A TWO‐DIMENSIONAL ACOUSTIC LOCALIZATION SYSTEM FOR MARINE MAMMALS

For the study of animal communication systems, ic is crucial co b~ able co identify che sender of a given call. T his nocion might seem crivial co investigators of many cerrescrial animals. However, there are situations in which simple visual observation cannot be used even in cerrescrial environments (e.g., observations at night or in dense vegetation). In aquatic environments sender detection is almost always a problem. As a result, many srudjes on marine mammals have only been able to link calls co che behavior of a whole g roup t:lCher chan particular individuals (Evans and Dreher 1962, Taruski 1979, Sjare and Smith 1986, Ford 1989. Weilgarr and Whitehead 1990, Smolker er al. L 993). A few scudies on wild marine mammals have been able co overcome the problem of sound source detection by using passive acoustic localization (Watkins and Scbevill 1974, Clark et al. 1986, M0hl et al. 1990, Frankel et al. 1995 ). This method uses differences 10 rimes of arrival of a signal ac different transducers of a microphone array to determine the source posicion. Ics main advantages are char it is completely non-invasive and that ic can be used ro monitor vocal behavior of several individuals at once. Alternatives, such as temporarily capturing individuals (e.g. , Sayigh et al. 1990) or arcaching telemetry devices (e.g .. Tyack 1986), are difficult ro apply in the wild and can affect rhe behavior of the animals. However, the accuracy of passive localization relies heavily on rhe acoustic topography of the study sire (Spiesberger and Frisrrup 1990) and data analysis can be very rime consuming. This is one reason why ir bas nor been used more widely in behavioral studies on marine animals. The acceptable error in such studies, however, varies with the quesrion under investigation. The study of close-coocact interactions berweeo individuals, for example, requires more accurate sound source localization chan thar of longdistance communication. In this study we present the localization accuracy of a three-element hydrophone array that was used to study different aspects of vocal behavior in bottlenose dolphins (Tursiops truncatus) and harbor seals (Phoca vitulina). The whole localization system, including the analysis software, was based upon readily available components and can be easily installed in any shallow-water