Decline of the Steller Sea Lion in Alaskan Waters Untangling Food Webs and Fishing Nets (2003) / Chapter Skim
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3. Identifying Clues and Testing Hypotheses
3. Identifying Clues and Testing Hypotheses
Pages 39-56
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From page 39... ...
Some blend of the above provides a useful conceptual framework for evaluating various hypotheses concerning the population collapse of Steller sea lions in western Alaska. Both population and ecosystem models are valuable as tools for identifying what types of factors could explain the population decline and for evaluating the available data for consistency with these potential causes.
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From page 40... ...
Each addresses different issues regarding population viability, population trends in the context of local populations, and regional metapopulations. Elasticity analyses of 44 population models based on 50 mammal life tables place Steller sea lions in a larger context.
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From page 41... ...
Hence, the shift in age structure suggests that juvenile sea lions were disproportionately affected by whatever factor (or factors) was responsible for the steep population decline.
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From page 42... ...
During the steepest decline in the Steller sea lion population from 1985 to 1990, estimates of groundfishing-related takes were 1,350 to 1,600
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From page 43... ...
Subsistence takes have been estimated at 350 animals per year in the l990s (Wolfe and Mishler, 1997; Wolfe and Hutchinson-Scarbrough, 1999~. Shooting of Steller sea lions by fishermen was legal prior to 1972 and until 1990 was allowed in order to prevent gear damage.
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From page 44... ...
Models developed for the eastern Bering Sea system have ranged from relatively simple and static "snapshot" models of trophic mass balance using Ecopath (e.g., Trites et al., 1999; www.ecopath.org) , to complex spatial models based on the Laevastu et al.
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From page 45... ...
and sand lance (Ammodytes hexapterus) that are most available to Steller sea lions versus offshore/deepwater species (such as myctophids)
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From page 46... ...
. As outputs, relative abundance series were provided, ranging from the Steller sea lion western stock total abundance estimates to total pollock, cod, and flatfish abundance estimates from single-species stock assessments.
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From page 47... ...
However, the committee was unable to find Ecosim parameter combinations that would predict a short period of rapid decline during the late 1970s or early 1980s for Steller sea lions, related only to changes in trophic interactions caused by measured fishery changes and fitted primary production changes. This is not because the Ecosim functional structure precludes such predictions; see Figure 3.4 for an example where it did predict rapid populaton decline for Hawaiian monk seals (Monachus schauinslandi)
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From page 48... ...
The base model does not predict as severe or rapid a decline in Steller sea lions as was observed; no parameter combination was found that predicted a rapid decline during the mid-1980s involving only trophic interactions and known impacts of fisheries. SOURCES: Figure created based on Ecosim simulation of Ecopath model by Trites et al.
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From page 49... ...
gradual and prolonged than indicted by the census data and that there are no measured rapid changes in prey abundance just prior to or during the rapid Steller sea lion decline. Of course, a more rapid ecosystem-scale regime shift could be forced on the model dynamics in order to produce a more rapid decline in Steller sea lions, but such scenarios result in poor fits to trend data for the fish species.
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From page 50... ...
Still another is to assume that physical regime changes in 1976-1977 led to reduced vulnerability of various fish prey to Steller sea lions, perhaps by affecting prey depth distributions and/or prey use of coastal areas near rookeries. There is always the possibility that future, more detailed, modeling
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From page 51... ...
(2001~; data on small pelagics, zooplankton, and jellyfish came from Anderson and Piatt (1999~. exercises, comparison of models for the eastern Bering Sea and Gulf of Alaska regions, and further comparisons to historical data may reveal a "complete" explanation for the Steller sea lion decline in terms of trophic changes.
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From page 52... ...
~ ~ , ~ , , ~ EVALUATING HYPOTHESES FOR THE CAUSE OF THE STELLER SEA LION DECLINE A variety of complex hypotheses involving changes in trophic interactions in the eastern Bering Sea and Gulf of Alaska have been suggested as possible explanations for the Steller sea lion decline. These hypotheses mainly involve factors that could have resulted in two types of immediate effects on the sea lions: (1)
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From page 53... ...
On a global scale (eastern Bering Sea plus western Gulf of Alaska) , the data from single-species stock assessments do not support the proposition that fisheries and/or climatic factors have led to a reduction in the total abundance of food available to Steller sea lions.
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From page 54... ...
EVALUATING MECHANISMS IN RELATION TO SEA LION POPULATION DYNAMICS Considering the natural complexity of the eastern Bering Sea food web, further complicated by the effects of fisheries and oceanographic conditions on the recruitment of individual species, a variety of other
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From page 55... ...
If so, fishery reductions and closures aimed at the groundfish stocks may help to reinforce benthic community dominance and prevent recovery of a community structure that favors Steller sea lions. Shifts in food web organization due to both fishing and environmental change might have altered feeding patterns of killer whales and other predators, resulting in higher predation rates than might be expected based on a straightforward change in relative prey abundance.
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From page 56... ...
The ecosystem models develop a varied menu of hypotheses to explain the historical decline. Thus, based on available trajectories for the population trends of many species, Ecosim is unable to reproduce the rapid decline in sea lion abundance.
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