Bridge Logo diver
An ocean of free teacher-approved marine education resources

NOAA SeaGrant

 
bullet Bridge DATA Series

Steller Sea Lion
A Stellar Animal

Written by: Lisa Ayers Lawrence, Virginia Sea Grant, Virginia Institute of Marine Science

Grade Level:
9-12

Lesson Time:
1 - 1.5 hrs.

Materials Required:
White Sisters data file (Excel), Amak Island data file (Excel), Metadata explanation (txt), graphing paper, ruler

Natl. Science Standards
Click here for a list of the aligned National Science Education Standards.

Related Resources
Marine mammal, Endangered species, Policy & Resource Management, Conservation

Summary
Analyze Steller sea lion population trends using data from the National Marine Mammal Laboratory's Alaska Fisheries Science Center.

Objectives

  • Identify the differences between seals and sea lions.
  • Name factors contributing to the population decline of Steller sea lions.
  • Compare and graph Steller sea lion population numbers.

Vocabulary
Otariid, Metadata

Introduction
Steller sea lions (Eumetopias jubatus) are certainly the stars of the Otariids, or eared seals. Steller sea lions, named for the German naturalist Wilhelm Steller, are the largest of the sea lions with males reaching up to 11 ft in length and weighing well over one ton. They are called sea lions because their thick neck resembles a lion's mane, and they differ from seals by the presence of ear flaps and by the shape of their flippers. Their long, fin-like front flippers enable them to swim far distances looking for food. With their large, paddle-like rear flippers, they can steer underwater and manuever on land.

Steller sea lions are found along the north Pacific rim from California to Japan. The Steller population is divided into two stocks that are split at 144 W longitude. These two stocks are not only genetically different, but are also very different in population status. Both populations experienced a dramatic drop in numbers in the 1970s, but the stock west of 144 W longitude continued to decline despite being officially listed as a threatened species since 1990. This continued decline resulted in their being listed as an endangered species in 1997 where they remain to this day.

It is widely believed that the drop in Steller numbers is due to a combination of disease, predation, environmental change and competition between the sea lions and commercial fisheries. Stellers feed on a variety of commercially important fish species such as pollock, mackerel, herring, capelin, cod and salmon. Some scientists believe that as the commercial fishing industry took increasingly more of the Steller's food items, the sea lions became nutritionally stressed, causing lower rates of reproduction and juvenile survival. This theory has been well publicized by the environmental group Greenpeace.

More research is being done on the Steller sea lion population and researchers believe that although direct competition with fisheries may have played a major role in the population decline in the 1970s, the decline from the 1990s through the present is far more complicated. According to a report by the Alaska Steller Sea Lion Restoration Team, the drop in western stock Steller sea lions may be a result of the following:

  • Low food availability near rookeries due to previous fisheries harvesting. This impacts the juveniles more than the adults, because they cannot travel as far looking for food.
  • The "junk food hypothesis" which says that the prey species near the rookeries are no longer the high fat fish, which the sea lions need, but low fat fish and therefore the sea lions are not getting proper nutrition.
  • The "predator pit hypothesis" which states that at low population numbers, the number of new sea lion pups born is lower than the number of sea lions eaten by predators such as killer whales.

Data Activity
For this activity, we will compare adult Steller sea lion populations from two locations, one from west of 144 W longitude, and one from east of 144 W longitude. The data are from the National Marine Mammal Laboratory's Alaska Fisheries Science Center. You can download the entire database by clicking on the NMML.SSL.ADULT.ZIP link and unzipping the file; however, for easier use we have downloaded the portions of the data we will look at and imported them into the following two Microsoft Excel files:

After downloading the data, be sure to read the metadata. Metadata give us important information about the data. For instance, data from the months of June and July are the ones generally used for population analyses, and data from aerial surveys are the most precise. For these reasons, we have selected only aerial count data from the months of June and July for our two locations. For years that had multiple counts conducted during those months, an average was taken.

Once you have downloaded the data, have your students graph the number of adult sea lions ("COUNT" column) versus the year and answer the following discussion questions.

  • What trends do you see in the populations? Are the trends similar between locations?
  • What factors do you think are influencing the population numbers?
  • Which population do you think is from the western stock? (The answer is at the bottom.) Why do you think that?
  • What do you predict will happen to the two populations over the next decade?

Answer: The Amak Island population is from the western stock.

 The Bridge is sponsored by NOAA Sea Grant and the National Marine Educators Association

Virginia Sea Grant Marine Advisory Program
Virginia Institute of Marine Science
College of William and Mary