Sea otters were originally found throughout the north Pacific including Japan, Russia, the United States, Canada and Mexico. By the late 1800s, they had been eliminated from most of their range due to over-harvest by fur traders. Sea otters came under international protection in 1911 and since then, their numbers have rebounded. Today, sea otters can only be harvested for subsistence purposes. Surveys of sea otters in the 1970s and 1980s indicated a healthy and expanding population in most of Alaska, including Prince William Sound.
More than a thousand otters became coated with oil in the days following the spill, and 871 carcasses were collected throughout the spill area. Estimates of the total number of sea otters lost to acute mortality vary, but range as high as 40 percent (2,650) of the approximately 6,500 sea otters inhabiting the western areas of the Sound. In 1990 and 1991, higher than expected proportions of prime-age adult sea otters were found dead in western Prince William Sound. Higher mortality of recently weaned juveniles in oiled areas was documented through 1993. Continuing studies of mortality rates, based largely on sea otter carcass recoveries, suggest that relatively poor survival of otters in the oiled area persisted for well over a decade.
Sea otters will have recovered when the population in oiled areas returns to conditions that would have existed had the spill not occurred and when biochemical indicators of hydrocarbon exposure in otters in the oiled areas are similar to those in otters in unoiled areas. An increasing population trend and normal reproduction and age structure in western Prince William Sound will indicate that recovery is underway.
No apparent population growth occurred for Prince William Sound sea otters through 1991. After 1993, the population in the western Sound began increasing at a rate approximately one-half of the pre-spill rate of increase. From 1993–2000, the number of otters increased by 600 animals which represents an annual growth rate of 4 percent. However, in areas that were heavily oiled, such as northern Knight Island, sea otter populations have remained well below pre-spill numbers, and population trends continued to decline through 2005. Moreover, the demographics within this group apparently are not stable as many of the females are below reproductive age and young, non-territorial males have moved into and out of the population.
The lack of recovery may reflect the extended time required for population growth for a long-lived mammal with a low reproductive rate, but likely reflects the effects of chronic exposure to hydrocarbons, or a combination of both factors. Food limitation does not appear to be a factor limiting recovery in the Knight Island group, because food resources are at least as plentiful there as they are at unoiled Montague Island. Productivity is also similar between oiled and unoiled sites. Exposure of sea otters to lingering oil is plausible because their foraging sites and prey species occur in habitats harboring oil. Additionally, biochemical responses (cytochrome P450) of oil exposure were elevated in animals from oiled sites through 2002. By 2004–2005, the response of this biomarker was similar in animals from oiled and unoiled areas. However, additional years of data areneeded to determine if the similarity is true convergence, and the apparent diminishing exposure to oil is a long-term trend.
Sea otters will have recovered when population levels, reproduction and productivity are within normal bounds in oiled and unoiled areas and have reached levels that would have existed without the spill. Recovery will also be substantiated when the biochemical indicators of hydrocarbon exposure are similar within the oiled and unoiled areas
Although there has been a slow increase since 2005 in the sea otter population within the heavily-oiled areas, there has been a greater rate of overall increase in the population within Prince William Sound. Therefore, sea otters are considered to be RECOVERING.
Click HERE for more information on Trustee Council funded studies of sea otters.