MORTALITY OF SEA OTTERS IN PRINCE WILLIAM SOUND FOLLOWING THE EXXON VALDEZ OIL SPILL

Abstract

This paper presents an estimate of the total number of sea otters that died as a direct consequence of the oil spill that occurred when the T/V Exxon Valdez grounded in Prince William Sound, Alaska on 24 March 1989. We compared sea otter counts conducted from small boats throughout the Sound during the summers of 1984 and 1985 to counts made after the spill during the summer of 1989. We used ratio estimators, corrected for sighting probability, to calculate otter densities and population estimates for portions of the Sound affected by the oil spill. We estimated the otter population in the portion of Prince William Sound affected by the oil was 6,546 at the time of the spill and that the post‐spill population in the summer of 1989 was 3,898, yielding a loss estimate of approximately 2,650. Bootstrapping techniques were used to approximate confidence limits on the loss estimate of about 500–5,000 otters. The wide confidence limits are a result of the complex scheme required to estimate losses and limitations of the data. Despite the uncertainty of the loss estimate it is clear that a significant fraction of the otters in the spill zone survived. We observed otters persisting in relatively clean embayments throughout the oil spill zone suggesting that the highly convoluted coastline of Prince William Sound produced refuges that allowed some sea otters in the oil spill area to survive.

Sea otter population status and the process of recovery from the 1989 'Exxon Valdez' oil spill

“…An age-specific reproductive and survival rate recovery model for only the oiled WPWS population of about 2000, produced by Udevitz et al (1996), projected recovery times ranging from 10 to 23 yr, dependent on assumptions regarding survival rates. Although Garrott et al (1993) and Udevitz et al (1996) calculated similar growth rates (about 1.10 yr -1 ), recovery times differed primarily because the number of otters assumed to contribute to recovery differed (13000 vs 2000). However, neither recovery model incorporated long-term spill related effects in projecting recovery times.…”

“…While several widely disparate estimates of sea otter mortality resulting from the spill in PWS have been published, all include recognized uncertainties (Garrott et al 1993, Bodkin & Udevitz 1994, Garshelis 1997. Although acute mortality estimates generated controversy (Eberhardt & Garrott 1997, Garshelis & Estes 1997, Garshelis 1997, it is clear that sea otter mortality was extensive and widespread regardless of the particular estimate.…”

“…Two independent modeling efforts projected recovery times for spillaffected sea otter populations (Garrott et al 1993, Udevitz et al 1996. Both sea otter recovery models implicitly assume that all otters remaining in the area of interest will contribute equally to the replacement of otters removed and that the study population is geographically closed.…”

“…Both sea otter recovery models implicitly assume that all otters remaining in the area of interest will contribute equally to the replacement of otters removed and that the study population is geographically closed. Garrott et al (1993) applied an estimated pre-spill annual population growth rate of 1.09 to the entire PWS 1989 post-spill sea otter population of about 13 000 and estimated a minimum recovery time of 3 yr, but recognized that population growth was not evident, based on post-spill surveys through to at least 1991. An age-specific reproductive and survival rate recovery model for only the oiled WPWS population of about 2000, produced by Udevitz et al (1996), projected recovery times ranging from 10 to 23 yr, dependent on assumptions regarding survival rates.…”

Food limitation and the recovery of sea otters following the 'Exxon Valdez' oil spill

“…One effect of the spill was the removal (either via death or for the purposes of rehabilitation and permanent placement in captivity) of a significant proportion of the sea-otter Enhydra lutris population in heavily oiled parts of PWS (Garrott et al 1993, Bodkin & Udevitz 1994, Garshelis 1997, Dean et al 2000). An estimated 165 sea otters were removed from the heavily oiled area of the western sound around northern Knight Island as a result of the spill (Dean et al 2000), and few if any sea otters remained there following the spill in summer 1989 (Bodkin & Udevitz 1994, J. L. Bodkin & D. H. Monson unpubl.…”