Alaska Sea Grant


Sea Otter Story Part 3

Sea urchins feed on kelp

After his boat surveys in the 1990s, Jim Estes worked with the biologists and managers of the Alaska Maritime National Wildlife Refuge to try to understand what was causing the sea otter population in the Aleutian Islands to once again be heading toward extinction.

It was their responsibility to find out the cause of the decline, which was the most rapid die-off of a marine mammal ever recorded. The cause of the decline was puzzling. Until the 1992 survey, the numbers had been stable or increasing in every area the biologists had surveyed earlier. All the reports they had received in the 1980s for specific areas were of continuing abundance. Something in the ecosystem had obviously changed. Based on the studies done by Jim Estes and his colleagues in the 1970s, the scientists and managers could predict that fewer otters in the ecosystem would result in major changes to other parts of the ecosystem. The results of the study showing Amchitka with lots of otters and Shemya with few suggested that fewer otters would mean the loss of kelp beds, which were habitat for fish, crabs, and other marine invertebrates. This could have a large impact on subsistence foods for people who lived in the Aleutians and the huge commercial fisheries that took place there. The only way to help the otters and the rest of the ecosystem to recover was to figure out what had changed that caused the otters to disappear.

Many more scientists were enlisted to solve the mystery and to determine the cause of the decline, so that wildlife managers could determine if there was anything people could do to reverse it and restore the ecosystem to an abundance of marine species.

Sea Otter Story Part 2 - A Comparison of Two Islands

When Jim Estes noticed fewer sea otters around Amchitka Island, his concern was based on his experience as a scientist who had been studying sea otters in the Aleutian Islands for 20 years. Remember, he arrived in 1970 to begin a study in the Rat and Near islands with another biologist, John Palmisano. They chose two sites to study, Amchitka in the Rat Islands, and Attu and Shemya in the Near Islands to the west, at the very end of the Aleutian chain of islands. They knew that Amchitka had an abundance of sea otters and that Attu and Shemya had few or none. Almost all of the sea otters from the Aleutian Islands to California had been harvested for their furs. Only 11 small groups of otters remained, and one was in the Rat Islands near Amchitka.

In a magazine article about their first study, they had this to say about their first impressions. "Upon arriving at the Amchitka Island in the Rat Islands group, we were immediately struck by the dense kelp beds. The kelp is so abundant that in many areas we could not see the rocky ocean floor either from the shore or when diving in the water. Yet at the Near Islands of Attu and Shemya, 250 miles to the west, there are only a few scattered kelp beds. What we did notice here was a dense carpet of large sea urchins, small invertebrates that live on the ocean floor or in rocky crevices and feed on the kelp. So completely have the sea urchins grazed the kelp that the ocean floor appeared light emerald, rather than dark brown as at Amchitka." (Palmisano and Estes 1976.)

Jim Estes and John Palmisano studied the otters and the ecosystems around the two islands for three seasons. Their results made it possible to understand the interconnections in kelp bed marine ecosystems and the important role of sea otters in the ecosystem. Their data and conclusions provided important clues to solve the mystery about the disappearing otters.

Consumers: Marine Mammals

Orcas, photo courtesy of Steve Trumble

Seals, sea lions, and killer whales live year-round in the waters near the Aleutian Islands. They eat fish, including salmon and small fish that live in schools. Gray whales and humpback whales migrate through the major passes in the Aleutian Islands to get to their summer feeding areas in the Bering Sea. These baleen whales eat small fish that live in schools and copepods, a type of zooplankton.

Two types of killer whales are found in these waters. Pods of resident killer whales stay together and stay around the same area and feed on fish, including a lot of salmon. Transient killer whales sometimes move through the area, roaming over long distances and preying on seals and sea lions. They may also prey on gray and humpback whales when they are migrating through the passes between the islands on their way to the Bering Sea.

Food chain diagram

Consumers: Marine Invertebrates and Fish in the Kelp Forest

Sea urchines, photo courtesy of Heloise Chenelot

Several types of animals depend on the kelp forest as their habitat. They find shelter there because the many tall blades of the kelp break the force of the waves. Inside the kelp forest, the water is calm and small animals can avoid being washed away by the waves. They also cling to the kelp and find hiding places from the other animals that want to eat them.

Sea urchins and snails eat the kelp, either when it is alive or after it has died back each year. Sea urchins feed at the base of the kelp, and large numbers of urchins can actually cut down a tall kelp. Crabs are the resident scavengers. Young salmon and other small fish that live in schools find shelter in kelp forests.

The Producers in the Ecosystem

Kelp, photo courtesy of Heloise Chenelot

The Producers in the Ecosystem

Only two types of living things in the ocean make food using sunlight. These are called producers. One type is phytoplankton. Huge numbers of phytoplankton, most of them only visible under a microscope, drift with the currents and are food for the zooplankton such as copepods and young urchins.

The second type are the seaweeds, which are large algae. The largest seaweeds grow low in the intertidal zone and extend out into the subtidal zone. These are the kelps, large brown seaweeds that are glued to the rocks with their holdfast. Their long blades float at or near the surface of the water where they get sunlight to make food, which allows them to grow very fast. Their holdfast glue is very strong, and it can keep the huge kelp in one place even when strong currents and waves occur during storms.

Snails and sea urchins eat kelp, and fish, crabs, and many other animals find shelter within the dense kelp stands—called a kelp forest. Like a forest on land, the kelp forest provides food, places to hide, and a calm place away from the force of waves and winds. Many kelps die back in the winter and grow again the following spring. Large amounts of dead kelp are recycled by scavengers, including sea urchins, and decomposers.

Ribbon Kelp 

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