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

A "bork" otherwise known as Pagothenia borchgrevinki swims just
  below the underside of the sea ice in McMurdo Sound.  This is one of 
        the species of fish that the Weddell seal hunts.  (Photo by R.W. Davis)

WEEK 6 – THE UNDERWATER WORLD OF WEDDELL SEALS

Lost Time and Animals in the Storm

The weather turned into a major challenge this week as we continued our search for Seal 19 (Ally McSeal) and the scientific instrumentation that she carried.  For three days we tried to track her by radio and satellite tags with little luck.  Constant overcast and blowing snow was keeping her in the water; the result was that the signals from the tags could not reach us.  (Unfortunately, the seal has to haul out on top of the ice for the tag signals to transmit through air.)

Then the Herbie hit.  And then another.  (See Week 3 for a description of Antarctic storms, called Herbies.)  We noticed the approaching storm in the early evening as we were conducting our seal surveys at the ice crack.  First, White Island disappeared in a snow cloud then Black Island was gone.  Two hours later the Herbie blew into camp.  The sky, ground and horizon quickly disappeared and it was too dangerous for us to leave the buildings.  There was no chance of finding Seal 19 now.

Winds howled at over 60 mph, sounding like a locomotive in the smaller huts.   Throughout the night the storm blew snow into every crack or hole in the walls and roof, and relentlessly shook the walls next to our sleeping bags.  The floors rumbled with the gusting winds.  Most of the team members got little sleep- partly out of concern for the camp and partly out of concern for Seal 19.    

Along with lots of snow the Herbie brought in several other visitors to our camp - Adelie penguins.  Two penguins arrived together sliding along the soft snow on their bellies.  Another little penguin walked into our backyard and seemed to be curious about the workings of the camp.  He walked slowly around the laboratory and huts.  Finally, he sat outside of our kitchen window watching us eat our lunch.  

Tracks in the snow gave us a clue that we had visitors.  To move quickly penguins will slide on their bellies using their feet like oars to propel themselves.  The central groove in the picture is where the penguin's chest slid along the snow.  On either side are the penguin's footprints and the thin edges of its wings slipping on the snow.  (Photo by T.M. Williams)

Discoveries Beneath the Surface of the Ice - a Seal's Tale

The exhausted research team was still up at midnight when we suddenly heard the rhythmic beep, beep, beep of Seal 19's radio transmitter echoing through the static of the radio receiver.  Several team members grabbed binoculars to look for her at the seal crack.  And there she was.  Seal 19 had returned to the haul out with the other seals and was rolling in the fresh snow.  

We rode excitedly on snow mobiles towards the crack only to have our high expectations sink into disappointment.  Seal 19 had returned but the instrumentation appeared to be in shambles.  The camera was dangling on a long cable that trailed behind her.  The neoprene blanket holding the VDAP was torn and the instrumentation barely hanging on.  

Seal 19 sat on the ice as the team members quickly removed all of our instruments.  As we rode back to the laboratory with the broken VDAP, Seal 19 went back to rolling in the snow.        

Back at the laboratory, the VDAP was washed in fresh water and the instruments inspected.  Fortunately, the damage was not as bad as we had first thought.  

It was now 3:00 in the morning.  Although curious about the data and the video tape in the VDAP housing, we had to wait for the instruments to warm up to room temperature before we could peak inside.  It would be later in the morning before we would know if Seal 19 was a success or a failure as a marine biologist.  

In the morning the computers begin to download the data and the research team studies the video tape from the VDAP.  With each passing minute our excitement builds.  Seal 19 has provided us with a whole new view of her underwater world.  Through the camera and data we now see beneath the Antarctic ice through the eyes of a Weddell seal. 

(ESB website)

We discover that it takes A LOT of fish to feed a Weddell seal.  During one dive Seal 19 ate over 100 Antarctic silverfish - each about the size of an anchovy.   She gulps them down like popcorn.

One of the most exciting discoveries is watching seal "conversations" on the video tapes.  The chirps, trills, and snorts provide a variety of signals to the seals.  (See Week 5 to hear some of the sounds that Weddell seals can make.)   Seal 19 seems to use her calls to echo off of the ice and ocean bottom to help her navigate.  She also uses sharp chirps to signal other seals to get out of an ice hole when she needs to breathe.  There are times that they answer her back with a series of trills or a  throaty chug.  

For six hours the research team watches the video, amazed at the images.  For the first time we  see Weddell seals vocalizing at deep depths and watch as alien looking plants and animals fly by the swimming seal.  Seal 19 shows us how she can capture small fish in the blink of an eye and hunt down giant Antarctic cod where there is no light or air, and the water temperatures are below freezing.  

It is time to celebrate our new discoveries.                              

As for Seal 19 (Ally McSeal to her friends):  
most of the time we see her sunbathing near the ice crack with the other seals.

 Next week - The Seals' Secrets

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