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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. | ||||||||||||||||||||||||||||||||||||||
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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 144Ê longitude (Figure 2). While they are the same species of sea lion, the two populations do not intermingle. | |||||||||||||||||||||||||||||||||||||
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) |
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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.
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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.
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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.
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Table 4 compares the nutritional value of fish that are both eaten by sea lions and harvested by fishermen.
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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 . |