back to data
glossary

Sea otters are found from the Gulf of Alaska around the Pacific Rim to northern Japan with translocated pockets in southern California, Washington, Vancouver, and southeast Alaska (Figure 1b). It is estimated that 90% of world's population is found in coastal waters of Alaska.

(biology.usgs.gov/s+t/noframe/s043.htm)
Figure 1a. Distribution of sea otters before fur harvest began in 1741 and populations that survived the harvest, providing the nucleus for recovery of the species.

Figure 1b. Current distribution of sea otters including locations of successful translocations. Surving otters were used to initiate translocated populations.

Graphs in Figure 2 show growth patterns observed in three sea otter population stocks in the north Pacific resulting from natural range expansion.

  1. Bering Island was recolonized by growing populations around Medny Island in 1970.
  2. Sea otters in Prince William Sound were from the original surviving 1911 population.
  3. In 1969 & 1970, 59 sea otters were released along the outer coast of Washington state. This represents a translocated population.

source: J. L. Bodkin, R. J. Jameson, J. A. Estes (http://biology.usgs.gov/s+t/noframe/s043.htm)
Figure 2. Population growth of sea otters in the Bering Sea, Prince William Sound, and Olympic Peninsula.

In March 1998, US FWS officially divided the Alaskan sea otters into three stocks based on differences in genetics, morphology, and geographic distribution. Figure 3 shows the general location of the three stocks. Recent aerial surveys suggest:

  • translocated Southeast stock continues to grow in numbers and expand in range,
  • Southcentral stock (including Prince William Sound) is either stable or increasing,
  • SOUTHWEST STOCK has declined dramatically in the past 10-15 years

(U.S. Fish & Wildlife Service, Marine Mammals Management Office, October 2001)

 

It is estimated that 90% of the world population of sea otters reside in the nearshore waters in Alaska. Since the 1911 international fur trade ban, sea otters have shown healthy recovery. Population growth rates into unoccupied habitat have been 17-20% per year. This suggests that something unique is negatively impacting the Southwest stock since the other stocks continue to prosper.
Table 1. Southwest Alaska sea otter survey results, 1986-2001.
Location Year Count or Estimate Decline
Aleutian Islands
1992
8,044
2000
2,442
70%
North Alaska Peninsula
1986
9,061-13,091
2000
5,756
36-56%

South Alaska Peninsula
1986
15,346-17,835
2001
1,344
91-92%
Kodiak Archipelago
1989
13,526
1994
9,817
2001
5,892
56%

Table 1 estimates the percent of population decline between the years indicated for each location.

  1. Could the geographic location of the Southwest stock contribute to the decline?

Figure 3. Map of Alaska indicating general locations of three genetically distinct sea otter stocks.
Table 2 summarizes data from different studies that investigated potential physiological and behavioral causes for the sea otter decline. The results suggest that there has not been a reduction in reproduction or a redistribution of sea otter populations. Exploring the possibilities for increased mortality show evidence against disease, toxins, and starvation. However, one study provides strong evidence for increased sea otter predation as a potential cause for population decline.
Table 2. Conclusions from investigations regarding physiological and behavioral causes for declining sea otter populations
Variable Results
Reduced fertility Birth rates of radio-tagged adult females and pup survival at Amchitka Island (1992-94) and Adak Island (1995-96) were similar to those of stable populations.
Redistribution There has been no population build up on any one island to account for losses on others. Radio-tagged otters on Amchitka and Adak show maximum distances moved were 4.31 and 6.95 km, respectively. Marked animals that were lost disappeared suddenly and without a trace.

Increased mortality

Population declines with no reduction in birth rate or evidence of redistribution suggest increased mortality. Mortality could be caused by disease, toxins, predation, and starvation.

 

 

 

 

 

 

 

 

 

(Estes et al, 1998)

Disease Few beach-cast carcasses have been found. Blood analysis of select otters on Adak, Amchitka, Kiska, and Kanaga Islands show no signs of diseases.
Toxins There have been reports of elevated contaminants in the Aleutian Island, but they are restricted to a few small areas.
Predation Although not a regular part of their diet, there have been at least 10 observed attacks on sea otters by killer whales. Population comparisons of two sites, Clam Lagoon, Adak Island (inaccessible to killer whales) and Kuluk Bay (open coastal environment), from 1993 to 1997 show 76% decline at Kuluk Bay and stable population at Clam Lagoon. Calculating caloric needs of killer whale with decline in sea otter population in this 6-year period suggest only 5.05 attacks are needed. Six of the ten killer whale attacks were observed during this time period.
Starvation There have been increases in sea urchin biomass during sea otter decline. There has also been striking changes in the kelp forest ecosystem accompanying sea otter decline.
back to data
glossary
 

This web site was created by Lynn Tran at the North Carolina State University, Department of Mathematics, Science, and Technology Education on 7/12/03. Faculty advisor Dr. David Eggleston, NCSU, Department of Marine, Earth, & Atmospheric Sciences. Last updated December 29, 2003 .