Recording common dolphin whistles aboard the Anna M

During August 2001 I spent three days aboard the Anna M. with skipper Joe Aston and Dr Simon Berrow of the Shannon Dolphin and Wildlife Trust. The aim of our trip was to record the communication whistles of common dolphins, which live off the coasts of Kerry and Clare. This work was carried out as part of my MSc project - a study of the effects of seismic surveying on the acoustic behaviour of common dolphins - supervised by Dr John Goold of the University of Wales, Bangor. Throughout this report you can listen to some of the underwater recordings I made whilst I was aboard the Anna M. Click here to listen to an example of some common dolphin whistles.

Dr Goold, my project superviser, had first taken an interest in common dolphin whistles in the mid-nineties when he was commissioned by the oil company Chevron UK to conduct an acoustic survey of cetaceans (whales and dolphins) to the west of Pembrokeshire. Chevron was about to prospect for oil in this area by conducting a seismic survey. Seismic surveying involves repeatedly producing intensely loud pulses of sound under the surface of the sea. Cetaceans are very sensitive to underwater noise because they rely on sound for communication and for sensing the world around them, and concern had been expressed that seismic survey noise might be damaging to these animals. Dr Goold's work was intended to monitor the extent to which cetaceans avoided the area affected by seismic noise (click here to hear common dolphins whistling during a seismic survey - you may need to turn your computer's volume down first!). Using a hydrophone (an underwater microphone) Dr Goold could detect and record the sounds made by whales and dolphins and so determine whether any of these animals were present in the survey area. Perhaps unsurprisingly, the results of the cetacean survey suggested that common dolphins - the species most often found near the Pembrokeshire coast - avoided the area during seismic surveying operations. This seemed like a sensible conclusion. If you or I were exposed to an uncomfortably loud noise, say from workmen digging up a nearby road, we might well avoid that part of town until the work stopped. In behavioural ecology, however, conclusions are rarely so clear-cut. Later research suggested that other factors, such as seasonal changes in water temperature and fluctuations in the amount of prey available, could have accounted for the decline in common dolphin numbers during the seismic survey.

This is where my work came in. As part of my MSc in Marine Environmental Protection at the School of Ocean Sciences, Bangor, I conducted a six month research project building on Dr Goold's findings. I was presented with the daunting task of re-analysing tens of hours of recordings of common dolphin whistles made before and during the seismic survey. My aim was to look for subtle differences in the whistles the dolphins used in these two contexts - such differences could indicate a vocal response to seismic surveying noise. The majority of this work was carried out in the lab, involving hours listening to whistles through headphones and staring at spectrograms (figure 1) on a computer screen. Thankfully, a few unanswered questions meant I also had to go into the field to make my own observations and recordings of common dolphins. For example, although I had hours of tapes of common dolphin whistles from Dr Goold's work, the majority of these recordings had been made without simultaneous visual observations. This meant that if I listened to one minute of the tapes and heard ten dolphin whistles I had no idea whether this represented ten dolphins making one whistle each or one dolphin making ten whistles! What I really needed was to be able to record different sized groups of common dolphins so that I could devise a simple relationship between the number of whistles recorded underwater and the number of animals likely to be present. It was with this aim in mind that I packed my oilskins and recording equipment and headed over to Ireland.

My supervisor had put me in touch with Dr Simon Berrow, who runs the Shannon Dolphin and Wildlife Trust based in Kilrush, earlier in the summer. He had assured me that there was a large, easily accessible population of common dolphins resident to the west of the Arran Islands and had arranged for his friend Joe Aston to take us out there in his yacht the Anna M. If you've had at look around this website you'll know a little about Joe and the Anna M. already. For three decades of his life Joe worked as a fisherman off the west coast of Ireland. He now spends most of his time sailing, either taking those with an interest out to watch dolphins on the Shannon, or just for the joy of being at sea. The Anna M. is a beautifully maintained 13.6m wooden schooner, well founded and an excellent sea boat.

I was glad of Joe's experience and the Anna M's sea-worthiness as we beat down the Shannon into the remnants of an Atlantic depression on the first day of our cruise. Despite the weather (a fine grey drizzle and a heavy swell) the trip started auspiciously when twenty minutes after leaving our mooring we were joined by four of the Shannon's resident bottlenose dolphins. As soon as we got past Loop Head and out into the Atlantic proper we began to search in earnest for common dolphins (figure 2). I couldn't have hoped for better companions in this task: Simon, who amongst other things spent two seasons studying sea birds in Antarctica, now conducts regular surveys of marine life in the area. His girlfriend Frances works locally as a sea fisheries inspector. Joe, himself a very experienced fisherman, has an almost prescient feel for the way that dolphins behave around boats.

During the early afternoon, once I had deployed my hydrophone on its long cable behind the Anna M. and set up my recording gear, we began to hear and see the occasional common dolphin. As its name suggests this species of dolphin has a very widespread distribution in both temperate and tropical oceans and is often sighted in large groups. Despite its ubiquity, surprisingly little research has been conducted into the behaviour and lifestyle of the common dolphin - a deficit I hoped my work would in a small way reduce. Around four o'clock in the afternoon a large group of diving gannets alerted us to the likely presence of dolphins. Altering course, Joe guided the Anna M. neatly into the area where the birds were feeding and soon we were among our first group of common dolphins (figure 3). Sharing the same school of fish as the gannets a group of perhaps twenty animals were breaching and diving, shooting in every direction in pursuit of their startled prey. With the hydrophone I was able to capture the excited whistles of the dolphins as they fed (click here). It was also a good opportunity to study the distinctive appearance of these animals as they charged around the boat. Their black, white and yellow striped markings and their playful, inquisitive behaviour make common dolphins easily identifiable at sea and it was nice to observe these features at such close quarters.

Click here to see common dolphins

As the afternoon wore on we had several such meetings with small groups of common dolphins, allowing me to make some nice recordings and simultaneous visual observations. Later, heading back east towards the Arran Islands we were approached by an Irish Navy vessel. A few terse questions over the VHF assured them that we weren't up to no good - indeed, when they heard Frances' name, we were greeted as old friends. In the course of her work for the fisheries inspectorate Frances had been on patrol aboard the ship, protecting Ireland's dwindling fish stocks. Not for the last time on the trip I was impressed by the closeness of the community of people on the west coast of Ireland whose livelihoods are dependent on the sea. That evening, as the sun set, the sea looked beautiful - almost benign - and the reasons for wishing to protect its diverse inhabitants were clear. Yet, at the same time, the sound of the Atlantic swell dashing itself on the cliffs of Inishmore was a reminder of the harshness of the sea. It isn't difficult to imagine how alien the prudence of scientists can appear to the fishermen who labour in this testing environment.

Following a night anchored in the lee of Kilronan, the next day saw us sailing south, close under the Cliffs of Moher. In the cabin of the Anna M. I used headphones linked to the hydrophone to monitor the sounds that echoed through the waters below. Our first few hours' sailing had been uneventful, with no visual sightings and only an occasional whistle faintly audible above the low rumbling of breaking waves. Then suddenly, at midday, the silence was broken by a chaotic cacophony of noise. I scrambled into the cockpit, where my eyes confirmed what my ears were telling me - the boat was surrounded by about fifty common dolphins bow-riding and leaping out of the water. With so many vocalising animals within range, individual whistles were inaudible above the high intensity squeals and clicks of dolphin sonar (click here). Joe, who must have seen a fair few dolphins over the years, was delighted to hear for the first time what a racket a large group of these animals can make underwater.

After half an hour or so the dolphins began to lose interest in us and headed off to the north-west. As they receded into the distance their sonar clicks, which although very loud have a relatively limited range, faded. Once again their whistles became audible, allowing me to make this recording of about twenty animals before they became too faint to hear (click here). Later, back in the lab, I was able to use digital techniques to remove much of the low frequency noise of breaking waves and the cable of the hydrophone itself from this recording. Playing the sample at a quarter of its normal speed, high pitched sounds are brought down into the range of everyday human hearing, making the whistles of individual dolphins clearly audible (click here). After such a spectacular show of dolphin sound and behaviour the rest of the day was relatively quiet. As darkness fell we rounded Loop Head once more before crossing the mouth of the Shannon Estuary. The wind, which had been moderating all day, became fickle as the sky filled with a dense drizzle and it was well into the night before we were finally able to flog into the tiny harbour of Fenit and enjoy a much appreciated glass of whisky.

The next day - the final day of the cruise - brought fine weather and a fresh south-westerly breeze. Leaving Fenit by mid-morning we were soon running downwind, making rapid progress back towards the Shannon. In the shallow waters south of Kerry Head I began picking up an echoing series of crackling and popping noises on the hydrophone. The overall effect was strangely reminiscent of the noise of frying bacon. This wasn't a hunger-induced illusion however (Joe had us fed us very well over the past few days) but the sound of large numbers of snapping shrimp (click here). Some species of shrimp produce sharp snapping or popping noises by generating a small bubble with their pincers, which then bursts under the high pressure found at depth. The intense snapping noise is used by the shrimp to stun their prey, defend their territory and communicate with one another.

Around lunchtime we were considering taking advantage of the following breeze by flying the spinnaker, when Simon spotted a group of about six harbour porpoises off to port. These tiny cetaceans are very wary of boats and more often than not are just glimpsed as they turn their dark backs and swim hurriedly away. This occasion was no exception but we counted ourselves lucky to have had even this brief encounter with this increasingly rare species. Bycatch of harbour porpoises is a growing problem world-wide and in European waters the use of bottom set gillnets is contributing to the decline of these elusive animals. Unlike dolphins, harbour porpoises do not produce whistle sounds but they do emit clicks. These clicks have very high frequencies, unfortunately well above the recording range of my equipment.

Heading up the Shannon Estuary, towards the Anna M.'s home port of Carrigaholt, we were once again visited by bottlenose dolphins. First, one large adult surged across our bow, then three more adults and a calf joined us. Listening to their approaching whistles on the hydrophone (click here) it was interesting to compare the vocal behaviour of this species with that of the common dolphins I had recorded the day before. Although the whistles of the two species sound very similar, bottlenose dolphins use their whistles more sparingly. It is now generally accepted that these animals do not communicate with a 'language' analogous to human speech. Rather, it is thought that each dolphin has its own individual whistle called a 'signature whistle' which it uses in the low visibility underwater environment to stay in contact with other members of its social group. If the previous sample is played at half the recording speed two bottlenose dolphins can be heard repeating their signature whistles. A few echo location clicks are also audible (click here).

Following our encounter with the bottlenose dolphins we headed back across the Shannon to Carrigaholt and tied up for the last time after three very productive and enjoyable days at sea. The recordings and observations I made during the trip subsequently allowed me to make a lot more sense of my project. At the moment it is only really possible to use hydrophones to determine whether dolphins are present or absent. My work shows that in the future it should also be possible to accurately relate the number of whistles detected to the number of dolphins in a particular area. Accurate monitoring of the size and distribution of dolphin populations is vital if scientists are to be able to identify and mitigate against any threats to the wellbeing of these animals.

Ewan Wakefield, January 2002.

Acknowledgements

I must offer my sincerest thanks to Joe Aston and Dr Simon Berrow for their invaluable help in arranging and conducting this fieldwork and to Dr John Goold for his guidance and advice throughout my project. The fieldwork was jointly funded by the Shannon Dolphin and Wildlife Trust, the UK Natural Environment Research Council (as part of my MSc studentship) and Joe Aston.