Alaska Sea Grant

Additional Teacher Background for Investigation 2

Data from Study:

Type of Data or Observation (Intertidal Zone)

Rat Islands (Amchitka)

Near Islands (Shemya)

Sea Otter Presence/Absence

Abundant (10-30/km2 of habitat)

Scarce or absent

Kelp Presence/Absence

Complete mat of kelp

Kelp sparse or absent

Average Intensity of Grazing

<1% of kelp plots grazed

50-75% of kelp plots grazed

Average Urchin Density (per m2.)

8

78

Urchin Size (Length)

<12 mm

Maximum of 21 mm

(Adapted from Estes and Palmisano 1974.)

Sea Otters Facts Known before Study Began

  1. One of only 11 remnant populations of sea otters in the world after 1900 was located in the Rat Islands. From the 100 animals that were estimated around Amchitka in 1911, the population had grown rapidly and expanded into all of the islands in the group. There had been a high density of sea otters in the Rat Islands for about 20-30 years and the numbers had been increasing.
  2. After the sea otters had disappeared, the first observations of otters in the Near Islands was in the 1950s. At the time they arrived in Attu in the 1970s, and in Shemya in the late 1980s or early 1990s, only a few sea otters had been seen anywhere in the Near Islands and no group of otters had become established around any of the Near Islands.
  3. Studies of kelp beds in other areas had shown that they provided food and cover for a variety of marine invertebrates and fish.

Hypothesis of Scientists Jim Estes and John Palmisano:

A dense population of sea otters reduces the sea urchins to a sparse population of small individuals. The result is a release from grazing pressure of the urchins on kelp and a significant increase in the size of kelp beds and the marine animals associated with them.

Difficulties of getting exact counts or measurements:

Sea Otters

  • They dive under water to feed.
  • The Aleutian Islands are remote and subject to very bad weather.
  • Plane surveys are expensive, small boat surveys can only be done safely in some areas, and numbers are hard to compare.
  • People have different abilities to see and count all of the otters.

Sea Urchins

  • Urchins live on the bottom. They are underwater much of the time even when they are in the intertidal zone and always underwater in the subtidal zone. The only way to count them is with diving gear.
  • Visibility is often not very good underwater.
  • It is difficult to get a count of a very dense group of urchins in a small area.
  • It is impossible to count all of the urchins over a large area.

Kelp Beds

  • At the time they did the study, they didn’t have the type of cameras and remote sensing technology that would allow them to take aerial photos from which they could measure the area of kelp beds in specific areas.
  • Diving was required.
  • The differences in the amount of kelp in the two areas were so great that they didn’t really need to determine if the two areas were different and by how much. They really wanted to know the effect of different densities of urchins on the kelp.

Methods Used by the Scientists:

Sea otter counts are done by airplane, by boat, and from land on islands that have road systems. Comparing the data and correcting for “sightability” is difficult. Studies have been done to develop correction factors, based on studies with marked animals locatable by radio-tracking. The percentage of animals that can be observed after being tracked to a general location can provide the correction factor. Because of these difficulties, the sea otter “counts” are estimates, not censuses, but the declining trend is clear even though the actual number of animals that were in the population and how many there are now cannot be determined.

For the urchin counts and kelp measurements:

25-40 sites were randomly located at each island.

20 randomly located 0.25 m2 quadrats were placed on the seafloor at each site.

Within each quadrat, divers:

  • Counted individual kelp plants (anchored to the bottom) and identified them to species.
  • Counted the number of urchins and measured their test (shell).

To measure grazing pressure:

Grazing experiments were conducted by anchoring the same biomass of kelp at different locations, placing one-half in cages that excluded animals that would graze on the kelp. After a set amount of time, the remaining biomass of the grazed kelp was weighed for comparison with the ungrazed kelp.

¼ m2 quadrats were used, so results were averaged and multiplied by 4 to express the data as number per m2.

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