Night Hunt

Scientists track seal predation behavior through the dark of Antarctica.

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“Since the seals’ behavior is done while hold their breath, the physiology is a pretty important part of understanding their ecology. Together the three of us make a really strong team in understanding what these seals are doing,” Fuiman said.

It worked. The technology not only offered insights into seal behavior, but also provided valuable data on two of its prey—the Antarctic silverfish and the Antarctic toothfish. They eventually published a paper on their findings in Marine Biology, along with numerous other papers on the seals. [See previous story in The Antarctic Sun, Dec. 22, 2002.]

“I don’t think anybody else has data like this because it is so difficult to collect data from the point of view of the predators. I think there is a goldmine there, and I suspect that there will be something publishable about seal prey in the future,” Fuiman said.

By a hair

In spite of their work on other critters, the focus of the group remains the Weddell seal.

The video data recorders glued onto the seals’ fur collect a variety of data that allow the researchers to create three-dimensional dive profiles, which they can then use to interpret the animal’s behavior. The data include depth, compass bearing and number of flipper strokes. The recorders also report on environmental conditions such as temperature, salinity and dissolved oxygen in the water. 

“It’s a suite of variables that allow us to look at diving and swimming performance, as well as the animal’s immediate environment in three-dimensional space,” Davis explained.

From the videos and other data, the seal biologists have learned a great deal about Leptonychotes weddellii behavior during the daylight hours. For example, after dropping away from a breathing hole in the ice, the seals become negatively buoyant in the first 30 to 50 meters, allowing them to dive with little effort as they make a “meandering descent,” according to Davis.

Davis said he believes that the seals rely primarily on vision when light is available, possibly detecting the silhouette of fish from below when they go in for a kill.

“They’re probably backlighting their prey, which means they’re relying on vision,” he said. “They have exquisite low-light vision. We’re pretty convinced of that.”

Fuiman politely disagrees with the “backlighting” assessment by his colleague. However, both men believe other senses are in play in low-light or dark conditions.

“If they’re catching fish in total darkness, then clearly they’re not using vision alone,” Fuiman noted. “It’s not likely that an animal is going to rely on one sense when it has so many available to it.”

Hearing is out. The seals don’t vocalize while hunting under water, and the fish don’t make noise.

Instead, the team believes the seals rely on vibrissae, or whiskers, which are not just hairs but very complicated sense organs with more than 500 nerve endings that attach to the animal’s snout.

“We think they could detect the wake of swimming fish and use that to capture prey,” Davis said.

Advances in technology

The video will provide some of the key evidence for the hypothesis, according to Fuiman. The camera is positioned in such a way that the scientists can observe the seal’s muzzle and the whiskers, allowing them to see if there is any movement prior to a capture.

“It will be interesting to see if their movement is more pronounced when it’s dark versus when it’s light,” Fuiman said.

So far, the team has been busy building the three-dimensional dive profiles, and is now combining them with the video-recorded behavior. The fourth generation of the video-data recorder system currently in use is about one-fourth the size of the instrument deployed back in 1997, according to Davis.  

While the instrument has shrunk in size, its memory capacity has increased tremendously with digital technology. (The original video was on 8mm tape.) That means more data—and more work.

“One of the curses of improving your technology is that we are deploying the animals for much longer periods of time, so now they’re out for many days,” Fuiman noted.