A new technique photogrammetry is used to determine the weight of nursing seal mothers. This research is part of an ongoing project which studies the Weddell seal population in McMurdo Sound.

PhD student Terrill Paterson (left) and field researcher Erika Nunlist gather data about a seal and her pup at the Turtle Rock seal colony. (Photo Credit: Jay Rotella)
PhD student Terrill Paterson (left) and field researcher Erika Nunlist gather data about a seal and her pup at the Turtle Rock seal colony. (Photo Credit: Jay Rotella)

Scientists working on the longest-running Antarctic seal population study are using a new technique that needs little more than an off-the-shelf digital camera to turn photos of Weddell seals into important new data in their continuing research.

The ongoing Erebus Bay Weddell Seal Population Study has been tagging and tracking seal populations around McMurdo for almost half a century. “We’re trying to understand how many animals are in that population and how that’s changing over time,” said Jay Rotella of Montana State University, one of the co-principal investigators for the project. “And within that population, we’re trying to understand the lives of individuals and why some live longer and produce more offspring than others.”

This year, they’ve started using a new technique called stereo photogrammetry to weigh seals without ever touching them. “The idea is you’re doing metrics with photos,” Rotella said. Kaitlin Macdonald, a master’s student at Montana State University has been spearheading the effort to glean the mass of a mother seal with a camera. “We can take photos around the female seal,” Macdonald said. “You create a 3-D image of the seal from all the photos that you take, so you can get a volume estimate from that 3-D model of that seal.”

A Weddell seal mother stretches out with her pup at the at Hutton Cliffs seal colony. (Photo Credit: Eric Boyd)
A Weddell seal mother stretches out with her pup at the at Hutton Cliffs seal colony. (Photo Credit: Eric Boyd)

After photographing the seal from all sides, those images are fed into a computer program which analyses where the seal’s edges are in each photo. It stitches all of those contours together, producing a virtual reproduction of the aquatic mammal. “It basically makes a 3-D wireframe model of the seal,” Rotella said. It’s a technique already used in research ranging from archaeology to topography to make virtual models of artefacts or terrain respectively. Knowing the seal’s total volume, it is then simple to multiply that by the average density of a seal to deduce the total mass.

a) Researchers surround a mother seal with specially designed measuring sticks and take pictures from all angles. b) A computer program defines the edge of the seal and the location of each measuring stick in each photo. c) Using the measuring sticks as a reference for size and placement, the program finds the seal’s volume by combining all of its outlines to create a 3-D model. Researchers then compute the seal's weight by multiplying the volume of that model with the known density of an average seal. (All images courtesy: Kaitlin Macdonald)
a) Researchers surround a mother seal with specially designed measuring sticks and take pictures from all angles. b) A computer program defines the edge of the seal and the location of each measuring stick in each photo. c) Using the measuring sticks as a reference for size and placement, the program finds the seal’s volume by combining all of its outlines to create a 3-D model. Researchers then compute the seal's weight by multiplying the volume of that model with the known density of an average seal. (All images courtesy: Kaitlin Macdonald)

The team had been testing the technique in the field in the last few years, but this is the first time they are integrating it directly into their research. “The beauty of this photography work is that we can now go and estimate weight loss of mothers,” Rotella said. “How much weight did the pup gain and how much weight did the mother lose?”

It’s a question that gets to the core of understanding how a seal’s health affects her offspring. Weddell seal mothers devote themselves to raising a fit pup, but producing huge quantities of milk takes its toll. “Females lose approximately 50 percent of their mass while they’re nursing their pup,” Macdonald said. “So they’ll typically come in around 1,000 pounds and we think they typically leave at 500. But we’ve never been able to weigh those females.”

It’s likely that older females lose more weight and give birth to smaller pups than younger mothers, but that’s difficult to quantify. Putting a 50-pound new-born pup on a scale is relatively easy; doing the same with a half-ton expectant mother has been anything but.

This adds a new dimension to an already extensive population dataset.

A curious pup investigates the researcher’s camera at the Turtle Rock Weddell seal colony (Photo Credit: Jay Rotella)
A curious pup investigates the researcher’s camera at the Turtle Rock Weddell seal colony (Photo Credit: Jay Rotella)

Source: Michael Lucibella, Antarctic Sun

The Antarctic Sun web site (http://antarcticsun.usap.gov) is the official online news site for the United States Antarctic Program.