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glossary
Steller's sea lions are found around the North Pacific Ocean rim from northern Hokkaiddo, Japan through the Kuril Islands and Okhotsk Sea, Aleutian Islands and central Bering Sea, southern coast of Alaska and south to the Channel Islands, California. The shaded area shows the extent of their distribution.
Figure 1. Map of Steller's sea lion distribution across northern Pacific Ocean including Bering Sea and Okhotsk Sea (nmml.afsc.noaa.gov/AlaskaEcosystems/sslhome/distrib.htm)
Figure 2. Division of Steller's sea lion population in Alaska. (www.marinemammal.org/whatsnew/2003/foodshortages.html)
Steller's sea lion in Alaska are divided into two distinct populations. The division between the western and eastern Steller sea lion populations. is shown by the dashed line at the 144 longitude (Figure 2). While they are the same species of sea lion, the two populations do not intermingle.

(Milette and Trites, 2003)

Figure 3 is estimated numbers of Steller's sea lions (all ages) in Alaska from 1956 to 2000. There was a rapid decline in western sea lion population from the 1970s to 80s. This population was still declining in the 1990s, but at a much slower rate. Meanwhile, the eastern population may have been increasing slowly during this same time period.

Think about the geographic location of the declining versus the stable populations.

(Trites and Donnelly, 2003; www.marinemammal.org/whatsnew/2003/nutritional_stress.html)

Figure 3. Population of two Steller's sea lion stocks in Alaska. (from Trites & Larkin, 1996; A.W. Trites, unpublished data)
Table 1. Reductions in axillary girth and weight in Steller sea lions aged 1, 7, and 14 in 1985-86 as compared to 1975-78.
Age Reduction in axillary girth (%) Reduction in weight (%)
1 10.4 26.9
7 6.29 12.3
14 1.7 3.0

Density-dependent population theories suggest that body size should increase as population density decreases since there are per capita food increases. Table 1 shows that there was a noticeable drop in sea lion length and weight from the 1970s to 1980s. Contrary to the theories, population growth rate was declining during this time period.

  1. What could be causing this inconsistency?

(Trites and Donnelly, 2003)

Table 2 compares the composition of food contents and scats in Steller's sea lions on Kodiak Island during three decades. It reveals that there was a shift in diet for sea lions in the declining population from predominantly small schooling fish to Gadids.

  1. What caused this shift, and has it been a change for the better?
(Trites and Donnelly, 2003)
Table 2. Proportion (%) of Steller's sea lion scats and stomachs containing five prey categories during the summer months in the declining Kodiak Island region in the 1970s, 1980s, and 1990s (Merrick et al., 1997)
Gadids
Salmon
Small schooling fish
Cephalopod
Flatfish
1990-93
85.2
18.5
18.5
11.1
13.0
1985-86
60.0
20.0
20.0
20.0
5.0
1976-78
32.1
17.9
60.7
0.0
0.0
Gadids = walleye pollock, Pacific cod, Pacific hake; small schooling fish = capelin, Pacific herring, eulachon, Pacific sand lance; Flatfish = arrowtooth flounder, rock sole
Table 3. Conclusions from studies investigating various physiological and behavioral causes for declining sea lion population, 1970s to 1980s (* studies conducted with sea lions in 1990s)
Variable Evidence
Birth weight & growth rates Insufficient data to conclude a reduction.
Reduced pup survival No evidence of high pup mortality.

Reduced juvenile survival

 

Reduced numbers of juveniles returning to rookeries of birth in 3-4 years for mating; reduced proportions of juveniles relative to adults at haulouts & rookeries.

Blood parameters

 

 

Blood chemistry test inconclusive. Haptoglobin levels (blood protein that increases markedly in response to infection, inflammation, trauma or tumors) in the western population is significantly high.

Behavioral & physiological modifications*

Maternal attendance patterns & foraging behavior suggest sea lions in declining population were not food-limited during summer months.

Table 3 summarizes conclusions from studies exploring physiological or behavioral causes for Steller's sea lion population decline in Alaska. The results show that there has not been evidence of high pup mortality, but there has been a decline in juveniles returning to rookeries for mating. Blood tests show that sea lions in the western population may be experiencing tramatic activities, but this is inconclusive.

 

Sea lion and sea otter populations in Alaska are both declining. Consider the geographic location and the time frame of the population declines.

  1. Where is the geographic range of population decline for the two marine mammal species?
  2. How do the time periods for the population declines relate to one another?
  3. Can the cause for sea lion population decline correlate with the sea otter decline?
(Trites and Donnelly, 2003)

Table 4 compares the nutritional value of fish that are both eaten by sea lions and harvested by fishermen.

  1. What do you think has caused the decline in the western sea lion population?
  2. Could it be a nutritional factor, i.e. the western population of sea lions are not as well nourished as those in eastern Alaska?
Table 4. Partial composition analyses and energy densities of herring and common gadids (Walford & Wilber, 1955; Perez, 1994)
Oil (%)
Protein (%)
Energy density (kJ/g wet mass)
Pollock
0.8
20
4.66
Cod
0.4
17
3.95
Whiting
0.4
17
-
Haddock
0.3
18
-
Herring
11.0
19
8.61
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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 .