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.
Were there any differences in movements between male and female seals?
Why wasn’t a satellite-linked time-depth recorder used to get locations?
Where can I find more information about this study? (References)
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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