Exxon Valdez Oil Spill Restoration Project Final Report: Trends in Sea Otter Population Abundance in Western Prince William Sound: Progress toward Recovery Following the 1989 Exxon Valdez Oil Spill.

摘要

Sea otter populations in western Prince William Sound (WPWS) were injured as a result of the 1989 Exxon Valdez oil spill (EVOS) (Ballachey et al. 1994). Effects included both acute mortality, resulting from contact with spilled oil in the days and months following the spill, and chronic effects, resulting from sub-lethal initial exposure or protracted exposure to lingering oil, or indirect effects, e.g., reduced prey populations or other ecosystem disturbances (Peterson 2000, Peterson et al. 2003). Estimates of sea otter mortality due to acute effects of the oil spill in WPWS ranged from 750 to 2,650 individuals (Garrott et al. 1993, Garshelis 1997). The disparity among acute mortality estimates largely reflects the lack of accurate pre-spill estimates of sea otter population size. Using population models, Udevitz et al. (1996) predicted recovery of the WPWS sea otter population in 10 to 23 years, with maximum annual growth rates from 0.10-0.14. Since 1993 we have conducted aerial surveys of sea otter populations in WPWS annually to track the progress of sea otter recovery, except in 2001, 2006, and 2011. Results of sea otter population surveys through 2000 demonstrated a significant increase of nearly 600 animals in WPWS, resulting in a statistically significant average annual increase of 4% per year since 1993 (Bodkin et al. 2002). This increase at the scale of WPWS was considered indicative of progress toward recovery, although the average annual rate of increase was about half the long-term growth rate experienced in PWS earlier in the 20th century (Bodkin et al. 2002). Additionally, Dean et al. (2000) and Bodkin et al. (2002) describe a situation at northern Knight Island in WPWS where there was no evidence of increase in sea otter abundance by 2000, more than a decade after the spill, and abundance was less than half of the population size prior to the spill. Recovery of the PWS ecosystem from the Exxon Valdez oil spill may not be considered complete until individual animals are no longer exposed to lingering oil from the spill, and when populations reach pre-spill levels of abundance (Exxon Valdez Oil Spill Restoration Plan, EVOS Trustee Council 2006). The results of aerial surveys of sea otters in WPWS provide a means to evaluate progress toward recovery goals and were summarized in detail by Bodkin et al. 2011. Our objective in this report is to update the results and interpretation of aerial surveys to estimate sea otter abundance in WPWS conducted through 2011. Here, we present those abundance estimates derived from the aerial surveys and evaluate the trends in abundance evident in the time series of data from 1993-2011.