1. The shift in fish populations would likely result in the Western stock of Steller sea lions having to adjust their diets to consume more pollock and less herring. This may have negative effects on their overall health and reproductive success, as pollock are more difficult for the sea lions to catch and digest compared to herring.
2. Hypothesis: The decline in the Western stock of Steller sea lions is due to the shift in fish populations, specifically the decrease in herring and increase in pollock, leading to changes in their diet and decreased food availability. The Eastern stock is increasing because they have access to more abundant herring populations.
3. The higher pup mortality rate in the Western sea lion stock could be attributed to the changes in their diet resulting from the shift in fish populations. With less herring available, the mothers may struggle to provide enough food for their pups, leading to higher pup mortality rates.
4. Experiment: To test the hypothesis, researchers could conduct a controlled feeding study where they provide different groups of Steller sea lions (representing the Western and Eastern stocks) with varying ratios of herring and pollock in their diets. The researchers would then monitor the health and reproductive success of the sea lions over time to see if there are differences between the two groups. This experiment would help determine if changes in diet due to shifts in fish populations could be causing the decline in the Western sea lion stock.
Steller sea lions are powerful predators, feeding on almost any smaller fish found in their habitat. However, two fish comprise a majority of the sea lions' diet: pollock and herring. Herring are small schooling fish that can be fed upon in large numbers. Contrastingly, pollock are larger, more solitary, and difficult for the sea lions to catch and digest.
In 1970, the Federal government, prompted by environmental groups that argued that the pollock population was declining in western Alaskan waters, introduced a ban on the commercial fishing of pollock from the southwestern coast of Alaska through the Aleutian Islands. This ban altered the western waters ecosystem, but left that of the waters along the southeast Alaskan coast largely unchanged Whilefish populations in the southeast remained unchanged, relative fish populations in the southwest flip-flopped, with herring becoming less plentiful and pollock more abundant. (A portion of the fish community was
salmon, and this remained unchanged with the change in fishing regulations in both the southwest and
southeast.)
As the ecosystem in southwest Alaskan waters changed, the population of the Western stock of Steller sea lions also changed. A census of the populations of pollock, herring, and the Western stock of Steller sea lions in southwest Alaska disclosed striking trends, which are illustrated in Figure 3. It was also observed that the
Western stock exhibited a far higher pup fatality rate than that observed in the Eastern stock (Rosen, )000). Steller sea lions give birth to a single pup. The age of weaning for pups is variable, ranging from one to three
years. While the pups suckle, they do not hunt and are dependent on their mothers for food (Alaska, (007).
The data in Figure 3 show the trends in the relative numbers of pollock, herring, and sea lions (Thorne, )009).
Figure 3. Population Trends
Relative Species Populations
180
160
140
Western
Stock
Pollock
Herring
2000
1995
1990
Year
Graph based on based on Thorne, 2009.
1985
1980
1975
1970
20
40
0
60
80
100
120
r. How would the shiftin fish populations (i.e., a decline in herring and increase in pollock in the southwest) affect the diets of the Western and Eastern stocks of Steller sea lions?
Questions
). Formulate a hypothesis that explains why the population of the Western sea lion stock is declining while that of the Eastern stock is increasing.
3. Would the hypothesis that you have developed account for the higher pup mortality rate observed in the Western sea lion stock? How?
+. Formulate and describe an experiment that could be used to test your hypothesis.
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