INJURY TO MUSKRATS (ONDATRA ZIBETHICUS) FROM THE CHALK POINT OIL SPILL, PATUXENT RIVER, MARYLAND¹ Open Access

Spill-associated mortality from the Chalk Point oil spill to muskrats (Ondatra zibethicus) was estimated using habitat and oiling extent data, muskrat population and home range estimates, local expert opinion, and hut count indices for three impacted creeks. Most of the 70 dead muskrats were collected from Swanson Creek. Muskrat density in this creek was estimated by local experts, and extrapolation from densities reported at a nearby wildlife refuge. Muskrat habitat acreage within Swanson Creek was determined using digitized aerial photography. Acreages of oiled marsh habitat for Swanson, Indian, and Trent Hall Creeks were estimated using spatial analysis. Oiling of muskrats likely occurred to those whose home ranges overlapped the spill area. Radius of a muskrat home range was reported to be 30 meters, and 30 meter buffers were circumscribed along borders of the oiled areas for these creeks. Exposure zone acreages were determined by summing the 30-meter buffers and oiled areas for Swanson, Indian, and Trent Hall Creeks, totaling 75.02, 19.55, and 9.12 acres, respectively. Muskrat population exposure within Swanson Creek was estimated by multiplying exposure zone acreage by muskrat density (4.2 per acre), totaling 315 individuals. Muskrat populations for Indian and Trent Hall Creeks were quantified using hut count indices and comparisons to the count index and population estimate for Swanson Creek. Muskrat exposures in Indian and Trent Hall Creeks were determined by multiplying estimated muskrat density by the exposure zone acreage for each creek. The exposed population was calculated to be 32 and 29 for Indian Creek and Trent Hall Creek, respectively. Assuming 100% mortality for muskrats potentially exposed to oiled marsh habitat, the total acute mortality in the spill area was estimated to be 376 individuals (i.e., 315 in Swanson Creek, 32 in Indian Creek, and 29 in Trent Hall Creek). Total muskrat years lost are calculated, and the strengths and uncertainties of this approach are discussed.