Search for the Missing Sea Otters: An Ecological Detective Story

Part II—“What Predator Could be Causing the Large Decrease in Otter Numbers?”

by
Mary E. Allen and Mark L. Kuhlmann
Department of Biology
Hartwick College, Oneonta, NY

A. How would you test a hypothesis?

Dr. Estes and his group hypothesized that increased predation by killer whales was the cause of the sea otter decline. This was an unusual idea, since killer whales and sea otters had been observed together in Alaska for decades with no obvious interactions occurring between them. The first time a killer whale was observed attacking a sea otter was in 1991. Nine more attacks were observed in the next seven years and it was these attacks that finally led Dr. Estes and his colleagues to propose their hypothesis. To test this hypothesis, the scientists needed to have information about the killer whale.

Make a list of the types of information about killer whales you believe the scientists might need to test their hypothesis that increased predation by the whales was the cause of the sea otter decline.
Describe two experiments that would allow you to test the hypothesis that increased predation by killer whales was the cause of the sea otter decline. Keep in mind the following key components of any good experiment: a control (something to which to compare the treatment), replication (do it more than once), and consideration of confounding factors (what might cause differences other than what you manipulate in your experiment?).

B. What do the data tell you?

Estes and his colleagues estimated the impact of killer whales on sea otter populations by comparing trends in population size and survival rates of individually marked otters between two adjacent locations on Adak Island—Clam Lagoon and Kuluk Bay. Kuluk Bay is on an open coast, so sea otters there are exposed to killer whales. In Clam Lagoon, the entrance from the open sea is too narrow and shallow for killer whales to get in. Based on Figures 3 and 4 below, what can you conclude about the effects of killer whales on sea otter populations? Why do you think the scientists both counted all the sea otters and did the tagging and radio tracking?

Graph

Figure 3. Changes in sea otter population size over time at Clam Lagoon and Kuluk Bay, Adak Island, Alaska. Redrawn from Estes et al., 1998.

Chart

Figure 4. Survival rates of sea otters individually marked in 1995 with flipper tags and radio transmitters at Clam Lagoon and Kuluk Bay, Adak Island, Alaska. Redrawn from Estes et al., 1998.

C. Could predation by killer whales account for a 90 percent decrease in otter numbers?

If increased predation by killer whales was the primary cause of the sea otter decline in Alaska from 1990-1996, as Estes and his group suspected, killer whales would have to have eaten 40,000 sea otters in six years! How many killer whales would it take to eat this many sea otters? We know that killer whales travel in groups ranging from five to 25 individuals. Would it take one such group or 100? This is an important question to answer to determine whether killer whale predation could account for all of the missing sea otters. Using data provided in the table below, calculate how many whales feeding exclusively on sea otters it would take to eat 40,000 sea otters.

**Table 1. Killer whale and sea otter energetics**
Adult sea otters
Estimated number of Aleutian Island sea otters eaten, 1990-1996	40,000
average caloric content	1.81 kcal/gram wet weight
average mass, male	34 kg
average mass, female	23 kg
Killer whales
average field metabolic rate	55 kcal/kg of whale/day
average mass, male	5600 kg
average mass, female	3400 kg

Data from Estes, J.A., M.T. Tinker, T.M. Williams, and D.F. Doak. 1998. “Killer whale predation on sea otters linking oceanic ecosystems.” Science 282: 473-476.

Go to Part III—“Why Are Killer Whales Eating Sea Otters Now?”

Originally published at http://www.sciencecases.org/sea_otters/sea_otters2.asp