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Sea Otters (Trophic Cascades)

Sea Otters

YouTube Video

The sea otter (Enhydra lutris) is the smallest marine mammal. Sea otters are distributed throughout the northern Pacific Ocean and are restricted to coastal regions because they collect their food (mostly crabs, clams, mussels, and sea urchins) from the ocean floor. They can remain underwater for only 30 to 90 seconds and so they inhabit areas where depths are shallow enough for short dives to the bottom. Once an otter brings food to the surface, it floats on its back, using its belly as a dinner tray. Otters often use rocks to smash open the hard shells of their prey, an activity that makes them unique among marine mammals.

Sea otters spend much of their time in water that can be as cold as -4°F. Consequently, they have evolved several mechanisms for maintaining a constant body temperature that is higher than that of their surroundings. Unlike other marine mammals (sea lions, for example), sea otters do not use an extra fat layer (blubber) to retain heat. Instead they have a double fur coat. The coat closest to the body of the otter, the underfur, is very fine and traps air. Heat released from the otter's body warms the trapped air, which serves as insulation. On top of the underfur are guard hairs, which keep the underfur dry. The guard hairs are much longer than those of the underfur and remain waterproof as long as they are clean, so sea otters spend 48 percent of the daylight hours grooming and cleaning their fur. This is the reason oil spills are so dangerous to sea otters. The oil coats the guard hairs and, since oil cannot be easily removed, the underfur becomes wet, loosing its insulative qualities, and the animal dies of hypothermia.

In addition to their two coats of fur, sea otters also keep their body temperature constant by maintaining high metabolic rates. It takes a lot of energy to maintain high metabolic rates. Consequently adult sea otters must consume 30 percent of their body weight each day. A human weighing 150 lbs would need to eat 45 lbs of food each day to do the same!

Sea otters used to be found all along the Pacific Rim coastline, from northern Japan, through the Kuril Islands (Russia), the Commander Islands (Russia), the Aleutian Islands (part of Alaska), and down the Alaskan, Canadian, and U.S. mainland coasts to the Baja Peninsula. In the mid-1700s, Russians began hunting otters for their pelts, and by the late 1700s, the English and Americans had also entered this fur trade. Sea otters were hunted nearly to extinction over the next 100 years. In fact, America purchased Alaska from the Russians in 1867 hoping to gain a greater share of the sea otter fur trade. What America did not realize was that Russia was willing to sell the territory because the otter populations had been reduced to a level where it was no longer economically productive to hunt them.

Before the onset of hunting in the mid-1700s, the estimated number of sea otters worldwide was 300,000 individuals. By the end of the 19th century, the number of remaining sea otters had dropped to approximately 1,000 in Alaska and 20 in California. Hunting of these animals ended with the signing of the International Fur Seal Treaty of 1911. The sea otters that remained in the 1900s were scattered amongst 13 relict populations. Several of these populations died out while those in Alaska flourished. The single population remaining in California has grown slowly and at its peak in 1995 consisted of 2,400 individuals. Presently there are sea otter populations on the Kuril Islands, Commander Islands, Aleutian Islands, and small portions of the mainland coasts of Alaska and California. The total worldwide population is currently estimated to be 150,000 individuals. 

What’s happening to the otters?

Sea otter Population by Island

Using the map above determine the current health of the Sea Otter population in the in the Aleutian Islands. Enter your data in the form linked below.
Check out the graphical results below. What do you think is happening to the Sea otter population in the Alaskan Island chain?

Sea Otter Population Study ‎‎(Responses)‎‎

Generating a hypothesis

As a class generate several hypothesis as to why the Sea Otters may be in decline. Record your hypothesis as directed by your instructor. You will vote on the several hypothesis you will investigate as a class.

Possible Hypotheses:
  • Sea Otters are not getting enough to eat.
  • Sea Otters are being hunted for fur.
  • Sea Otters are being eaten by predators.
  • Sea Otters are getting sick.
  • Sea Otters are being poisoned from toxic pollutants
  • Sea otters are failing to reproduce.
As it turns out Dr. James Estes from the University of California at Santa Cruz was very interested in determining the reason for the decline of the Sea Otters in the Aleutian Islands. Him and a team of researchers set out to collect data from their own observations as well as those from other research teams tackling similar questions. In their research they discovered a small Lagoon off of Kuluk Bay on the island Adak, Alaska. In this lagoon the otters seemed to be thriving and data suggested they were not under the same pressures as those in the nearby bay. Using Data collected in the Lagoon and Kuluk Bay, Dr. Estes and his team began to unfold the story of the Sea Otter decline. As a class we will be using data collected from Estes and other researchers to tell the story for ourselves. 
  • Pick a hypothesis (generated above) to investigate.
  • Using the Claims, Evidence, Reasoning template and the data linked below, support or refute your chosen hypothesis
    • Claim, evidence, Reasoning
      • Claim: We think...
      • Evidence: 
        • three pieces
      • Reasoning:
        • How does your data support your claim
        • Explain your thinking in detail

Why does this all matter?

Are Sea Otters important? Sure they are cute and you want to hug them, but are they really that important to the marine ecosystem? Take a look at the Food webs below, determine as a class the role Sea Otters play in the nearshore food web.


Keystone Species and Trophic Cascades

Sea otters are not the only organism that has large impact on their ecosystem. A keystone species, named after the stone found at the apex of a masonry arch, is a  a species which has a large impact on their local ecosystem. Another marine keystone species was discovered by Robert Paine in the 1960's. 

Watch the Excerpt from the video linked below, on how Paine conducted his experiment on Sea Stars. 

YouTube Video


Robert Paine Sea Star Data