12 March 1999
Kaikoura, New Zealand

My role in this expedition is to track sperm whales during their 45-minute long dives in the depths of Kaikoura Canyon. But why track sperm whales if you are looking for giant squid? Because these whales eat squid, including Architeuthis. In fact, much of our knowledge of giant squid comes from the stomach contents of stranded or hunted sperm whales. By knowing where these sperm whales are diving (and by assuming that they are hunting for huge calamari meals), we can send DEEP ROVER to the same places and depths, and have a better chance of finding Architeuthis.

To track the whales I am using an array of hydrophones (underwater microphones). Sperm whales are very cooperative in a study like this, because during their dives they almost constantly produce short sounds called clicks (136kb .wav file). Each click is received at each hydrophone at slightly different times, depending on the geometry of the situation. Not only can you record the click from the direct propagation path, but also from various echoes; the most prominent of which is the reflection from the ocean surface (see spectrogram of click and echo). By comparing the signals received at each of the hydrophones you can work out the whale's position. To construct a dive profile, I simply string together many of these locations into a track. If there is more than one whale on a recording (and the whales are spatially separated), then individually located clicks will fall into distinct paths representing each whale.

Recording naturally occurring whale sounds falls under the general heading of passive acoustics. One advantage of this technique is that I do not have to tag the whales with some device that sends back signals, or use active acoustics like sonar, both of which are more likely to effect the behavior of the whales. While this study is helping to predict where we might find giant squid, I am also hoping to use this dive track data to better understand how sperm whales find their food, and what they are doing with all those sounds they produce.

Spectrograms of a single sperm whale click as recorded from a vertical line array of 4 hydrophones. Why does it look like two clicks on the deeper hydrophones? Actually you are seeing one click as it arrives from its direct propagation path, and from its reflection off the ocean surface. The time between these two arrivals gets longer the deeper your hydrophone. Also notice that the click is first received by the deepest hydrophone. Why? Where must the whale be? Somewhere deeper than the deepest hydrophone, right?

Large swells unfortunately prevented the launching of DEEP ROVER, but, by reducing the demands on the smaller boat Rukuwai, this turned into a wonderful opportunity for me to get a large block of recording time. With Derek Parkinson as skipper, we steered for the head of Kaikoura Canyon, where we saw three Whale Watch Kaikoura boats (a good indication of where the whales are). Ever helpful, they advised us that there were two whales in the area, known as "Droopy Flukes" and "Tiaaha". It seemed a good place to set up, so we cut the motor and deployed the hydrophone array. Aside from the swells, this was an ideal day. The rare combination of almost no current and less wind allowed us to drift with minimal movement for hours.

After setting up the recording computer, I flicked on the speaker, and there they were: two loud streams of sperm whale clicks. This sound may get boring to most people fairly quickly, but it always makes me excited, happy, and relieved. It means that all the equipment is finally working right, that the whales and the weather are cooperating, and that the time, money, and effort to collect good data has been worth it. When you start to listen carefully to the clicking, you begin to pick up on variation in the click timing. Suddenly the clicks are a bit faster, or slower, or the whale goes silent for a moment. What are they doing? Why do they produce clicks anyway? This is a subject of seemingly endless debate, and I won't attempt to answer it here. But, by correlating this click variation with dive track data (that is, being able to observe when and where sperm whales are producing what sounds), I am hoping that this study will help shed some light on questions like these.

 

As it turned out we did pick a good place to drift. Droopy Flukes politely continued to dive and surface just inshore of us, and Tiaaha held his position slightly offshore of our boat. For about four hours I was able to continuously record complete dives! (Usually they swim out of range in that time.) Whale Watch skippers helped by confirming the identity and position of the whales at each surfacing, which will serve as a nice visual corroboration of the acoustic dive tracks.

This great recording day finally ended with the loud unwelcome alarm of my computer power supply. We had used up all of our battery power. Needless to say, I bought another car battery on the way home, so that next time we get such good conditions, it will only be a change to bad weather or darkness that brings us in.

Bernard Brennan


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