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Where do marine mammals go? 

The map and graphs below represent what scientists are able to learn from data they collect.  This is an example of a track line for an elephant seal that spent months with a scientific instrument attached to his back.   

This map illustrates what scientists can find out about the movements of an elephant seal (a male, in this case), by attaching a geographic location time-depth recorder to its skin.  If you are wondering about where the seal went or what kind of data the instrument collected, more information is available.

contdivgraphmd.jpg (45978 bytes)

This figure shows the diving behavior of a male elephant seal. The elephant seal has a continuous diving pattern, with only 2 to 3 minutes between dives. Dives are 20 to 40 minutes long, with the seal descending rapidly to a certain depth (in this case, some dives are down to 900 m, but elephant seals can dive to 1500 m). Several minutes are spent at that depth, then the elephant seal ascends to the surface. During the four months that the elephant seal is at sea, it is underwater for about 90% of the time.

What does this map mean?

This is a track of the a 4 month-long migration of an adult male elephant seal from San Miguel Island, California to a foraying area in the North Pacific and return to San Miguel Island based upon daily positions from a geolocation time-depth recorder.

Where did the seal go?

This male elephant seal was instrumented on San Miguel Island, California in March. After the instrument was attached, the seal swam along the west coast of North America, from San Miguel Island up to the Aleutian Islands. He spent a month feeding (elephant seals feed primarily on squid), and returned to San Miguel Island, across the ocean, in July.

Were there any differences in movements between male and female seals?

Scientists put instruments on both male and female elephant seals, and found that males migrate north (from California to the Gulf of Alaska or the eastern Aleutian Islands) twice a year: once after the breeding season, in March, and once after the molting season, in August. During the molting season, elephant seals shed their old hair and a layer with skin, and grow new hair; while they are molting, they stay on land. Female elephant seals also do a double migration, but the do not move so far north, spending their time in the oceanic waters far off the shore of Oregon and Washington.

What kind of data did the instrument collect?

The geographic location time-depth recorder, attached to the seal’s back with epoxy glue, monitored the depth and duration of the elephant seal’s dives, and monitored light levels and surface water temperature during the 3 to 4 minutes when the seal came up to the surface between dives.  The data were analyzed and the analysis produced dive depth profile charts and statistics on dive duration, depth, time submerged, as well as latitude and longitude positions. 

Elephant Seal Diving Duration

Animals

Dives

Duration (min)

Surface Interval (min)

 

 

Mean   Max

Mean

21 Females

116,555

24         89

2

15 Males

100,662

23         80

3

 

Elephant Seal Diving Depth

Animals

Dives

Depth (m)

 

 

Mean 

Max

21 Females

116,555

493

1567

15 Males

100,662

365

1581

 

Elephant Seal Diving Time Submerged

Animals

Dives

Percent of Time

 

 

Submerged 

Surface

21 Females

116,555

91

9

15 Males

100,662

88

12

Elephant seal diving duration, depth, and time submerged data tables based on slides by Robert DeLong based a geographic location time-depth recorder instrument study.

The light level and surface water temperature measurements were measured with a time-depth recorder which allowed scientists to figure out the location of the seal.

Why wasn’t a satellite-linked time-depth recorder used to get locations?

They were not available when this study began.  However using satellite-linked time-depth recorders  would still be difficult because a satellite must be able to "fix" on the instrument.  This requires that the instrument is at the surface for longer than two to three minutes and elephant seals do not spend very much time at the surface of the water. Therefore, scientists had to figure out a way to calculate the location of the seal, and developed the geographic location time-depth recorder.

Where can I find more information about this study? (References)

Stewart, Brent S. 1996. Uncommon Commuters. Natural History 105(2):58-63.

Stewart, B.S. and R.L. DeLong. 1995. Double Migrations of the Northern Elephant Seal, Mirounga angustirostris. Journal of Mammalogy 76(1):196-205.

Stewart, B.S. and R.L. DeLong. 1994. Postbreeding foraging migrations of northern elephant seals. P. 290-309 in: (LeBoeuf, B.J. and Laws, R.M. eds.) Elephant Seals: Population Ecology, Behavior and Physiology. Berkeley: University of California Press.

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