I thought I’d start this off by telling you a little bit about what I’m doing for the summer. I am a Master’s student at the University of Victoria. I am collecting data for my thesis, which will ask the question, “Do individual Southern Resident orcas have different “voices”?” Can we tell individuals apart by sound?
It’s an easy question to ask, but a difficult one to answer. If orcas were land animals, then I could observe a group of individuals talking to one another, and use their facial movements to infer which individual was speaking at what time. But orcas spend an estimated 95% of their time underwater. Underwater video won’t help either, because orcas don’t need to move their mouths to speak. Rather than passing air over vocal chords in their throats to make sounds, as we do, orcas pass air back and forth in air sacs in their sinus cavities. The sound is focused by the fatty tissue in an organ called their “melon,” and comes out of their foreheads.
So with no reliable visual cues to indicate who is speaking, how does one figure out who is saying what?It’s a little complicated, and does involve a few different steps, but I believe it’s doable.
The first step is the most expensive. To find out where a sound came from, you need acquire an array of underwater sound receivers, called hydrophones. You need at least 3, because, after some fancy math, each pair of hydrophones will give you one line of bearing. In other words, if we imagine a line drawn between a pair of hydrophones, we can calculate the angle at which an incoming soundwave crossed that line, giving us a line from the receivers out to infinity upon which the sound source must be. With three receivers, we can calculate up to three lines of bearing, which intersect at the location that a sound came from. This age-old method is called triangulation, or localization.
So for this first and crucial step, I will be using the array of Ocean Sonics digital hydrophones that is being deployed on Saturna Island by the Saturna Island Marine Research and Education Society (SIMRES). We have two hydrophones in the water now, and one to go. This means that I cannot triangulate the source of a sound right now, but I can draw a line on which the sound source must be. So, until the third hydrophone is deployed, I can’t localize all of the sound sources (orcas), but I can localize the leaders and trailers of a group – the whales who are far enough ahead or behind that no other whales will be in the same line of bearing at any given time. If the whale I’m focusing calls multiple times as it passes the array, then I can make a “track” of the whale’s movements to refer to after step two.
Step two is a little less passive than step one (pun intended for those of you into passive acoustics!). This step requires that I watch the whales (woohoo!). I stand on the shore near the hydrophones with my handy-dandy theodolite. This instrument is actually one used for construction and surveying, but is easily appropriate for visual data collection. It looks like a telescope suspended between two posts on a rotating base. It measures the vertical and horizontal angles of the position of the telescope. If I aim the telescope at a whale, then I can measure the vertical angle that whale is at relative to myself, and the horizontal angle it is at relative to a reference point (in this case, true North). With this information, as well as knowledge of my height above the waterline and my GPS location, I can use trigonometry to find out where the whale is. If I take these measurements on the same whale several times as it passes through the research area, then I can make a visual “track” of the whale’s movements. If this track matches the acoustic track from step one, then I can reasonably assume that the whale I was looking at is the whale that was speaking. At the same time, my assistant videos the whales so that I have a record of what happened, which individuals were there, and the movements of other whales. Since every Southern Resident orca has been photographed and is recognizable by differences in their dorsal fin shapes and saddle patches, I can use all of this information to determine which individual was making all the noise.
With a collection of calls from several individuals, I can then analyze calls of the same call type for differences after the data collection season is over. And, if I’m lucky and there are differences, then long term monitoring projects may be able to make more and better use of passive acoustic techniques to follow this at-risk population, allowing us scientists to reduce our dependence on boats for data collection and therefore our impact on the whales themselves.
So that is what I’m doing here on Saturna Island this summer. If you want to know more about the orcas, and about this community specifically, check out the Center for Whale Research at http://www.whaleresearch.com, and the Whale Museum at whalemuseum.org. Or check back here for more updates and educational blog posts!