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Migration and Life at Sea

The northern elephant seals come to the rookery twice a year – once in the late spring and summer for approximately one month to molt and once in the fall or winter to rest if they are young or to give birth and breed if they are mature. The rest of the year, ten months of the year for most of the animals, they are at sea. While at sea they are solitary and spend 80% to 90% of their time deep under water. They dive routinely to 1000 to 2000 feet (300 to 700 meters) staying down typically for periods of one-half hour before returning to the surface to breathe for 2-3 minutes. Individual seals have been known to dive to over 5000 feet and to stay down as long as 2 hours. This behavior continues, uninterrupted by visits to land or longer periods on the surface, 24 hours a day for periods of several months.

The male seals typically forage for food on the continental shelf from Oregon to as far west as the Aleutian Islands – 3000 miles from the Central Coast of California. Their diet consists primarily of bottom dwelling fish – hakes, rays, small sharks – and squid. The density of food on the shelf is much higher than at intermediate depths so that they are able to obtain the approximately 100 pounds of fish per day that their great size requires. Feeding over the shelf, however, puts them relatively near the coast, a region most heavily populated by their major predator, the great white shark, but the need for food in large quantity justifies the greater risk. Their two foraging trips are approximately equal in length at four months.

The female seals, who do not grow so large as the adult males and who, as adults, spend two more months at sea each year, forage in deeper water well south of the continental shelf. There the ocean is so deep that they cannot feed off the bottom. They have a diet of squid and fish, and range widely over a considerable area. Unlike the males, the two foraging trips of adult females are of quite different length – approximately two months between weaning their pup and returning to molt and eight months between the end of the molt and birthing. Only during this longer second period is the female nourishing a fetus as well as herself.

The foraging patterns noted above are not always followed. One tracked seal from Ano Nuevo was observed to spend all of her foraging time in the neighborhood of Monterey Bay, and at least one female has been seen near the Alaskan continental shelf.

 

The Tracking of Pacific Pelagics - TOPP - web site has maps showing the current, day by day paths of individual seals. Click on the picture to go to that site.

To see an animation of the foraging trips of some female elephant seals during the two months between breeding and molting and the eight months between molting and return for birthing, click HERE.
 

 

 

 

Image courtesy of Google Earth mapping service

Elephant seals do not sleep in any conventional sense while at sea. Their frequent but brief trips to the surface preclude that. Electronically monitored seals have been observed, on occasion, to go into a shallow, gliding dive rather then their typical steeper foraging dive and it has been suggested that this is a resting behavior. Some marine mammals have developed special resting strategies including “turning off” portions of their brain for a time but it is not known whether the northern elephant seal has such an adaptation.

In its foraging dive, the northern elephant seal first exhales, emptying its lungs of almost all air. Per pound of mass, at the start of the dive an elephant seal will have less than 40% the amount of oxygen in its lung that a human diver would have but it has almost seven times as much oxygen in its blood and over eight times as much in its muscles. Within a short time into the dive, the lungs are collapsed expelling any remaining air. This both reduces buoyancy and protects the seal from the bends – an affliction caused by the absorption of gases by tissue at high pressures and the resulting damage on ascent when that pressure is rapidly decreased. All of the oxygen used to provide the energy needed during the remainder of the dive is stored in the red blood cells and the muscles. Only at the beginning of the dive does the seal swim with continuous use of its tail fins. During the rest of the descent, almost 90% of the time the seal simply glides. Intermittent stroking dominates the ascent.

To enable the long period without breathing and the rapid recharge at the surface, the northern elephant seal has very different surface and underwater metabolism. At the surface the seals take between 15 and 19 breaths per minute and have a pulse rate of between 80 and 110 beats per minute. On diving, the pulse rate drops typically to one-third the surface rate and occasionally as low as 3 beats per minute. Further, circulation is limited almost completely to the heart and the brain so that oxygen consumption is minimized. Time spend at the surface between dives is approximately two minutes, throughout the period they are at sea, so they spend 90% of their time under water.

Dive profiles are approximately V-shaped with descent and ascent at 40 - 50 degrees from the vertical at speeds of 2 - 5 mph (4 - 8 kph). As that is a rather continuous rate of speed for the time at sea, a seal will swim approximately 15,000 - 20,000 miles (25,000 - 32,000 km) each year. They are capable of swimming at speeds up to 10 mph (16 kph). Seals away from the continental shelf dive deeper during daylight hours than at night. This probably reflects the behavior of the prey rather than the effects of reduced light on the seals.

 

Tracking of elephant seals began in the early 1980's. Because the seals spend so little time at the surface, standard Global Positioning Satellite techniques would not work for location determination.

Early location techniques used a photocell, measurements of day length for latitude and time relative to sunrise which provided longitude. Later, he development of the Argos satellite provided accurate location determination a few times per day.

Newer, modified GPS systems now promise a few hundred locations per day including dive profiles.

 

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