By María Vanesa Reyes, Miguel Iñíguez and Mariana Melcón
In recent years, there has been a considerable increase in the use of passive acoustic techniques to study cetaceans. One of the main advantages of listening over watching is that light in water attenuates within a shorter distance than sound. For instance, even in clear waters, visibility rarely exceeds 30 meters, while low frequency sounds in the ocean can be heard thousands of kilometers away from the source. This feature of aquatic environments was probably at least one factor responsible for leading cetaceans, over millions of years of evolution, to develop a sophisticated hearing and sound production system. Cetaceans tend to rely mostly on hearing as their primary sense for almost every aspect of their lives.
However, the efficiency of sound propagation can also be disadvantageous for cetaceans. Over the last decades, the increase in aquatic human activities has introduced many sources of noise that include recreational activities, tactical sonar, dredging, construction, oil exploration and drilling, and geophysical surveys. This noise, like the sounds produced by cetaceans, can be propagated over large regions. The result seems clear: man-made noise has the potential to interfere with the cetacean's ability to detect relevant sounds, leading to changes in behavior and/or impairment of hearing.
Commerson's dolphin (Cephalorhynchus commersonii) is one of the four species of the genus Cephalorhynchus that only inhabit the Southern hemisphere. Its distribution is restricted to the coastal waters of Argentina, Chile, and around the Islas Malvinas/ Falkland Islands and Kerguelen Islands. These dolphins prefer shallow waters and also live in estuaries. They are fast, active swimmers and are often seen bow-riding and surfing on waves. They are very inquisitive and social animals.
Commerson's dolphins, like all toothed cetaceans, produce high-frequency sounds. The sound travels through the water, reflects off objects and returns as echoes that provide dolphins with an acoustic image of their environment. This use of echolocation allows the dolphins to recognize their environment, find prey and avoid obstacles.
Bahía San Julián, province of Santa Cruz, Argentina, is an area used by Commerson's dolphins to breed, give birth, socialize, forage and rest. However, as in other areas of Argentina, dolphins in Bahía San Julián are also exposed to motorized nautical activities that include dolphin watching, artisanal fisheries, and various other uses of small ships, boats and personal watercraft.
Since 2011, researchers at Fundación Cethus have used passive acoustics to study Commerson's dolphins in Bahía San Julián. A hydrophone is deployed from a platform (a boat or a pier), enabling us to record dolphins' vocalizations and ambient and anthropogenic noise.
The main question that motivates this research is whether the noise generated by nautical activities in Bahía San Julián has an effect on Commerson's dolphins' behavior. The first step, therefore, was to analyze the sounds emitted by the dolphins to better understand what "normal" vocalizations are like. In a second phase, we wanted to categorize the types of man-made noise that occur in the study area to better understand the possible consequences of the noises for the dolphins. Finally, we want to evaluate effects of this anthropogenic noise on Commerson's dolphins.
During the first year of research, we were able to collect enough data to start characterizing Commerson's dolphins' vocalizations. Currently we are preparing a first manuscript about the acoustic repertoire of Commerson's dolphins in Bahía San Julián.
In addition, we acquired data necessary to study the potential effects of anthropogenic noise on Commerson's dolphins in the area. Preliminary results show that boat noises in the proximity of echolocating animals overlap with their high-frequency sounds called "clicks." This overlap may prevent dolphins from detecting relevant sounds, such as the echoes reflected off prey. Thanks to financial support from AWI and the Whale and Dolphin Conservation Society, it was possible for us to present our results to an Acoustic Communication course of the Graduate School SNAK, Institute of Biology, University of Southern Denmark.
During fieldwork we also offered lectures to elementary school students about cetaceans, the threats they face, and the work carried out by Fundación Cethus in Bahía San Julián. We consider it very important to share the knowledge generated by our research with the local community to raise people's awareness of the need to conserve these dolphins.
This is the first time in Argentina that passive acoustics have been used to identify potential effects of anthropogenic noise on cetaceans. However, more data is needed for a complete evaluation of the potential effects of vessel noise on Commerson's dolphins. Our aim is to extend this study to other areas inhabited by the dolphins but with more boat traffic, in order to compare different circumstances and gain better understanding of the impact of anthropogenic noise, as well as possible strategies used by Commerson's dolphins to compensate.
María Vanesa Reyes, is a PhD student in Biological Sciences, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, and has worked as a volunteer with Fundación Cethus since 2005. Miguel Iñíguez is the founder and president of Fundación Cethus, an organization established in 1992 to study, disseminate information about, and conserve the dolphins and whales in Argentine waters. He is also the IWC Alternate Commissioner of Argentina, a position he has held since 2003. Mariana Melcón holds a PhD degree in Animal Physiology, University of Tübingen, Germany, and is currently a Postdoctoral Scholar at the Marine Physical Laboratory of the Scripps Institution of Oceanography at the University of California, San Diego. She is a mentor, acoustic consultant, and has worked with Fundación Cethus since 1998.