Decline of the Steller Sea Lion in Alaskan Waters Untangling Food Webs and Fishing Nets (2003) / Chapter Skim
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4. Review of Steller Sea Lion Biology
4. Review of Steller Sea Lion Biology
Pages 57-76
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From page 57... ...
. Steller sea lions are members of the Otariidae family.
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From page 58... ...
. According to Gentry, only Steller sea lions and southern sea lions (Otariaflavescens)
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From page 59... ...
Higher mortality rates for older males are suggested because the competition for breeding among males creates wounds that might contribute to shorter lives, but there are very little data on survival rates at older ages. During the period of rapid decline of Steller sea lions in western Alaska, the population in southeastern Alaska showed a slow but steady increase.
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From page 60... ...
Hazy 36. Forrester FIGURE 4.1 Distribution of the Steller sea lion in Alaska.
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From page 61... ...
. NMFS concluded that natal site fidelity made it unlikely that declining rookeries in the western stock would be supplemented by recruitment from more distant healthy populations in southeastern Alaska.
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From page 62... ...
POPULATION TRENDS Counts of Steller sea lions began as early as 1959 at rookery sites from islands just north of Kodiak Island out the Aleutians to Attu (see Tables 4.1 and 4.2~. At this time there was no reason to believe that any decline in the number of sea lions would be possible since it was reported that there were probably over 300,000 in the world.
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From page 64... ...
(1996~; additional data for 1991-2000 came from Anne York, National Marine Fisheries Service, National Marine Mammal Laboratory, Seattle, personal communication, 2002. As depicted in Figure 4.2, a rather dramatic decline occurred in the region between about 157° W and 165° W longitude (western Gulf of Alaska to eastern Aleutians)
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From page 65... ...
The homing behavior of female Steller sea lions suggests that rookery sites may represent a metapopulation (Raum-Suryan et al., 2002~. A metapopulation is comprised of local populations with low levels of exchange with one another.
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From page 66... ...
The heterogeneity in population trends since 1990 suggests there may have been a change in the predominant sources of mortality from the earlier to the current pattern of decline. VITAL RATES All Steller sea lion population models use vital rates data derived from samples taken near Marmot Island from 1975 to 1978 and again from 1985 to 1986 (Catkins et al., 1998; Pitcher et al., 1998~.
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From page 67... ...
PHYSIOLOGY, BEHAVIOR, AND FEEDING ECOLOGY To understand the potential impacts of food limitation or changes in food quality on the biology of Steller sea lions, it is necessary to consider their physiology, nutritional demands, and behavioral feeding patterns. This is usually conducted by equating their metabolic demands with the amount or quality of food available and the costs of obtaining that food.
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From page 69... ...
Some of these factors can be measured in free-living species but others cannot and must be estimated or modeled. In all cases the goal is to obtain data that can be used to estimate the nutritional requirements of Steller sea lions.
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From page 70... ...
These data are important for defining critical habitat and understanding if changes in prey base move some fish stocks out of sea lion foraging range. For example, male northern elephant seals migrate into the same area of the North Pacific to feed but do so at depths far exceeding those of routine depths by Steller sea lions (Le Boeuf, 1994~.
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From page 71... ...
Current methods of diet analysis include collection and analysis of scat samples from rookeries or haulouts and analysis of chemical signatures in Steller sea lion tissues that are indicative of certain prey items (stable isotope and fatty acid analysis)
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From page 72... ...
The recovery plan notes that there may be seasonal, spatial, and individual variations in diet but concludes that "diet studies over the last 15 years show that Steller sea lions eat a variety of fishes and invertebrates; demersal and off-bottom schooling fishes predominate." A summary table of stomach content studies can be found in the 1999 Biological Opinion (BiOp #2; Table 5 of National Marine Fisheries Service, 1999) and in condensed form in an overview report prepared by the Alaska Department of Fish & Game (Kruse et al., 2000, Table 3.2~.
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From page 73... ...
(2001) examined bone samples from Steller sea lions, northern fur seals, and harbor seals collected in the Bering Sea and the Gulf of Alaska from 1951 to 1997 for possible changes in stable isotope ratios of nitrogen (BAN, which varies with trophic level)
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From page 74... ...
(1997) concluded that the "high correlation between area-specific diet diversity and population changes supports the hypothesis that diet is linked with the Steller sea lion population decline in Alaska." This study forms the basis for the "diet diversity" hypothesis (see Chapter 6~.
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From page 75... ...
Adult females can be captured on rookeries when they are moving back and forth from the sea while nursing their pups. Adult female Steller sea lions, like all otariids, make foraging trips to sea and then return to the beaches to nurse their pups.
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From page 76... ...
A final caveat is that the endangered status of Steller sea lions and increased legal, political, and economic ramifications of research associated directly with the species have made the Marine Mammal Protection Act permitting process cumbersome. Many handson research projects approved during the spring 2001 in the multimillion dollar Steller Sea Lion Research Initiative had still not been approved for permitting as of spring 2002.
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