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WILDLIFE INJURY ASSESSMENT PLAN FOR

THE CHALK POINT OIL SPILL

Prepared by

 Representatives of the Natural Resource Trustee Agencies 

and Potomac Electric Power Company

October 5, 2000

1.0 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. The pipeline is 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 resulted in the oil being spread approximately 17 linear 

miles downstream in the Patuxent River and into several downstream tributaries, including Indian Creek, 

Trent Hall Creek, Washington Creek, and Cremona Creek, oiling approximately 40 miles of shoreline. 

 

The spill zone includes several types of habitat including openwater, tidal wetlands, fringing wetlands and 

sandy shoreline. Shoreline and aerial avian surveys indicated that over 1000 migratory and resident 

birds were present in the impacted area during the spill event, including ruddy ducks, grebes, 

buffleheads, cormorants, osprey, great blue herons and bald eagles. 

Physical oiling often results in the loss of the insulative properties of the feathers and fur of wildlife; this 

can lead to hypothermia, stress, starvation and can ultimately result in the death of an animal. 

Consequently, those animals that spend a majority of their time in or on the water, e.g., diving ducks, 

are particularly susceptible. Ingestion of oil due to preening of feathers and fur can also result in toxic 

effects to wildlife. Approximately 180 oiled birds, mammals, and reptiles died as a result of the Chalk 

Point spill (USFWS 2000). Ruddy ducks and muskrats suffered the highest mortalities. Many more 

animals, particularly birds, were observed to be oiled but were too active to be retrieved. Based on 

experience at previous oil spills, it is suspected that some portion of these living yet oiled animals 
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eventually died due to oiling. In addition, there was potentially a large number of dead and oiled animals 

that were not found by the responders. 

In addition to the direct impacts to wildlife, the Trustees are also concerned about the potential effects 

on the reproductive success of avian species, including bald eagles, osprey and great blue herons, that 

were nesting in the area when the spill occurred. The foraging strategy of these species and willingness 

to enter water and wetlands make them susceptible to oiling. Adults that become oiled may transfer the 

oil from their plumage to their eggs during incubation. Refined oil is highly toxic to avian embryos. In 

addition, the oil spill may have reduced or contaminated the prey base of these species, with ultimate 

effects on reproductive success.

2.0 Objective

The objective of the wildlife injury assessment is to determine the magnitude and extent of injury to 

wildlife receptors nesting or foraging in or near the spill zone during or shortly after the spill. Results will 

be used to assist in determining the level and form of compensation necessary to replace wildlife losses 

resulting from the spill. 

3.0 Study Approach

The methodology for this assessment plan was developed cooperatively by the wildlife assessment team 

consisting of personnel from PEPCO, the U.S. Fish and Wildlife Service (USFWS), the Maryland 

Department of Natural Resources (MDNR), the U.S. Geological Survey’s (USGS) Patuxent Wildlife 

Research Center, and the Nanjemoy Environmental Education Center (NCEEC). The National 

Oceanic and Atmospheric Administration (NOAA) and the Maryland Department of the Environment 

(MDE) also provided input to this document. 

Wildlife species included for injury assessment were chosen based on proximity of breeding or foraging 

populations to the spill zone, frequency of observed oiling to individuals and their habitats, and/or 

availability of historical data and assessment methods. Based on these criteria the wildlife assessment 

focuses on the following species and species assemblages: ospreys (Pandion haliaetus), great blue 

herons (Ardea herodias), bald eagles (Haliaeetus leucocephalus), furbearers, and waterfowl.

The injury assessment includes two approaches, one based on observations of oiled wildlife, the number 

of rehabilitated wildlife and mortality counts to estimate direct impacts (furbearers and waterfowl); the 

second, monitoring studies designed to evaluate the potential impacts on reproductive success (ospreys, 

great blue herons and bald eagles).

3.1 Furbearers
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An independent wetland consultant characterized Swanson Creek marsh and provided benchmarks for 

restoration success as it relates to furbearers and other wildlife. In addition, representatives from a local 

furbearer organization inspected the spill impacted areas of the marsh and provided the MDNR with 

estimates concerning pre- and post-spill populations in the spill zone. 

Furbearer losses in wetlands (focusing mainly on muskrats [Ondatra zibethica]) will be estimated using 

oiled wildlife mortality counts maintained by the USFWS during the response, post-spill furbearer 

organization survey results, historical surveys from other areas, and the literature. The NRDA wildlife 

assessment team expects to rely heavily on NRDA Wetland Habitat Assessment results, including 

reference marsh comparisons, to determine the period of injury and recovery time for furbearer 

populations. 

3.2. Waterfowl 

Aerial and shoreline surveys indicated that many species of resident and migratory waterfowl were 

present in the impacted area during the spill, including cormorants, coots, mallards, canvasbacks, mute 

swans and over 700 ruddy ducks. In addition, shoreline surveys also provide an estimate of the 

proportion of observed animals that were oiled. As part of response efforts, the USFWS produced a 

technical report summarizing these wildlife observations, the aerial overflight data and wildlife mortality 

counts (USFWS 2000). This report, in conjunction with information in the literature on the recovery 

rates of dead and oiled waterfowl in other spills, will serve as the foundation for the injury assessment to 

local populations of waterfowl species, particularly ruddy ducks.

3.3 Ospreys

Steve Cardano of the Nanjemoy Creek Environmental Education Center (NCEEC) and Ann 

Wearmouth of PEPCO located and inspected 37 active nesting pairs from Truman point to Cremona 

(the “middle river” area including most of the spill zone) several times immediately following the spill. 

During their inspections, they observed 6 oiled adults, four of which were later rehabilitated and 

released. The four rehabilitated ospreys were from nests at Chalk Point and the electrical towers 

upstream of Swanson Creek on the Patuxent River. One of the rehabilitated tower ospreys was later 

found dead. A total of six oiled eggs were found in the two nests. The fifth and sixth oiled adults were 

observed at Hunting Creek and Indian Creek. 

In order to evaluate potential impacts on reproductive success, nests were revisited every two weeks 

until fledging occurs (usually in August). Nest inspections consisted of counting and recording live young 

and eggs. Counts were accomplished by visual examination from a boat or using a mirror attached to a 

pole using methods similar to those used by Reese (1977), Woodford et al. (1998), and proposed by 
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McGowan (USFWS 1998). Results for # of young, # of eggs/nest, % hatchability, and fledging 

success will be statistically compared to 25 years of historical data for the middle river area and to 2000 

data from the upper river (above the spill zone and extending to Jug Bay) to determine whether the spill 

impacted the nesting population. Data from the approximately twenty upper river nests were collected 

by PEPCO and Greg Kearns of the Maryland Capitol Parks and Planning Commission (NCPPC). The 

locations of all nests were recorded using Global Positioning System units and mapped using GIS 

ARCVIEW. The only major deviation from methods described in the published literature is that the 

eggs were not numbered immediately following egg laying. Had this been done, it may have resulted in 

an easier, more accurate assessment of osprey productivity. However, comparisons to 25 years worth 

of historical data obtained using the same methods proposed herein should provide enough useful 

information to enable statistical analysis and the formulation of conclusions regarding effects of the spill 

on osprey productivity.

In late June, the young were banded. During banding, chicks will be weighed, subjected to Body 

Condition Indexing (BCI), and examined for external abnormalities. This information will be used as an 

adjunct to population assessment. The BCI is a subjective measure of body condition or robustness of 

an individual bird. The pectoral muscles and keel (breast bone) are palpated and assessed on a numeric 

scale of 1-5: 1 = emaciated, 2 = thin, 3 = average, 4 = above average, 5 = robust. Used in 

conjunction with weight and visual inspection, it offers additional information to assess overall condition 

of the bird. Data sheets for health assessment will be included for ease of recording this information. 

Used with population, hatching, and fledging success this information will assist trustees in overall 

assessment of osprey status in the area of the oil spill and at a reference site above the spill site.

Statistical Analysis: Nest and fledging success will be analyzed using the Mayfield method(Bart and 

Robson 1982) to compare middle and upper river areas to historical data. Categorical measures of 

productivity (# young per nest, # eggs per nest, % hatchability) will be analyzed with contingency table 

methods (Stokes et al. 1995). Differences between the mean BCI and weight data for the impacted and 

reference areas will be evaluated via ANOVA or the non-parameteric equivalent. Future monitoring 

needs will be determined based on the results of the 2000 monitoring. If statistically significant effects are 

identified, it may be necessary to perform similar monitoring in 2001.

3.4 Great Blue Herons

Craig Koppie (USFWS), Ann Wearmouth, and Mark Causey (heron consultant/climber) inspected the 

heronry on Swanson Creek on April 19 and April 20, 2000. A total of 34 nests were counted. Of the 

34 nests, 17 were accessible to the climbers. Thirteen out of 17 nests contained chicks and/or eggs for a 

total of 18 chicks and 15 eggs. Three adult birds were observed to be oiled and all eggs appeared to be 

un-oiled. The Swanson Creek heronry was revisited by Craig Koppie, Dan Murphy (USFWS), and 

Mark Causey on of May 12, 2000 to begin nesting success monitoring. Of the 17 nests monitored, 10 
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contained a total of 20 living chicks and 7 were empty. No eggs were found. One dead chick was 

found on the ground and another was found in a nest. These were collected and frozen for future 

analysis (they were both judged to be too decomposed for necropsy). 

A reference heronry on Swamp Creek, a tributary to the Patuxent River located four miles upstream of 

the mouth of Swanson Creek was visited on May 15, 2000 to establish reference conditions for 

statistical comparisons. A total of twenty nests were selected for monitoring. Fourteen of the nests were 

located in a single tree and were easily accessible to the climbers, and six, located in nearby trees, were 

chosen because the contents could be easily observed by the climbers. Eighteen of the twenty reference 

nests contained a total of 41 chicks.

The heronries were revisited weekly until fledging to monitor nesting success. Methods employed are 

similar to those used by Blus et al. (1997), Erwin et al. (1996), Parsons (1996), and Parsons and 

McColphin (1995). Results for # of young, eggs/nest, and fledging success at Swanson Creek will be 

compared to the literature and statistically to results from the reference heronry at Swamp Creek to 

assess detectable impacts on the nesting population. 

There may be some difficulties associated with evaluating nesting success in the heronries. First of all, 

since heron egg laying began prior to the spill, the actual number of eggs initially laid in each nest is 

unknown. Secondly, great blue heron nests are relatively difficult to access, so marking individual eggs 

and nestlings to follow their progress over time is not feasible. However, Parsons (1996) and Parsons 

and McColphin (1995) were able to statistically compare productivity of two heronries using the average 

maximum number of young observed per nest and average number of young observed (per nest) at the 

age when they are capable of sustained flight. Hence, if results for the traditional parameters described 

above prove to be statistically unusable, we propose to use these metrics to compare productivity of the 

heronries at Swanson Creek and Swamp Creek.

In addition, the number of eggs lost due to oiling will be estimated by using the average number of eggs 

expected per nest (from the literature), the number of oiled adults and, a multiplier, determined by the 

Trustee Council.

Statistics: Nest and fledging success will be analyzed using the Mayfield method (Bart and Robson 1982) 

to compare Swanson Creek to Swamp Creek, if a sufficient number of visits are realized. At least 3 visits 

are required per nest in which the number of eggs or chicks are counted in each nest until the time of 

fledging. Categorical measures of productivity (# young per nest, # eggs per nest, % hatchability, average 

maximum number of young, and (if possible) average number of young at time of sustained flight) will be 

analyzed with contingency table methods (Stokes et al. 1995).
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Adult site fidelity will be monitored in 2001 by counting the number of breeding pairs that return next 

spring. A large reduction in return breeders may be a result of the spill. Future monitoring needs will be 

determined based on the results of the 2000 and site fidelity monitoring. If statistically significant effects 

are identified, it may be necessary to perform similar monitoring in 2001. 

3.5 Eagles

Three eagle nests located within the spill zone were inspected by USFWS following the spill. Upon initial 

inspection, the nest in Prince George’s County on Swanson Creek contained two chicks that appeared 

normal and un-oiled. The adults at this nest were also un-oiled. During the week of May 1, this nest 

was revisited and appeared to have been blown from the tree by high winds. All that remained of the 

fledglings were several bloody feathers. 

Although an adult pair of eagles had been observed to be nesting earlier in the year, the nest on the 

Charles County side of Swanson Creek contained no young. Eggs had been observed in the nest prior 

to the spill. It is assumed that the eggs failed to hatch and were eventually removed from the nest. The 

third nest, at Cremona in St. Mary’s County, contained two fledglings that appeared healthy despite an 

odd coloration and two healthy, un-oiled adults. 

Ann Wearmouth continued to observe the Cremona nest weekly until the fledglings left the area. Both 

nestlings successfully fledged and no signs of oiling were observed.

4.0 References:

Bart, J., and D.S. Robson. 1982. Estimating survivorship when the subjects are visited periodically. 

Ecology 63(4):1078-1090.

Blus, L.J., B.A. Rattner, M.J. Melancon, and C.J. Henny. 1997. Reproduction of black-crowned 

night-herons related to predation and contaminants in Oregon and Washington, USA. Colonial 

Waterbirds 20(2):185-197.

Erwin, R.M., J.G. Haig, D.B. Stotts, and J.S. Hatfield. 1996. Reproductive Success, Growth, and 

Survival of black-crowned night-heron (Nycticorax nycticorax) and snowy egret (Egretta thula) 

chicks in coastal Virginia. The Auk 113(1):119-130.

McGowan, P.M. 1998. Study proposal titled: MD/Washington, D.C.- Contaminant exposure and 

potential effects on reproduction of ospreys nesting in Chesapeake Bay regions of concern. USFWS, 

Annapolis, MD.
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Parsons, K.C. 1996. Significant Wetlands of the Upper Delaware Bay: Habitat Status and Relationship 

to the Pea Patch Island Wading bird Colony.

Parsons, K.C. and A.C. McColpin. 1995. Great blue heron reproductive success in the upper 

Delaware Bay. J. Field Ornithol., 56(2):184-191.

Reese, J.G. 1977. Reproductive Success of Ospreys in Central Chesapeake Bay. The Auk 94:202-

221.

Stokes, M.E., C.S. Davis, and G.G. Koch. 1995. Categorical data analysis using the SAS system. SAS 

Institute, Inc., Cary, NC.

U.S. Fish and Wildlife Service (USFWS) 2000. U.S Fish and Wildlife Service Response During the 

Chalk Point Oil Spill, August 25, 2000 Draft technical report prepared by the Chesapeake Bay Field 

Office, Annapolis, MD.

Woodford, J.E., W.H.Karasov, M.W. Meyer, and L. Chambers. 1998. Impact of 2,3,7,8-TCDD 

exposure on survival, growth, and behavior of ospreys breeding in Wisconsin, USA. Environ. Toxicol. 

Chem. Vol.17, No.7: 1323-1331.