Search for the Missing Sea Otters: An Ecological Detective Story

Around 1991, Dr. James Estes and his colleagues at the University of California, Santa Cruz, noticed that the otter populations they had been studying for over 20 years were beginning to shrink. Sea otter populations inhabiting several of the Aleutian Islands had declined as much as 90 percent in fewer than 10 years (Figure 1). What could cause such a sharp drop in sea otter numbers in this island chain of Alaska?

Read the following excerpt from an article published in the New York Times and see if you can determine where all of the missing otters had gone.

Could the otters simply have migrated from one part of the region to another? To find out, the researchers analyzed populations over a 500-mile-long stretch of the Aleutians from Kiska to Seguam …. By 1993 otter numbers in that whole stretch had been cut by half. Here the geographical scope of the research effort became critical; a smaller region would not have been large enough to reveal the decline. In 1997, they … found that the population decline had worsened, to about 90 percent ….

“That told us for sure it was a very large-scale decline, but we were still trying to understand the cause,” Dr. Estes said …. The researchers … ruled out reproductive failure. Their studies enabled them to keep track of how often otters gave birth and how many young survived, and this revealed that reproduction was continuing to re-supply the population.

With other possible causes eliminated, … mortality had to be the explanation. In the past, they had seen temporary declines in otter populations because of starvation, pollution or infectious disease. “In all those cases,” Dr. Estes said, “we find lots of bodies. They get weak and tired and come ashore to die.” This time not a single dead otter was found—a clue, he said, that “something really weird was going on.”

Part I—“What Could be the Cause of Decreasing Otter Numbers?”