Question

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.