Mystery in Alaska by Dinan, Stabler, and Larson

Part III – Diet vs. Energy Requirements

An experiment was conducted at the Vancouver Aquarium Marine Science Centre to compare the relative effects of eating pollock and herring on Steller sea lions (Alaska Fisheries Science Center, 1994). Releasing live, swimming prey for sea lions to chase and capture in two very large tanks simulated the natural habitat of the Steller sea lions. The first group of sea lions was placed in a tank where they preyed upon herring as their only food source. A second sea lion group preyed exclusively on pollock in an otherwise comparable second tank. The sea lions were allowed to eat as much of each of these types of fish as they desired. Those fed herring all gained weight during the course of the experiment while those consuming exclusively pollock all lost weight.

This experiment was then extended for the second group. This group’s diet was switched so that they ate strictly herring rather than pollock. Under these conditions, the sea lions reversed their weight loss and experienced a steady weight gain. It was also noted that sea lion pups exhibited the most dramatic weight losses and gains during each of these experiments.

The researchers compared the amount of energy available to the Steller sea lions when eating pollock to that available when they fed on herring. Pollock is a lean, low fat fish; it contains 1% fat and 20% protein and 79% carbohydrate. Herring is a fatty fish; it contains 10% fat, 19% protein, and 71% carbohydrate (Donnelly, 2003). It was also found that the larger pollock is a more difficult fish for the sea lions to digest than the smaller herring.

This experiment highlighted the importance of meeting energy requirements for marine mammals to live healthy lives. There are three general types of foods from which sea lions obtain energy: fats, carbohydrates, and proteins. Fats contain 9 calories per gram while proteins and carbohydrates afford only 4 calories per gram. To thrive, any species must consume food that provides enough net energy to sustain a healthy body.

Calculations

1. Calculate the number of calories a sea lion would consume when eating 100 grams each of pollock and herring. Base your calculations on the fat, carbohydrate, and protein content of each of these fish.

   1. Total calories per 100g of pollock = fat contribution + protein contribution + carbohydrate contribution

   2. Total calories per 100g of herring = fat contribution + protein contribution + carbohydrate contribution

2. Calculate the percent of calories obtained from fat, carbohydrate, and protein in both herring and pollock.

Questions

1. Based on your calculations, which fish—pollock or herring—better meets the energy needs of the Steller sea lions?

2. Since the sea lions in each tank were allowed to eat unlimited amounts of prey, why did they lose weight when eating pollock? Why could they not just eat more pollock to maintain or increase their weight?

3. What is the significance of the fact that the sea lion pups were affected more than the adults? Why were the pups especially impacted? Explain your reasoning.

4. Using all of the information and the inferences you have drawn above, clearly explain why the Western stock of the Steller sea lion population is decreasing while that of the Eastern stock has remained steady in the 1970s and has increased modestly.

5. Propose a measure or measures that could be implemented to reverse the decline in the Western stock of Steller sea lions. Would you expect both support and opposition to the measure that you propose? If so, where would the opposition and support for your proposal come from?

References

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 33: 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.

Note

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). Time considerations prevent us from exploring these alternative proposals.

 

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