2001 Antarctic Expedition

The Foraging Biology of Weddell Seals

Notes from the Field

  (from Terrie M. Williams, Ph.D.)
text and photos copyright to T.M. Williams

 The Flag road leading into Weddell World.  Blowing snow and constant sunlight
 characterize our days and nights on the sea ice.  (Photo by R.W. Davis)

WEEK 5 – DIVING SEALS

Working with Weddells

This week the research team finally completed the instrument calibrations and seal surveys.  It is time to begin our studies on the hunting behavior of Weddell seals.  Our goal is to discover how these seals are able to navigate, catch fish and feed under the Antarctic sea ice.  

Fortunately, the Weddell seal is very cooperative when it comes to science.  They are large and confident - as a result they rarely bite and are easy to work with.  Few animals will prey on Weddell seals; only killer whales are a threat to them in open water.  However, the whales will not venture under the solid sea ice.  That makes the area around our camp a safe haven for diving Weddell seals.    

                 Selecting a Weddell seal for our study.  The research team uses data that we 
                 have collected on the seals at the ice crack during the past few weeks to select
                 a healthy adult female.  The seal is then transported to our camp in a special
                 sled connected to a Spryte.  At the back door of the laboratory the seal is placed
                on a soft sling that is used to weigh her.  (Photos by T.M. Williams) 

Once the seal is in camp, we take a wide variety of measurements.    The most difficult is trying to find out how much a Weddell seal weighs.   It takes all of us encouraging the seal to slide onto a soft sling net to move her into position.  We then lift her with a winch attached to a hanging scale.  It is like measuring fruit at the grocery store only a lot heavier.  Our measurements show that the seal weighs 316 kg (over 695 lbs) which is equivalent to approximately 4 - 5 of your teachers or 2 grand pianos.  That is A LOT of seal.  To find out how much of that weight is fat and how much is muscle we measure blubber thickness with a portable ultrasound machine, and body length and girth using a tape measure.

One Weddell seal = four Antarctic Explorers

As we fill out the data sheets we realize that we need to give the seal a name.  Her scientific name for the project is Seal 19 because she is the nineteenth Weddell seal that our team has studied in the past 5 years.  It soon becomes apparent that Seal 19 also has a lot of personality; some of the members of the research team begin to call her Ally McSeal.   

Once the body measurements are completed we prepare the instrumentation that Seal 19 will carry with her during her dives.  A special neoprene patch is fitted on her back like a blanket on a horse.  The VDAP camera/instrument pack is then attached to the patch. (See Week 4 for details about the VDAP.)  Seal 19 will wear the instruments for the next week .   Afterwards they will be removed and we will retrieve the data.   It is possible that Seal 19 will decide to swim away from the area with all of our instruments.  If she does that, the neoprene patch will fall off and the instruments with all of their data will be lost.  Naturally, the research team is counting on Seal 19 wanting to stay with the other seals near the crack by our camp.     

The Big Plunge for Seals and Scientists

After a good night's rest Seal 19 is ready to go for a dive.  We open up the floor boards of the laboratory and show her the ice hole that had been hidden beneath.  If you recall from Week 2, we built our camp around this diving hole that we drilled through the ice.  Now our seal can go for a dive without the team transporting her in the seal sled back to the ice crack.  Instead Seal 19 can simply swim back to the other seals on her own. 

An hour goes by and suddenly Seal 19 comes back to the dive hole in our laboratory for a brief visit.  She looks terrific!  She calmly floats in the water for a few minutes while we inspect her instruments- everything appears to be in working order.  

Then after taking several huge breathes Seal 19 goes for a dive and is gone.   

Now begins the hardest part of the expedition - waiting.  Seal 19 has just swum off with thousands of dollars of scientific instrumentation on her.  She will record the ocean temperature as she swims and the sounds of other Weddell seals calling in the area.  During her travels the instruments will record her swimming speed and the whoosh of every stroke she takes.  The research team hopes that she will hunt for giant Antarctic cod and enter schools of little Antarctic silverfish.  Through the video camera we hope to see everything that a seal sees when it is diving to the great dark depths of the Ross Sea.  Right now Seal 19 is the Chief Scientist.

The BIG question on everyone's mind is........ 

                                                     will she come back with the instruments and the data?

Suddenly we face an unexpected setback.  For three days blowing snow and high winds roll across the sea ice.  Our camp is buried under 8 foot deep snow drifts.  The ice bends under the weight of the snow and the floorboards of the laboratory start to buckle.  We wonder how much strain the wood can take.  The foul weather also causes all of the seals to hide below the surface of the ice.  Like the other seals in the area, Seal 19 is no where to be found.    

A snow drift climbs over the side and top of our laboratory.  (Photo by T.M. Williams) 

Our only chance of finding Seal 19 now depends on two tags included in the instrumentation that she carries.  One tag emits a radio signal that can be picked up with an antenna.  The other is a satellite tag that will send a location signal to an overhead satellite.  Both tags will be able to tell us if Seal 19 has hauled out on the ice.  For days we listen for the slow rhythmic beep, beep, beep of the radio tag and try to download satellite locations on our computers.  But the radio and computer remain silent.

Tracking the elusive Seal 19.  Randy Davis uses a radio antenna to 
listen for the pulse of the radio tag while Jesse Purdy monitors a computer
for data from the satellite tag.  (Photos by T.M. Williams and R.W. Davis)  

In addition to the tags, we place a hydrophone (underwater microphone) into the dive hole.  Although we can not see the seals, we can hear their "conversations" below us in the water.  Weddell seals chirp, click, trill and create booming sounds that echo off of the ice.  It appears that there are many seals swimming all around the camp.

                          Somewhere below the ice surface Seal 19 (alias Ally McSeal)
                     is making some of these calls and swimming with our instruments.  

                                                                                (Photo by R.W. Davis)

The research team tries to be patient as we await her return.

 Next week - The Weddell Seal's Underwater World

You can write to members of our Research Team at williams@biology.ucsc.edu