Ny Ålesund and Kongsfjord. May 2000. Part VI

[Read the first part of this post here.]

Running marine research from tiny boats in remote places usually doesn't go smoothly, so it's always wise to have a Plan B. Preferably something that involves working from shore.

Do male bearded seals call 24 hours a day? Are there times of day when they call more than others? To answer questions on temporal patterning, Sofie set up a program of recording 15 minutes every hour over either 12 or 24 hours every third day. As we couldn't go out on the boat to do this – too many days missed due to bad weather – she recorded from the harbour wall. A little hut there meant we could keep warm while recording. A five minute walk from the Polar Hotel to the harbour wall, press record, and we were doing science. We'd take it in turns to sneak out for the recordings in the small hours of the morning, leaving the other warm and (in my case) snoring.
To localize individual seals from their calls, Sofie used a simple but elegant little system. She'd set three hydrophones from the harbour wall. At the start of each recording period, she'd carefully place the hydrophones in a predetermined position, then run their cables back to the hut. All three hydrophones connected to a multitrack cassette tape recorder.

The theory behind localization is pretty simple. A seal calls underwater, the sound travels through the water to the hydrophones. The nearest hydrophone detects the call first, the most distant hears it last. Knowing the speed of sound through water, it's then a matter of triangulation to work out where the seal was when it called. Friends from the Cornell Lab of Ornithology had written computer programs to do the triangulation automatically, making Sofie's life a lot easier than it could have been.

The precision of this system depends on a few factors – how far away the seal is and the distance between the hydrophones probably being the most important. Still, for male seals calling underwater, even a rough idea of where they are is better than nothing. The hydrophones were spaced about 40 yards apart, and it's about two and a half miles from the harbour wall to the nearest point on Blomstrandhalvøya. So we needed to know just how rough Sofie's rough triangulation was.

To resolve this, we calibrated the array. One calm day near the end of May, I popped out in the Buster by myself, while Sofie recorded from the hut. Sofie had calculated a grid of 30 positions and input them to our handheld GPS. I puttered around in the Buster, going to each position, then implementing our highly technical calibration system – banging on the side of the boat with a hammer. There was only time for a couple of quick bangs at each position, given the current in the fjord. We coordinated this highly technical process by radio. Using these bangs, and cross checking our little GPS with equipment available at the research station, Sofie worked out that the array was precise to under five yards at best (very close to the harbour wall), and out by roughly 200 yards at two miles. Good enough to track seals in Kongsfjord.

As it turns out, some males hold small territories within which they display, but others roam the length of Kongsfjord, calling. Maybe they're looking for an in to their own patch? And from our work from the Buster, observing males we could identify, Sofie could identify individual males by their calls. She eventually showed that some individual males had called in the same parts of Kongsfjord over two years. They probably held the same residences over several years, but we'll never know that now.

Read the next part of this post here.