10/24/00 Control Jack Bay 1 3.04 2.81 2.50 1.73 7.90 

49 10/24/00 Heavy Caney Creek 2 3.14 2.72 2.60 1.61 7.30 

50 10/24/00 Light Playground 1 3.11 2.74 2.63 1.45 6.30 

51 10/25/00 Control Jack Bay 1 2.78 2.35 2.34 1.52 6.30 

52 10/25/00 Heavy Caney Creek 1 2.94 2.68 2.57 1.45 5.90 

53 10/25/00 Heavy Caney Creek 1 3.03 2.66 2.62 1.71 7.60 

54 10/25/00 Heavy Caney Creek 1 2.93 2.53 2.57 1.53 5.50 

55 10/25/00 Heavy Caney Creek 1 3.00 2.61 2.49 1.64 6.00 

56 10/25/00 Heavy Caney Creek 1 3.17 2.80 2.76 1.67 8.20 

57 10/27/00 Control Jack Bay 1 3.05 2.65 2.74 1.69 7.60 

58 10/29/00 Light Playground 2 3.15 2.65 2.80 1.57 6.50 

59 10/30/00 Heavy Hoyer Farm 1 3.10 2.62 2.72 1.57 8.40 

60 10/30/00 Light Playground 2 3.21 2.89 2.67 1.57 9.50 

61 11/1/00 Heavy Caney Creek 2 3.02 2.64 2.59 1.62 6.90 

62 11/3/00 Control Jack Bay 1 2.98 2.62 2.45 1.56 7.20 

 
 

 

vii

 

Table 5. Summary statistics (mean ± 2 SE, sample size) of all terrapin measurements (carapace 

length [CL], plastron length [PL], maximum width [MW], maximum height [MH], and weight 

[Wt]) by oiling category and by exclosure. Some measurements were not made on a limited 

number of terrapins early in the experiment. All length measurements are given in centimeters 

and were made to the nearest mm; weight was measured to the nearest 0.1 g. (- indicates that no 

terrapins were collected in that exclosure) 

Oiling category & site CL PL MW MH Wt

Heavy (h) to moderate (m) 3.02 ± 0.044, 33 2.64 ± 0.054, 33 2.57 ± 0.044, 31 1.58 ± 0.38, 31 7.06 ± 0.43, 31 

 Caney Creek (h) 3.01 ± 0.060, 7 2.70 ± 0.082, 7 2.60 ± 0.090, 5 1.60 ± 0.096, 5 6.64 ± 1.06, 5 

 Caney Creek (h) 3.04 ± 0.044, 19 2.65 ± 0.070, 19 2.57 ± 0.048, 19 1.61 ± 0.040, 19 7.33 ± 0.49, 19 

Long Point (h) - - - - - 

Long Point (h) 2.73 ± 0.41, 2 2.36 ± 0.29, 2 2.40 ± 0.20, 2 1.41 ± 0.21, 2 4.85 ± 1.70, 2 

Hoyer Farm (m) 3.03 ± 0.15, 2 2.53 ± 0.18, 2 2.56 ± 0.33, 2 1.49 ± 0.17, 2 7.15 ± 2.50, 2 

Hoyer Farm (m) 3.06 ± 0.09, 3 2.67 ± 0.16, 3 2.66 ± 0.17, 3 1.54 ± 0.10, 3 7.50 ± 0.12, 3 

Light (l) to very light (vl) 3.09 ± 0.11, 9 2.68 ± 0.11, 9 2.60 ± 0.011, 9 1.55 ± 0.042, 9 7.08 ± 0.75, 9 

Grandfather's Pond (l) - - - - - 

Grandfather's Pond (l) 3.28 ± 0.07, 2 2.81 ± 0.11, 2 2.61 ± 0.41, 2 1.62 ± 0.080, 2 7.60 ± 0.40, 2 

Trent Hall (l) - - - - - 

Trent Hall (l) - - - - - 

Playground (vl) 2.97 ± 0.28, 2 2.61 ± 0.26, 2 2.62 ± 0.030, 2 1.47 ± 0.040, 2 6.10 ± 0.40, 2 

Playground (vl) 3.06 ± 0.12, 5 2.66 ± 0.13, 5 2.60 ± 0.16, 5 1.55 ± 0.036, 5 7.26 ± 1.22, 5 

Non-oiled, control (c) 2.96 ± 0.058, 20 2.55 ± 0.068, 20 2.53 ± 0.072, 17 1.60 ± 0.054,15 6.81 ± 0.48,15 

Prison Point (c) - - - - - 

Prison Point (c) - - - - - 

Sheridan (c) 3.04, 1 2.56, 1 2.68, 1 1.49, 1 6.30, 1 

Sheridan (c) 3.14, 1 2.73, 1 - - - 

 

Jack Bay (c) 2.95 ± 0.060,18 2.54 ± 0.074, 18 2.52 ± 0.074, 16 1.60 ± 0.056, 14 6.85 ± 0.51,14 

Jack Bay (c) - - - - - 

 
 

 

viii

 

Table 6. Frequency of occurrence (first number) and number of terrapins (second number) examined for 

various developmental features (egg tooth [ET], yolk sac [YS], occurrence of axial scutes [AS], occurrence of 

inguinal scutes [IS], and split cervical scutes [CS]) of terrapins by oiling category and by exclosure. The mean 

number ([AS(#)] ± 2 SE) of axial scutes is also indicated. (- indicates that no terrapins were collected in that 

exclosure; other abbreviations as in Table 2.) 

 

Oiling category & site ET (%) YS (%) AS (%) AS (#) IS (%) CS (%)

Heavily & moderately oiled 74, 31 42, 31 84, 31 1.55±0.28 10, 31 32, 31 

 Caney Creek 1 (h) 100, 5 20, 5 100, 5 1.80±0.40 0, 5 40, 5 

 Caney Creek 2 (h) 84, 19 63, 19 95, 19 1.74±0.26 11, 19 42, 19 

Long Point 1 (h) - - - - - - 

Long Point 2 (h) 50, 2 0, 2 50, 2 1.00±1.00 50, 2 0, 2 

Hoyer Farm 1 (m) 50, 2 0, 2 50, 2 1.00±1.00 0, 2 0, 2 

Hoyer Farm 2 (m) 0, 3 0, 3 33, 3 0.67±1.33 0, 3 0, 3 

Lightly to very lightly oiled 44, 9 11, 9 89, 9 1.78±0.44 11, 9 11, 9 

Grndfthr's Pond 1 (lt) - - - - - - 

Grndfthr's Pond 2 (lt) 50, 2 0, 2 100, 2 2.00 0, 2 0, 2 

Trent Hall 1 (lt) - - - - - - 

Trent Hall 2 (lt) - - - - - - 

Playground 1 (vl) 50, 2 50, 2 100, 2 2.00 0, 2 50, 2 

Playground 2 (vl) 40, 5 0, 5 80, 5 1.60±0.80 20, 5 0, 5 

Non-oiled, control (c) 80, 15 69, 16 65, 17 1.29±0.48 0, 20 5, 20 

Prison Point 1 (c) - - - - - - 

Prison Point 2 (c) - - - - - - 

Sheridan Point 1 (c) 0, 1 0, 1 100, 1 2.00 0, 1 0, 1 

Sheridan Point 2 (c) - - - - 0, 1 0, 1 

 

Jack Bay 1 (c) 85, 14 73, 15 63, 16 1.25±0.50 0, 18 6, 

18 

Jack Bay 2 (c) - - - - - - 

 
 

 

ix

 

Table 7. Number of excavated nests (# Nests), number of recovered eggs (# Eggs, with mean nest size ± 2 SE 

in parentheses), and the number of all recovered eggs and/or egg remains in different conditions (hatched [H], 

viable [V], and inviable [dead, D) recovered from those excavations at two non-oiled, control sites and two 

heavily oiled sites. The number of eggs equals the sum of the number hatched, the number viable, and the 

number inviable (# Eggs= H + V + D). * indicates that nest size could not be determined from three nests; thus 

data from only 15 nests were used to compute averages. 

Heavily oiled sites # Nests # Eggs # Hatched # Viable # Dead

 Caney Creek 1 0 0 0 0 0 

 Caney Creek 2 18* 135 (9.0 ± 2.77) 69 48 18 

 Long Point 1 9 55 (6.11 ± 2.72) 43 2 10 

 Long Point 2 14 121 (8.64 ± 2.22) 65 0 56 

Non-oiled, control sites

 Sheridan Point 1 1 6 6 0 0 

 Sheridan Point 2 0 0 0 0 0 

 

 

 Jack Bay 1 26 227 (8.73 ± 1.71) 126 63 33 

 Jack Bay 2 0 0 0 0 0 

 
 

 

x

$

2

$ 4

#

3

$ 5 %

7

%

8

%9

Study Sites

#

Playground

Caney Creek

Long Point

Grandfather's Pond

Trent Hall 

Hoyer Farm

Sheridan Point

Prison Point

Jack Bay

$

#

$T

$T

#

%

%

%

1.

2.

3.

4.

5.

6.

7.

8.

9.

Figure 1: Location of nine study sites to assess the hatching 

success of northern diamondback terrapins along the Patuxent River.

N

#S

1 # Heavily/Moderately Oiled

$ Lightly/Very Lightly Oiled

% Non-Oiled

Patuxent River

Swanson 

Creek

0 2 4 6 Kilometers

Rte. 231

Benedict

#6

N

Chalk 

Point
 

 

xi

Figure 2. Construction of terrapin enclosures, September, 2000. (a) Installation of a wooden frame at the 

Hoyer Farm site for the 50m2 exclosure. (b) Attaching vertical panel of 1.5 cm2 mesh plastic fencing at the 

corner of the wooden frame. The plastic fencing is attached with plastic cable ties and metal staples, and is 

buried 15 to 20 cm into the ground to discourage potential predators. (c) Close-up view showing the border of 

an exclosure with 2.5 cm2 mesh plastic deer cloth attached. (d) A completed exclosure at the Playground site. 
 

 

xii

Figure 3. Excavation of exclosures, December, 2000. (a) Edge of a terrapin nest at Jack Bay with one egg 

exposed. In the months subsequent to the deposition of eggs into this nest, shifting sands have roughly doubled 

the thickness of sand overlying the nest. (b) Four terrapin eggs removed from a nest at Jack Bay. 

 
 

 

xiii

Appendix 1. Soil temperature (degrees Celsius) collected inside (temp in) and outside (temp out) exclosures 

_______________________________________________________________________________________ 

Date Time Location Temp in (C) Temp out (C)

9/28/00 1310 Caney Creek 1 22 22 

9/27/00 1120 Hoyer Farm 1 17 17 

9/27/00 1120 Hoyer Farm 2 19 19 

9/27/00 900 Jack Bay 1 14 14 

9/27/00 900 Jack Bay 2 14 14 

9/27/00 945 Prison Point 1 15.5 15.5 

9/27/00 945 Prison Point 2 15.5 15.5 

9/27/00 1030 Sheridan point 1 16.5 16 

9/27/00 1030 Sheridan point 2 16 15 

9/27/00 1145 Trent Hall 1 20 21 

9/27/00 1145 Trent Hall 2 20 21 

9/28/00 1310 Caney Creek 2 22 22 

9/28/00 1235 Grandfather's pond 1 24 24 

9/28/00 1235 Grandfather's pond 2 24 24 

9/28/00 1210 Long Point 1 22 22 

9/28/00 1210 Long Point 2 24 25 

9/28/00 1145 Playground 1 23 22 

9/28/00 1145 Playground 2 23 22 

9/30/00 1010 Caney Creek 2 15 16 

9/30/00 945 Grandfather's pond 2 16 17 

10/2/00 930 Caney Creek 1 12 12 

10/2/00 930 Caney Creek 2 12 12 

10/2/00 1230 Jack Bay 1 17 17 

10/2/00 1230 Jack Bay 2 20 20 

10/2/00 1150 Prison Point 1 22.5 23 

10/2/00 1150 Prison Point 2 22.5 23 

10/2/00 1130 Sheridan point 1 21 22 

10/2/00 1130 Sheridan point 2 20 20 

10/2/00 1100 Trent Hall 1 21 21 

10/2/00 1100 Trent Hall 2 21 21 

10/3/00 1030 Grandfather's pond 1 15.5 16 

10/3/00 1030 Grandfather's pond 2 15.5 16 

10/3/00 1015 Long Point 1 15 14 

10/3/00 1015 Long Point 2 14.5 15 

10/3/00 1000 Playground 1 14 14 

10/3/00 1000 Playground 1 14 14 

10/6/00 1035 Jack Bay 1 15.5 15.5 

10/9/00 1015 Caney Creek 1 8 7.5 

10/9/00 1200 Grandfather's pond 1 12 12 

10/9/00 1200 Grandfather's pond 2 12 12 

10/9/00 1230 Hoyer Farm 1 12 12 

10/9/00 1230 Hoyer Farm 2 11 10 

10/9/00 1200 Jack Bay 1 7 7 

10/9/00 1130 Long Point 1 9.5 8.5 

10/9/00 1130 Long Point 2 10 10 

Appendix 1 continued. 

Date Time Location Temp in (C) Temp out (C)
 

 

xiv

10/9/00 1330 Sheridan point 1 12 12 

10/9/00 1330 Sheridan point 2 12 12 

10/9/00 1040 Playground 1 9 9 

10/9/00 1040 Playground 2 9 9 

10/9/00 1300 Trent Hall 1 14 15 

10/9/00 1300 Trent Hall 2 14 15 

10/10/00 1025 Jack Bay 1 7 7 

10/10/00 1110 Jack Bay 2 8 7.5 

10/10/00 1110 Jack Bay 2b 7 7 

10/10/00 1300 Prison Point 1 11 11 

10/10/00 1300 Prison Point 2 11 11 

10/12/00 1415 Long Point 2 17 18 

10/13/00 1245 Jack Bay 1 12 11 

10/16/00 1450 Caney Creek 1 15 14 

10/16/00 1450 Caney Creek 2 15 14 

10/16/00 1335 Grandfather's pond 1 18 17 

10/16/00 1335 Grandfather's pond 2 18 17 

10/16/00 1400 Hoyer Farm 1 14 14 

10/16/00 1410 Hoyer Farm 2 16.5 15 

10/16/00 1030 Jack Bay 1 11 11.5 

10/16/00 1030 Jack Bay 2 11 11 

10/16/00 1030 Jack Bay 2b 11 11 

10/16/00 1510 Long Point 1 18.5 16 

10/16/00 1450 Long Point 2 15.5 17 

10/16/00 1135 Prison Point 1 13.5 13 

10/16/00 1135 Prison Point 2 13.5 13 

10/16/00 1220 Sheridan point 1 14 13 

10/16/00 1220 Sheridan point 2 13.5 13.5 

10/16/00 1525 Playground 1 16 16 

10/16/00 1525 Playground 2 16 16 

10/16/00 1335 Trent Hall 1 15 15 

10/16/00 1335 Trent Hall 2 15 15 

10/18/00 1320 Sheridan point 1 14.5 14.5 

10/19/00 1240 Caney Creek 2 15 15 

10/19/00 1140 Playground 2 13 13 

10/20/00 1430 Caney Creek 2 10 10 

10/24/00 1305 Caney Creek 1 11 11 

10/24/00 1305 Caney Creek 2 11 11 

10/24/00 1435 Grandfather's pond 1 15 13 

10/24/00 1435 Grandfather's pond 2 15 13 

10/24/00 1230 Hoyer Farm 1 10 10 

10/24/00 1230 Hoyer Farm 2 11.5 11.5 

10/24/00 1130 Jack Bay 1 9.5 10 

10/24/00 1110 Jack Bay 2 10 10 

10/24/00 1110 Jack Bay 2b 10 10 

10/24/00 1455 Long Point 1 15.5 13.5 

 

Appendix 1 continued. 

Date Time Location Temp in (C) Temp out (C)

10/24/00 1445 Long Point 2 15 15 
 

 

xv

10/24/00 1100 Prison Point 1 10 10 

10/24/00 1100 Prison Point 2 10 10 

10/24/00 1040 Sheridan point 1 9.5 9.5 

10/24/00 1040 Sheridan point 2 10.5 10.5 

10/24/00 1515 Playground 1 15 14 

10/24/00 1515 Playground 2 15 14 

10/24/00 1245 Trent Hall 1 15 15 

10/24/00 1245 Trent Hall 2 15 16 

10/25/00 1330 Caney Creek 1 12 12 

10/25/00 1140 Jack Bay 1 10 10 

10/27/00 1300 Jack Bay 1 12 12 

10/29/00 1110 Playground 2 9 6 

10/30/00 1415 Grandfather's pond 1 12 13 

10/30/00 1415 Grandfather's pond 2 12 13 

10/30/00 1300 Hoyer Farm 1 11 10 

10/30/00 1300 Hoyer Farm 2 11 10 

10/30/00 1205 Jack Bay 1 7 6.5 

10/30/00 1145 Jack Bay 2 8 7 

10/30/00 1430 Long Point 1 6.5 7 

10/30/00 1440 Long Point 2 11 12 

10/30/00 1100 Prison Point 1 8.5 6.5 

10/30/00 1100 Prison Point 2 8.5 6.5 

10/30/00 1015 Sheridan point 1 9 8 

10/30/00 1015 Sheridan point 2 7 6 

10/30/00 1355 Playground 1 10 9 

10/30/00 1355 Playground 1 10 9 

10/30/00 1320 Trent Hall 1 11.5 13.5 

10/30/00 1320 Trent Hall 2 11.5 13.5 

11/1/00 1345 Caney Creek 2 8 7 

11/3/00 1210 Jack Bay 1 10 10 

11/8/00 1345 Caney Creek 1 8.5 8 

11/8/00 1345 Caney Creek 2 8.5 8 

11/8/00 1100 Grandfather's pond 1 11 11 

11/8/00 1100 Grandfather's pond 2 11 11 

11/8/00 1130 Hoyer Farm 1 8.5 8.5 

11/8/00 1130 Hoyer Farm 2 8.5 8.5 

11/8/00 1245 Jack Bay 1 8.5 8.5 

11/8/00 1245 Jack Bay 2 7.5 8.5 

11/8/00 1320 Long Point 1 9 8 

11/8/00 1320 Long Point 2 11 11.5 

11/8/00 1220 Prison Point 1 7 7.5 

11/8/00 1220 Prison Point 2 7 7.5 

11/8/00 1200 Sheridan point 1 9.5 9.5 

11/8/00 1200 Sheridan point 2 9.5 9.5 

11/8/00 1115 Trent Hall 1 9 9 

11/8/00 1115 Trent Hall 2 9 9 

Appendix 2. Information collected from excavated exclosures: date, site, identification of individual nests, 

numbers of eggs in each nest (Egg N), numbers of terrapins that hatched (H), numbers of viable embryos (V/L), 

and numbers of inviable embryos and/or remains of dead terrapins (I/D). P* indicates that the number of eggs 

could not be determined and could not be included in determination of average clutch size. 
 

 

xvi

Index # Date Location Nest # Egg N H V/L I/D 

1 12/7/00 Caney Creek 2 1 P* 

2 12/7/00 Caney Creek 2 2 P* 

3 12/7/00 Caney Creek 2 3 16 0 14 2 

4 12/7/00 Caney Creek 2 4 4 4 0 0 

5 12/7/00 Caney Creek 2 5 1 0 0 1 

6 12/7/00 Caney Creek 2 6 P* 

7 12/7/00 Caney Creek 2 7 9 9 0 0 

8 12/7/00 Caney Creek 2 8 5 3 0 2 

9 12/7/00 Caney Creek 2 9 2 2 0 0 

10 12/7/00 Caney Creek 2 10 16 0 16 0 

11 12/7/00 Caney Creek 2 11 12 10 0 2 

12 12/7/00 Caney Creek 2 12 14 8 0 6 

13 12/7/00 Caney Creek 2 13 8 0 4 4 

14 12/8/00 Caney Creek 2 1 12 12 0 0 

15 12/8/00 Caney Creek 2 2 4 4 0 0 

16 12/8/00 Caney Creek 2 3 16 1 14 1 

17 12/8/00 Caney Creek 2 4 4 4 0 0 

18 12/8/00 Caney Creek 2 5 12 12 0 0 

19 12/9/00 Long Point 1 1 12 12 0 0 

20 12/9/00 Long Point 1 2 9 9 0 0 

21 12/9/00 Long Point 1 3 5 5 0 0 

22 12/9/00 Long Point 1 4 2 2 0 0 

23 12/9/00 Long Point 1 5 12 0 2 10 

24 12/9/00 Long Point 1 6 2 2 0 0 

25 12/9/00 Long Point 1 7 3 3 0 0 

26 12/10/00 Long Point 1 1 3 3 0 0 

27 12/10/00 Long Point 1 2 7 7 0 0 

28 12/11/00 Long Point 2 1 6 4 0 2 

29 12/11/00 Long Point 2 2 15 2 0 13 

30 12/11/00 Long Point 2 3 8 0 0 8 

31 12/11/00 Long Point 2 4 5 4 0 1 

32 12/11/00 Long Point 2 5 10 9 0 1 

33 12/11/00 Long Point 2 6 1 0 0 1 

34 12/11/00 Long Point 2 7 6 6 0 0 

35 12/11/00 Long Point 2 8 11 0 0 11 

36 12/12/00 Long Point 2 1 17 15 0 2 

37 12/12/00 Long Point 2 2 10 3 0 7 

38 12/12/00 Long Point 2 3 5 0 0 5 

39 12/12/00 Long Point 2 4 8 4 0 4 

40 12/12/00 Long Point 2 5 11 10 0 1 

41 12/12/00 Long Point 2 6 8 8 0 0 

42 12/14/00 Sheridan Point 1 6 6 0 0 

43 12/18/00 Jack Bay 1 1 4 0 2 2 

Index # Date Location Nest# Egg N H V/L I/D

44 12/18/00 Jack Bay 1 2 5 5 0 0 

45 12/18/00 Jack Bay 1 3 14 2 9 0 

46 12/18/00 Jack Bay 1 4 6 6 0 0 

47 12/18/00 Jack Bay 1 5 3 3 0 0 
 

 

xvii

48 12/18/00 Jack Bay 1 6 10 10 0 0 

49 12/18/00 Jack Bay 1 7 13 13 0 0 

50 12/18/00 Jack Bay 1 8 12 7 5 0 

51 12/18/00 Jack Bay 1 9 10 2 8 0 

52 12/18/00 Jack Bay 1 10 20 20 0 0 

53 12/18/00 Jack Bay 1 11 12 2 9 1 

54 12/18/00 Jack Bay 1 12 3 3 0 0 

55 12/18/00 Jack Bay 1 13 12 12 0 0 

56 12/18/00 Jack Bay 1 14 9 1 7 1 

57 12/18/00 Jack Bay 1 15 11 2 7 2 

58 12/18/00 Jack Bay 1 16 4 4 0 0 

59 12/18/00 Jack Bay 1 17 10 1 4 5 

60 12/18/00 Jack Bay 1 18 13 0 12 1 

61 12/18/00 Jack Bay 1 19 11 11 0 0 

62 12/19/00 Jack Bay 1 1 12 4 0 8 

63 12/19/00 Jack Bay 1 2 4 4 0 0 

64 12/19/00 Jack Bay 1 3 6 6 0 0 

65 12/19/00 Jack Bay 1 4 3 0 0 3 

66 12/19/00 Jack Bay 1 5 5 5 0 0 

67 12/19/00 Jack Bay 1 6 5 5 0 0 

68 12/19/00 Jack Bay 1 7 10 0 0 10 
1

September 7, 2001 

Norman Meade 

NOAA Damage Assessment Center (N/ORR3) 

1305 East-West Highway 

Room 10357 

Silver Spring, MD 20910 

Dear Norman: 

 The following are my comments on "Comparison of northern diamondback terrapin 

(Malaclemys terrapin terrapin) hatching success among variably oiled nesting sites along the 

Patuxent River following the Chalk Point Oil Spill of April 7, 2000" by authors anonymous to 

me. 

I would also like to take this opportunity to comment on the highly professional manner 

in which the review process itself was handled by you and others involved in the transfer of 

materials and the giving of instructions for the review. Objectivity and impartiality were 

maintained at all levels. If any of the parties involved wanted a supportive review or a dissenting 

one, it was not revealed to me. Hence, my comments are as objective as I can be based only on 

the written report that I received. 

My review of the report ("Comparison of northern diamondback terrapin (Malaclemys terrapin 

terrapin) hatching success among variably oiled nesting sites along the Patuxent River following 

the Chalk Point Oil Spill of April 7, 2000") addresses the following queries: 

1. Are the methods used appropriate for undertaking the stated goals of the study? 

2. Were the methods employed properly implemented? 

3. Are the conclusions adequately supported by the data and the statistical analyses? 

In addition, comments are given regarding specific statements in the report that could be made 

more succinctly, might be confusing, or that would benefit from editorial modification. 

General Statement Regarding Study and Report: 

 Overall, the study appears to have used a satisfactory approach and methodologies for 

determining "the hatching success of terrapins at nesting beaches that were variously oiled 

compared to beaches that were not oiled . . . for assessing the magnitude and extent of injury to 

the year 2000 hatchling cohort" in consideration of the constraints on the timing of when the 

study was initiated. The report gives an understandable presentation of the approach and most of 

the findings, interpretations, and conclusions seem reasonable. 
2

Answers to Specific Questions: 

1. Are the methods used appropriate for undertaking the stated goals of the study? 

 In consideration of the timing of the study (approximately five months after the oil spill), 

using exclosures to locate hatchings in the early fall and excavating enclosures during late fall 

were reasonable approaches for assessing the impact of the oil spill on reproductive success of 

nests during the first year. The Objective section of the report states that the emphasis of the 

survey was on the 2000 hatchling cohort, so assessing hatchling success was appropriate since 

there was no opportunity to observe the behavioral responses of females to nesting beaches 

within the spill zone or to determine comparatively whether females in the spill zone showed 

different clutch, nesting, or other reproductive patterns than females from non-spill areas. The 

plan to search for previously hatched terrapins and for overwintering nests by excavating 

enclosures was a reasonable extension of the exclosure comparisons, in order to obtain a more 

accurate estimate of reproductive output by female terrapins the previous nesting season. 

2. Were the methods employed properly implemented? 

 Based on the seasonal timing (late summer/autumn after many hatchlings would have 

departed nests), the selection of nesting beaches and the construction of exclosures were 

apparently completed in less than a month's time. Continuing the surveys for hatchlings until 

November 10 following a week with no hatchlings being found was rational. Thus, the study 

plan of using exclosures appears to have been implemented in a reasonably efficient manner. The 

recording of hatchling emergences and the body measurements taken also appear to have been 

done properly. The plan to excavate at each of the nine study sites was thwarted by cold weather, 

but the alternate plan at least provided a comparison between two sites receiving heavy oil and 

two control sites without oil. The comparison, therefore, provided a complement to the exclosure 

studies for 4 of the 9 sites and permitted a comparison between extreme sites in regard to the oil 

spill. 

3. Are the conclusions adequately supported by the data and the statistical analyses? 

Interpretation of nest densities at sites: 

 The conclusion that "terrapin nest density on oiled and unoiled beaches downriver from 

the Chalk Point generating station did not differ during the 2000 nesting season" is supported by 

the data. Based on hatchlings captured in exclosures, on hatchlings, hatched eggs, and eggs 

present in the excavated enclosures, and on the combination of samples from the two approaches 

the heavily oiled beaches did not differ significantly from those of control beaches in numbers of 

nests per area. 

Interpretation of hatchling emergence in the fall: 

The conclusion that "the hatching of terrapins in fall did not differ between oiled and 

unoiled nesting beaches downriver from the Chalk Point generating station" is supported by the 

data available. The finding that a total of 33 live hatchlings were encountered in 5 of the 6 

exclosures in the heavily oiled sections compared to only 21 from 3 of the 6 in the control sites 
3

can be interpreted as being insufficient evidence that the oil spill hindered hatchling success the 

year of the spill. The statistical tests used (nested analysis of variance) are adequate for the 

particular purpose of comparing whether difference in hatchling numbers occurred among or 

within the three oil categories. A conclusion that the findings were not significant is warranted, 

and the fact that the highest absolute numbers came from the heavily oiled sites further reduces 

cause for question. 

Interpretation of hatchling developmental anomalies and size measurements: 

The statements that 1) "no significant differences in any size measurements of 

hatchlings" occurred among the two oil sites and the control sites and that 2) "fall hatchlings 

recovered from variously oiled and unoiled sites were comparable in size and weight" are 

supported by the analysis of variance and Table 5. The sample sizes are adequate to be 

convincing that no appreciable differences exist. 

Interpretation of nest size comparisons: 

 The statement that "best estimates of nest size at oiled and unoiled sites did not differ" is 

supported by the data and by the interpretation of the partial nest phenomenon within the report. 

Interpretation of egg conditions from selected sites: 

 The conclusion that "the condition of eggs recovered at selected sites revealed a 

significantly higher frequency of dead embryos and a lower frequency of presumably springemergers at oiled sites compared to unoiled sites" seems equivocal. The report itself even makes 

the statement on page 14 that “the cause of mortality of these embryos is unknown and may be 

due to many factors other than the effects of the embryos’ exposure to residual oil present in 

sediments after cleanup.” This does not provide convincing evidence of an oil-based impact. See 

additional comments (#4) below. 

The following points should probably be clarified in the final report: 

1. The statement on page 4 that the designation of the Playground site from “heavily oiled” to 

“moderately to lightly oiled” altered the “original grouping of beaches in the experimental 

design” is confusing based on the subsequent presentation of three beaches in each “oiling” 

category in tables 2, 3, 5, 6, etc. That is, if Playground was originally designated in the “heavily 

oiled” category, what beach did it replace in the “lightly oiled” category and which beach 

replaced it in the “heavily oiled” category. The statements on page 4 that address this point are 

not particularly lucid. 

2. The report states on page 10 (lines 14-15) that "Analysis of variance indicated that there were 

no significant differences in any size measurements of hatchlings among oiling categories." 

Carapace length and maximum height are included among the measurements. On lines 23-24, the 

report states "carapace length and maximum height also differed significantly among sites (see 

Table 5)." This seems contradictory. The latter statement should probably be either dropped or 

reworded to indicate that "sites" include both oil categories and controls and that the statement 

has no bearing on a comparison of the impact of oil sites versus control sites (other than to show 

the high variability in the characters measured). Including the comparison obfuscates the issue. 
4

3. The suggestion made in the summary (page 16) that the oil spill may have contributed to a 

reduction in nest size seems totally unsupported by the evidence presented. Numerous 

publications in turtle ecology have demonstrated that reproductive output, including both clutch 

size and frequency, can vary among years as a consequence of environmental factors unrelated to 

oil pollution. Also, the interpretation that nest sizes were lower in 2000 appears to be based on an 

unsupported assumption that clutch sizes determined for females by x-radiography in previous 

years are applicable to nest sizes in another year. And finally, if the presumed reduction in nest 

size were a function of oil pollution, why would it not be more apparent on the “oiled” beaches 

compared to the controls? 

4. Table 7 is troublesome for several reasons that collectively cause the results and 

interpretations presented on this aspect of the study to be questionable and confusing. Listing the 

most salient points is probably the most efficient way to address the problems associated with 

this part of the study. 

 a. At least two or three of the mean number of eggs per nest seem miscalculated in Table 

7. For example, under Jack Bay 1, 222 eggs divided by 26 nests yields 8.54 eggs/nest. 

The table has the mean as 8.73. However, in Appendix 2, the total number of eggs is 227, 

which would give a mean of 8.73. Which is correct? 

 b. The intra-nest variability is presented in Appendix 2 but it is not clear how or if it is 

considered in the analysis that compares the heavily oiled sites with the control sites. 

 c. In the G-tests on pages 13-14 an implicit assumption is made that the #Dead category 

represents animals that are comparable to the #Viable category (i.e., that they would have 

overwintered if they had lived). In reality, a dead embryo could have been comparable to 

one in the #Hatched category (i.e., it could have emerged in the fall if it had not died). 

The analysis is based on an assumption that all dead embryos were in the overwintering 

category. Although there would be no way to know which category a dead embryo 

should be assigned to, the analysis becomes somewhat confounded, and this point needs 

to be considered. 

d. An inconsistency appears to be present between the statement on page 14 that “At least 

28 inviable eggs from several different nests at one Caney Creek exclosure (#2) are of 

particular interest” and the data on inviable embryos presented in Appendix 2. I only 

count 18 inviable embryos in Table 7 and Appendix 2. 

e. The clutch size of terrapins ranges from 4-18 according to Ernst, Lovich, and Barbour. 

Appendix 2 indicates several nests with fewer than 4 eggs (2 with only 1) and one nest 

with 20. The report refers to having concerns about whether those with 1 or 2 eggs 

represented a complete clutch, and they were thus eliminated from the analyses. Should 

there also have been concerns about the 5 nests with only 3 eggs each? Should they have 

been eliminated also? Should there also be concerns about whether the nest with 20 eggs 

represented a clutch laid by a single female? If other reports exist of clutch sizes/nest 

sizes outside the range reported by Ernst et al., these should probably be mentioned. This 
5

would seem important in an analysis that is based on comparing intra- and inter-nest size 

among sites, and the justification for using some eggs and not others bears careful 

consideration. 

5. The report should explain how "oiled" beaches would be expected to result in a higher 

mortality of eggs than control beaches in the same area. The oil presumably would not be on the 

beach itself above the high tide mark, yet all eggs would be laid in nests above the high tide 

mark. Therefore, the eggs would not be exposed to oil during incubation. As female terrapins in 

some areas are known to move more than a mile to nest in some instances, the report should 

make it clear that a female that nested on an "oiled" beach presumably lived and fed in the 

affected area, which is extensive, and that the control beaches are sufficiently distant from the 

oil-impacted areas to have assurance that nesting females did not live and feed in oil-impacted 

habitat. These considerations should probably be mentioned in the report. 

 

6. The meaning and purpose of a couple of sentences in the report were not clear to me in the 

context they were presented, as follows: 

 a. page 12, line 2 from bottom – "It is unknown if data from any nests included in this 

analysis were complete . . . ." Not sure exactly what this would mean. 

 b. page 15, lines 1-2 from bottom – "Spring emergers would experience the potential 

effects of any residual oil contamination for a protracted period compared to fall 

hatchlings." I assume this means because they would be in the soil for a longer period, 

but this would be based on a premise that the sand itself was contaminated above the high 

tide mark. How could that happen? 

The following points are for consideration only and not necessarily required as part of the report 

on hatching success among variably oiled nesting sites in 2000. 

1. Open-literature references are minimal in the report, although citations may not be an 

important aspect. A variety of references could be cited about terrapins and about reproductive 

traits in emydid turtles that would be pertinent for the topics addressed. 

2. Could mammalian predators have been deterred from entering some of the nesting areas (due 

to olfactory aversion), resulting in a reduction in predation and therefore a nest density that 

would be higher in the fall? This could perhaps explain the statement on page 12 that "three of 

the four sites we excavated far exceeded the maximum nest density . . . previously reported." 

An examination of predator density based on tracks or using more sophisticated approaches with 

break-beam photography could address this question, but was not possible during the nesting 

season of 2000 due to the seasonal timing of the study. However, the point should perhaps be 

mentioned in explaining the higher nest densities. 

3. Will the most dramatic impact on reproductive output of terrapins result from a decrease in 

prey base productivity (e.g., crustaceans and mollusks) in the heavily oiled areas which in turn 

leads to reduced growth rates of individuals and smaller (or less frequent) clutch sizes of 
6

females? If this were true, the evidence would not become apparent until the nesting seasons of 

2001 or later. 

Sincerely, 

J. Whitfield Gibbons 

Professor of Ecology 

Office 803 725-5852 

Email gibbons@srel.edu