ACUTE MORTALITY OF DIAMONDBACK 

TERRAPINS FROM THE CHALK POINT OIL SPILL 

By Jacqueline Michel, Richard Greer, Mark Hoffman, 

Peter McGowan and Roger Wood1

November 9, 2001

This report was prepared for the Natural Resource Trustee Council responsible for assessing 

injuries to natural resources and restoring losses resulting from the April 7, 2000 oil spill at Chalk Point, 

Maryland. It presents an estimate of the number of diamondback terrapins (Malaclemys terrapin) killed 

directly by the oil in the weeks immediately following the spill. This injury is described as direct or 

"acute, " in contrast to, for example, the intergenerational loss of terrapins as estimated in Hinkeldey, et 

al. (2000). A subgroup of the Wildlife Injury Workgroup, established by the Natural Resource Trustee 

Council, developed the estimates and prepared this report. 

EXISTING INFORMATION 

Site-specific Information 

Site-specific information available on terrapins includes observations reported as part of the oil 

spill, i.e. extent of shoreline oiling, reported mortality, and rehabilitation of live terrapins. In addition, 

terrapin population estimates for the late 1980s for the Patuxent River (Roosenburg 1990a, 1990b) were 

available. 

In the weeks following the spill, the extent and degree of shoreline oiling was documented 

through surveys by Shoreline Cleanup Assessment Teams (ENTRIX, 2000). Based on the Shoreline 

Assessment Teams' (SCAT) data, three oiled (exposure) zones were defined: 1) heavily oiled - between 

Chalk Point and Teague Point (oiled shoreline of 11.3 km); 2) moderately oiled - from Teague Point 

downstream to Long Point (oiled shoreline of 8.2 km); and 3) lightly oiled - from Long Point downstream 

to Spring Cove (oiled shoreline of 24.7 km). 

During wildlife mortality and SCAT surveys, which began shortly after the spill, 7 dead 

diamondback terrapins were collected and sent to the wildlife morgue. The wildlife morgue was 

established for the purpose of processing dead animals found in the spill zone. Only two of the dead 

terrapins showed visible signs of oiling, while four had damaged carapaces. An additional 8 oiled, live 

terrapins were also captured in the spill zone. Seven of them were rehabilitated and returned to the wild, 

while the eighth died in captivity. Table 1 lists the information known about each of these 15 terrapins. In 

addition, Roger Wood (Wood et al., 2000) reported that waterfront landowners in the spill zone told him 

that they had also collected an additional 4 dead terrapins and buried them. 

 

1 Michel: Research Planning, Inc.; Greer, ENTRIX, Inc.; Hoffman: Maryland Department of Natural 

Resources; McGowan: U.S. Fish and Wildlife Service; Wood: Richard Stockton State College. 
2

Table 1. Summary information on the dead and live diamondback terrapins 

collected during SCAT and wildlife mortality surveys following the PEPCO oil spill 

Individuals Collected Dead Individuals Collected Alive 

4/17 off Chalk Point, has carapace markings, 

assumed to be oiled 

4/16 Golden Beach, heavily oiled, 328 g, 

rehabilitated 

4/26 pipeline break marsh, carapace cracked open, 

dead for a while, assumed to be oiled 

4/20 Indian Creek, lightly oiled, 55g, rehabilitated 

4/26 Buena Vista, bloated, no visible oil., damaged 

carapace 

4/21 Chalk Point (pipeline break marsh), heavily 

oiled, 51 g, rehabilitated 

4/30 Washington Creek, head missing, damage to 

carapace, notched, no visible oil 

4/22 Indian Creek, moderately oiled, 9g, 

rehabilitated 

5/4 Swanson Creek, no visible oil 5/2 Chalk Point (pipeline break marsh), 115 g, 

rehabilitated 

5/7 Buena Vista, no visible oil 5/6 Buena Vista, lightly oiled, 5g, rehabilitated 

5/11 Buena Vista, no visible oil, cracked carapace 5/11 Swanson Creek, moderately oiled, 7 g, 

rehabilitated 

 5/18 Washington Creek, lightly oiled, deep 

abrasions, died during rehabilitation 

Based on the above information, the number of known dead diamondback terrapins associated 

with the spill is 12 (7 collected dead, 1 died in rehabilitation and 4 were reported dead by Wood). It is 

important to note that during oil spills, some terrapins can also die from the increased boat traffic 

associated with the cleanup, so mortality estimates should include both mortality from contact with the oil 

and from interactions with oil spill response craft. This assumption is supported in part by the fact that 

there were no terrapins collected until over a week after the spill, as boat operations increased. 

Furthermore, while it is admittedly a small sample, twice as many dead terrapins were found to have 

damaged carapaces as were believed to be oiled. 

The most recent population estimate available for diamondback terrapins in the Patuxent River 

was based on a 1987-1989 mark-recapture study by Roosenburg (1990b). The study area extended from 

Long Point to Spring Cove, which includes approximately 26.6 km of shoreline based on a shoreline 

length estimate reported by ENTRIX (2000). Roosenburg (1990b) estimated the mean of the population 

in the study area to be 2,293 adult and juvenile animals (with a 95% confidence interval of 1,717 - 2,895). 

This estimate indicates there was a mean of 86.2 terrapins per kilometer of shoreline (with a 95% 

confidence interval of 64.5 - 108.8). 

General Information on Oil Impacts to Terrapins 

A comprehensive literature search for information on the effects of oil on terrapins was 

conducted. In addition, turtle experts were contacted to identify any previous studies on the impacts of 

oils spills on turtles (Kiviat, 2000; Lutcavage, 2000; Wood ET al., 2000). Only one publication was 

found that describes the impacts of oil on terrapins following a spill. Burger (1994) described the 

behavior of eleven female diamondback terrapins that were oiled during the January 1990 spill of No. 2 

fuel oil in Arthur Kill, New York. The terrapins were hibernating at the time of the spill, and when they 
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emerged from hibernation, were found to be oiled. The terrapins voided oil from their digestive tracks for 

two weeks in rehabilitation. They developed edema, scored low on strength tests, and were slow to right 

themselves when placed on their backs. Eight of the eleven died; these animals "had oil traces in their 

tissues and exhibited digestive-tract lesions consistent with oil exposure" (Burger, 1994). 

 There have been several turtle and oil studies involving sea turtles. The most applicable study for 

extrapolation to terrapins was a laboratory study of the impact of oil on loggerhead turtles (Vargo, ET al., 

1986). In that study, juvenile turtles were exposed to floating oil for 48-96 hours. There was no mortality, 

but the individuals exhibited some skin sloughing, changes in blood chemistry, and oil was observed in 

the feces. One of the researchers indicated that impacts of oiling could be more severe for diamondback 

terrapins due to their smaller size (<1 kg) compared to juvenile loggerheads (8-12 kg; Lutcavage, 2000). 

A representative from Tri-State Bird Rescue stated they had good success in rehabilitating turtles oiled 

during spills (Gilbert, 2000). 

Additional background information on the life history and habitat utilization of diamondback 

terrapins in the mid-Atlantic region was obtained from Drs. Roger Wood and Stan Hales (Wood, et al., 

2000). 

MORTALITY ESTIMATES 

After reviewing the extent of available information to support an estimate of acute mortality to 

adult and juvenile terrapins, the trustees elected to take a risk assessment (i.e., population at risk) 

approach. The population at risk from exposure to the oil was based on the mean population estimate of 

2,293 adults and juveniles, or 86.2 terrapins per km of shoreline, from Roosenburg (1990b). The total 

length of shoreline between Chalk Point and Spring Cove is estimated to be 54.5 km, including both oiled 

and unoiled shoreline (note that only the oiled portions are used to estimate the mortality figures, below, 

however). Based on the 86.2 terrapins/km of shoreline mean estimate from above, the total population of 

terrapins between Chalk Point and Spring Cove is estimated (54.5 km x 86.2) to be 4,698 (range of 3,515 

to 5,930). 

The shoreline was partitioned into the same three oiled (exposure) zones defined above. Total 

acute mortality was then estimated based on best professional judgement concerning the mortality risks 

posed by the differential degrees of oiling and on the length of oiled shoreline and population estimates, 

as follows: 

1. Chalk Point to Teague Point - heavy oiling. This zone had the highest degree of oil exposure. Oil 

slicks/sheening persisted in this area for months following the spill. The mortality rate, based on best 

professional judgment, for diamondback terrapins is estimated to be 10 percent in this zone because of 

the degree and persistence of oiling. The total shoreline length of this zone is 14.0 km and the oiled 

portion was 11.3 km long. Mortality is estimated (11.3 km x 86.2 animals/km x 10% mortality) to be 97 

animals (range of 73 to 123). 

 

2. Teague Point to Long Point – moderate oiling. This zone had relatively moderate amounts of oil 

exposure, with most of the oil confined to a narrow band along the outer fringes of marsh. The degree and 

persistence of oiling was much reduced, compared to the Chalk Point to Teague Point zone. The mortality 

rate, based on best professional judgment, for diamondback terrapins is estimated to be 2 percent in this 

zone. The total shoreline length for this zone is 13.9 km and the oiled portion was 8.2 km long. Mortality 

is estimated (8.2 km x 86.2 animals/km x 2% mortality) to be 14 animals (range of 11 to 18). 

3. Long Point to Spring Cove – light oiling. This zone had relatively light amounts of oil stranded on the 

shoreline and little sheening. The mortality rate, based on best professional judgment, for diamondback 
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terrapins is estimated to be 0.5 percent in this zone. The total shoreline length for this zone is 26.6 km 

and the oiled portion was 24.7 km. Mortality is estimated (24.7 km x 86.2 animals/km x 0.5% mortality) 

to be 11 animals (range of 8 to 14). 

Thus, total acute mortality of terrapins from the Chalk Point spill is estimated to be 122 (with a 95% 

confidence interval of 92 to 155) adults and juveniles using the risk assessment approach. 

REFERENCES CITED 

Burger, J. 1994. Immediate effects of oils spills on organisms in the Arthur Kill. In. J. Burger, ed., Before 

 and After An Oil Spill: The Arthur Kill. Rutgers University Press, New Brunswick, New Jersey, pp. 

 115-130. 

ENTRIX. 2000. Summary of SCAT activities and data management, Swanson Creek incident. October 

16, 2000. Report prepared by ENTRIX, Inc., Walnut Creek, CA. 

Gilbert, E., personal communication, 2000. Conversation with Tri-State Bird Rescue regarding experience 

with turtle mortality and rehabilitation on oil spills. November 2000. 

Hinkeldey, H., J. Michel, T. Tomasi, R. Greer, W. Kicklighter, R. Wood and N. Meade. 2001. Estimate of 

Total Injury to Diamondback Terrapins from the Chalk Point Oil Spill. Report by a subgroup of the 

Wildlife Injury Workgroup to the Chalk Point Oil Spill Natural Resource Trustee Council, 13pp. 

Kiviat, E., personal communication, 2000. Conversation with turtle expert from Bard College regarding 

the availability of any information on the impacts of oils spills on turtles. November, 2000. 

Lutcavage, M., personal communication, 2000. Conversation with turtle researcher from the New 

England Aquarium regarding study of the impacts of oil on loggerhead turtles. November, 2000. 

Roosenburg, W.M., 1990a. The diamondback terrapin: population dynamics, habitat requirements, and 

opportunities for conservation. Proc. Conference on New Perspectives in the Chesapeake System: A 

Research and Management Partnership. Chesapeake Research Consortium Publication No. 137, 

Solomons, MD, pp. 227-234. 

Roosenburg, W.M., 1990b. Final Report: Chesapeake diamondback terrapin investigations for the period 

1987, 1988, and 1989. Chesapeake Research Consortium Publication No. 133, Solomons, MD, 83 pp. 

Sperduto, M., C. Hebert, J. Myers, and G. Haas. 1998. Estimate of total acute mortality to birds resulting 

from the North Cape oil spill, South Kingstown, Rhode Island, January 19, 1996. Report by U.S. Fish 

and Wildlife Service and Rhode Island Department of Fish, Wildlife, and Estuarine Resources. 

Vargo, S., P. Lutz, D. Odell, E. Van Vleet, and G. Bossart, 1986. Effects of oil on marine turtles. OCS 

Study MMS 86-0070. Prepared by Florida Institute of Oceanography. 

Wood, R., and S. Hales, personal communication, 2000. Multiple conversations with diamondback 

terrapin experts from The Wetlands Institute, Stockton State College, New Jersey regarding terrapin 

life history and potential impacts of oil on terrapins. 
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November 11, 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 combined comments on two reports, "Acute Mortality of 

Diamondback Terrapins from the Chalk Point Oil Spill" and "Estimate of Total Injury to 

Diamondback Terrapins from the Chalk Point Oil Spill." Both reports address the quantification 

of natural resource injury to terrapins under the 1990 Oil Pollution Act (OPA) following the 

April 2000 Chalk Point oil spill in the Patuxent River. 

 The purpose of the Acute Mortality study was to estimate the direct mortality to 

diamondback terrapins (Malaclemys terrapin) from the oil spill. The goal of the Total Injury 

report was to estimate the number of lost diamondback terrapin years, based on both acute 

mortality and the loss of production of the next generation, following the oil spill. My review of 

these reports addresses the following queries: 

1. Were the assumptions used and the data and methods employed by the investigators 

appropriate for undertaking the stated goals of the study? 

2. Were the methods employed properly implemented? 

3. Do you have suggestions on how to improve the substance and exposition of the report? 

General Statement: 

 My overall assessment of the two reports is that a fair treatment of the situation was given 

considering the absence of thorough knowledge of the disposition of terrapins throughout the 

system and the limited data that were available. In essence, as good a job as possible was done 

with the material available, and the approach used of building scenarios based on known 

biological facts was reasonable and appropriate. To the best of my knowledge the model 

quantifying total lost terrapin years has been properly implemented. A few specific points bear 

mentioning. 

Specific Comments 

Acute Mortality Study 
2

Table 1 – The authors need to be consistent in use of "pipeline break marsh" – if this is a specific 

locality it should be capitalized; need to clarify "notched" on April 30 – does this mean a marked 

animal from a study or an animal injured by a boat? 

page 2 – The point about increased boat traffic is a good one that should be included in the final 

report, as such activities are a consequence of the spill. 

page 3 – The last sentence in first paragraph under Mortality Estimates needs to provide 

confidence limits (range) for the total population estimate. 

page 3 – The most difficult assumption in the acute injury report is the 10%, 2%, and 0.5% 

estimates of mortality. My suggestion is that these be referred to as professional opinions based 

on limited availability of data. The rationale for such an estimate is that higher proportions would 

not be expected because of the minimal number of oiled or dead terrapins found after the spill. 

However, the finding of several individuals affected by the oil spill suggests that at least a small 

proportion of the population was affected, which would presumably be reduced in habitats 

receiving lesser amounts of oil. 

Total Injury Study 

Table 1 – This table needs to be partitioned into two parts (1. Population parameters and 2. 

Parameter values) in order to keep the columns clear and consistent. 

Table 2 -- The citations need to be provided for Wilbur 1975, Tinkle et al. 1981, and Mitchell 

1988. Defining values as both the "mean" and the "average" is confusing as the numbers should 

be the same for either. 

page 5 - The series of assumptions are tedious to follow but seem to be best estimates that are 

accurate within the context of available data on the species for the Patuxent River area. 

page 11 - In the Summary, the maximum of "39 eggs per year" is used in the final calculation. In 

other parts of the reports, both the mean and range have been given. Explanation should be given 

for why the maximum is being used for the potential number of eggs. 

 I hope these comments are useful. Please let me know if you need additional information. 

 

Sincerely, 

J. Whitfield Gibbons 

Professor of Ecology 

Office 803 725-5852 

Email gibbons@srel.edu