The National Zoo's Antarctica Expedition is sponsored by the National Science Foundation's Office of Polar Programs.
All photographs depicting Weddell seals were taken under NMFS Permit No. 763-1485-00 issued under the authority of the Marine Mammal Protection Act.
There's More Than One Way to Catch a Seal
August 7, 2006
People often ask me, how do you catch a seal? Especially, one that's about six times your mass and has teeth like an oversize pitbull? Well, the actual catching of these large carnivores in the wild is nothing compared with the challenge of finding the right gear to catch them with. A lot of field equipment we use cannot be bought, but must be painstakingly handcrafted by scientists themselves or obliging master tradesmen.
Bagging a Seal
There are basically two ways to catch a big seal. The first is to put a bag over its head. In the 1960s, Ian Stirling working in McMurdo Sound (where we are going) pioneered the method of catching Weddell seals by putting a mail sack over their head. Stirling published his method in 1966, and everybody has been using it ever since. The idea behind this is that once the bag is over its head, the seal thinks it's completely trapped and gives up, allowing you to collect your samples.
When I was working with colleagues at the University of Tromsø, Norway, a few years ago, they were still using actual mail sacks. But today, most seal scientists have abandoned this technique in favor of high-tech, funnel-shaped canvas bags with ropes attached to the opening. The ropes are used to throw the bag over the head of an unsuspecting seal and hold the bag tight over the animal's shoulders while you wait for it to get used to the idea of being "bagged." The ropes keep you from getting your hands near the seal's teeth, a real plus in my opinion. Skilled seal catchers can bag a large Weddell seal single-handedly with one of these capture bags.
Ideally, a good seal bag fits snugly over the seals' shoulders, pinning the front flippers, but doesn't touch its face, which would upset it. You can see already that precision engineering is required. When it came to ordering seal bags for our expedition, I realized I did not have any specifications so I sent emails to fellow seal scientists all over the world pleading for information on their bags. Almost all replied but didn't have specs either and, on the verge of giving up, I finally managed to track down the magic numbers with the help of a colleague, who—replying from the Canadian Arctic—gave us the address of a bag manufacturer. Just in time—the ship with our cargo is supposed to leave California in three weeks!
Using a Pole Net
The second way to catch a seal—and the preferred method whenever we are planning something more involved such as weighing—is to catch the seal in a pole net. A pole net consists of two poles linked by a short length of chain at one end, with netting suspended between the poles. You walk up behind a seal and place the pole net over it so that it is facing into the sharp angle of the “V.” If the seal is cooperative, it will scoot into the closed end of the net and so entangle itself; otherwise, you pull back and drop the poles down to the ground and then close the poles to completely surround the seal.
A netted seal is a nice, compact package that can be weighed with a winch and tripod. I don't know who invented the pole net, but I was taught the fine art of using it by Drs. Hindell and Harcourt, and it has proven an essential career skill for me ever since.
We got the net specs from a most obliging fellow seal scientist and gave them to the National Zoo's skilled welding crew—Benny Turner and Joe Smith—who kindly agreed to make us a pole net: They did an incredible job: apart from the poles, everything is handcrafted from scratch, including the rings for attaching the chain to the poles.
Over the years, the Zoo's welders, carpenters, and sheet-metal artisans have helped scientists like us translate vague ideas into state-of-the-art, â€œone-of field gear that in many cases have been make-or-break for our projects. Scientific research often depends on the ingenuity and skills of people who help design and make the equipment needed for research. Sometimes scientists can do it themselves, but more often carpenters, mechanics, welders, and other skilled professionals come to our rescue: this essential support is rarely acknowledged and we deeply appreciate it.
Stirling I (1966) A technique for handling live seals. Journal of Mammalogy 47 , 543-544.