Case Teaching Notes
for
“Mystery in Alaska: Why Have All the Sea Lions Gone?”

by
Frank J. Dinan, Thomas R. Stabler, and Renee A. Larson
Department of Chemistry and Biochemistry
Canisius College, Buffalo, NY

Introduction / Background

This case study is designed for use in a non-science major course in chemistry or biology or in an environmental science course. It also could be used in an introductory level chemistry or biology course by stressing the quantitative, energy balance aspects of the case.

The case was inspired by the PBS Nature film, A Mystery in Alaska (PBS Nature, 2008). This program describes the drastic population decline of Steller sea lions along the western Alaskan coast beginning in the 1970s, and offers several hypotheses to account for this decline. We recommend that this program be viewed before teaching this case.

Prior to the 1970s, the commercial pollock fishery operating in southwestern Alaskan waters had expanded to become the largest in the world. The environmental activist group, Greenpeace, began protesting what they believed to be the over-fishing of pollock in this region, which is also the habitat of the Western stock of Steller sea lions. Subsequently, a federal ban on pollock fishing was imposed along that stretch of the Alaskan coast. The sea lion population decline began soon after this ban was introduced.

Steller sea lion populations are strongly dependent on both the abundance and nature of the prey available in their habitat. This is the so-called nutritional-stress hypothesis (Trites, 2003). This hypothesis states that “changes in the diet of Steller sea lions have had an undefined detrimental effect on their health” which requires a “good nutritional balance.” The ban on the commercial fishing of pollock caused a shift in the availability of the sea lions’ prey. Pollock became more abundant and herring less abundant along the southwest Alaskan coast where the Western stock of sea lions live, but the relative abundance of pollock and herring remained constant for the Eastern sea lion stock along the southeast coast.

These two fish comprise the major part of the Stellers’ diet. Pollock are larger than herring and are solitary, aggressive fish that prey upon smaller fish, including herring. The smaller herring are schooling fish that are therefore readily eaten in large numbers. Steller sea lions are powerful hunters that eat any fish available to them. After the pollock fishing ban was introduced in the 1970s, pollock became increasingly abundant to the Western stock of Steller sea lions and the herring stock diminished since they were preyed upon by both pollock and sea lions. However, this type of population shift did not occur along the southeast coast of Alaska, the home waters of the Eastern stock of Steller sea lions. In these waters, pollock fishing was not banned and the abundance of both herring and pollock was maintained and their ratio remained constant.

This increase in pollock abundance and decrease in herring abundance in Western waters coincided with a severe decrease in the population of the Western stock of Steller sea lions. The population of the Eastern sea lion stock increased gradually due to the stability of the herring and pollock populations. A direct link was established between the amount of pollock and amount of herring in the sea lions’ diet to their overall fitness: the junk food hypothesis (Rosen, 2000). This hypothesis asserts that “Steller sea lion populations are declining because they eat too much pollock, which contains fewer calories, and not enough fattier high energy fish.” The key, but not explicitly stated, point is that the sea lions expend a greater amount of energy catching and digesting pollock than the energy that they obtain when feeding on pollock. Thus, the sea lions have an overall negative net energy balance when consuming pollock. In contrast, sea lions have an overall positive net energy balance when consuming the fattier, easier to catch and digest herring.

This case study highlights the importance of energy considerations within food chains. It points out how energy considerations within these chains can lead to devastating, unpredictable outcomes.

Note: In this case, we have explored only one of the hypotheses that could account for the decline in the Western stock of Steller sea lions. Others exist that could also account for this decline. For example, a climate shift occurring in the region has also been proposed as a contributing cause (see Fritz and Hinkley, 2005). In developing this case, class time considerations were a factor in our decision to not explore these alternative proposals. Other instructors may wish to expand the case to include them.

Objectives

Upon completing this case study, students should be able to:

• Explain why the Western stock of Steller sea lions is decreasing while the Eastern stock is increasing.
• Infer that the problem with the Steller population is due to the nutritional and energy needs of these sea lions.
• Interpret data from graphs and other visuals and apply them to a real-life situation.
• Recognize the special vulnerability of sea lion pups and the key role they play in maintaining the population.
• Do calculations that allow them to determine the contribution that fats, proteins, and carbohydrates make to sea lions’ diets when eating pollock and herring.
• Recognize the importance of proper energy balances within elements of a food chain.

Classroom Management

This case study has been used with college freshmen in introductory science courses for both non-science and science majors. Fifty minutes, an average college class period, is sufficient time to discuss and complete the case study. Students work in four-person cooperative teams to facilitate cooperative discussion of the problems posed by the case.

The case is divided into three parts:

• Part I – West vs. East is a general introduction to the Steller sea lion population decline, with a focus on the sea lion pups. Students should be encouraged to actively brainstorm to generate ideas that might account for the population changes reported for the sea lion populations.

• Part II – Pollock vs. Herring reveals more information about the diet of the Steller sea lions, including the importance of pollock and herring. Students should determine that the population decline is due to changes in the availability of pollock and herring. Students also need to create their own hypothesis explaining the sea lion population trend and create an experiment to test their hypothesis.

• Part III – Diet vs. Energy Requirements focuses on the energy content and needs of the pollock, herring and Steller sea lion populations. Students discover how the health of sea lions is affected by eating either pollock or herring. Students are asked to apply what they have learned in the previous portions of the case to proposing a practical solution. After this has been done in each group, the groups present their findings and solutions to the class. A class discussion analyzing the Steller sea lions’ disappearance and possible solutions to this problem follows.

Answer Key

Answers to the questions posed in the case study are provided in a separate answer key to the case. Those answers are password-protected. To access the answers for this case, go to the key. You will be prompted for a username and password. If you have not yet registered with us, you can see whether you are eligible for an account by reviewing our password policy and then apply online or write to answerkey@sciencecases.org.

References

Alaska Department of Fish and Game. Steller Sea Lion: Wildlife Notebook Series. 2007.

http://www.adfg.state.ak.us/pubs/notebook/marine/sealion.php. Last accessed 6/10/09.

Alaska Fisheries Science Center. National Marine Mammal Laboratory: Marine Mammal Species. 1994.

http://www.afsc.noaa.gov/nmml/species/species_steller.php. Last accessed: 5/20/08.

Donnelly, C.P., A.W. Trites, and D.D. Kits. 2003. Possible effects of pollock and herring on the growth and reproductive success of Steller sea lions. British Journal of Nutrition 89(1): 71–82.

Fritz, L.W., and S. Hinckley. 2005. A critical review of the regime shift-“junk-food” nutritional stress hypothesis. Marine Mammal Science. 21: 476–518.

North Pacific Universities Marine Mammal Research Consortium. Steller Sea Lion Research—The Problem.

http://www.marinemammal.org/research/research.php. Last accessed: 5/28/08.

PBS Nature. A Mystery in Alaska.

http://www.pbs.org/wnet/nature/alaska/index.html. Last accessed: 8/7/08.

Pilcher, K.W., and D.G. Calkins. 1981. Reproductive biology of Steller sea lions in the Gulf of Alaska. Journal of Mammalogy 62(3): 599–605.

Rosen, D.A.S., and A.W Trites. 2000. Pollock and the decline of Steller sea Lions: Testing the junk-food hypothesis. Canadian Journal of Zoology 78(7): 1243–1250.

Thorne, R.E. 2009. The Role of Pacific Herring as Critical Winter Forage for Steller Sea Lions Final Report.

http://www.pwssc.org/research/biological/Stellar/StellerSeaLionFinalReport.pdf. Last accessed: 6/8/09.

Trites, A.W., and P.A. Larkin. 1996. Changes in the abundance of Steller sea lions (Eumetopias jubatus) in Alaska from 1956 to 1992: How many were there? Aquatic Mammals 22.3, 153–166.

http://www.marinemammal.org/pdfs/TritesLarkin1996-abundance.pdf

Trites, A.W., and C.P. Donnelly. 2003. The decline of Steller sea lions Eumetopias jubatus in Alaska: A review of the nutritional stress hypothesis. Mammal Review 33(1): 3–28.

Acknowledgements: This case was published with support from the National Science Foundation under CCLI Award #0341279. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Date Posted: November 3, 2009.

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