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ages are called auditory evoked potentials (AEPs). They are measured via electrodes on the surface of the animal’s head or small needle electrodes inserted just beneath the skin (Fig. 4). This method has been applied to study hearing of human infants and land animals. Adaptation of this methodology to fish and marine mammals occurred almost simultaneously. But marine mammal scientists quickly developed field portable systems (Fig. 5) because of the need to obtain data on multiple species of non-captive animals. New hearing data for a number of species is now being collected. The state-of- the-art for AEP measurements in marine mammals is sum- marized in a special issue of Aquatic Animals published in 2007 (Vol. 33, No. 1).
Beaked whales and sonar
Understanding why beaked whales are unusually sensi- tive to mid-frequency sonar is necessary to manage and mit- igate its potentially adverse effects (Cox et al., 2006). The col- lective knowledge about beaked whales presented and dis- cussed at the Marine Mammal Commission’s April 2004 Beaked Whale Technical Workshop, was published in a spe- cial issue of the Journal of Cetacean Research and Management in 2006 (Vol. 7, No. 3). After the workshop, much more became known about the deep diving foraging behavior of these animals because of successful field studies using acoustic data logger tags (Johnson et al., 2004, Madsen et al., 2005). In addition AEP measurements have been made on one stranded juvenile beaked whale (Cook et al., 2006)— a very small amount of data, but better than nothing at all.
At the 2004 Workshop, participants discussed several potential mechanisms for the stranding behavior and subse- quent deaths of beaked whales. The consensus was that the most plausible mechanism was an acoustically induced change in their normal deep diving foraging behavior, which caused them to surface too quickly and develop significant gas bub- bles that damaged multiple organs or interfered with normal physiological function, similar to a human diver getting the “bends” or decompression sickness (Jepson et al., 2003). Thus workshop participants recommended that CEE’s to determine beaked whale behavioral responses to mid-frequency sounds should be a top research priority (Cox et al., 2006).
Subsequently an international research team was formed and plans were made for a multi-year Behavioral Response Study (BRS) of beaked and pilot whales (pilot whales were involved in the 2005 North Carolina mass stranding). Last year marked the first field season of the BRS, which took place in the Bahamas’ Tongue of the Ocean and utilized the M3R passive acoustic monitoring system in place at the Navy’s Atlantic Undersea Test and Evaluation Center (AUTEC) on Andros Island. During this landmark study acoustic data logger tags will be attached to whales to record their sound exposure and track their response to mid-fre- quency active sonar and other playback sounds. It is a huge undertaking with funding provided by CNO N45, CNO Submarine Warfare Division, ONR, the oil and gas E&P Sound and Marine Life JIP, the DoD/DOE Strategic Environmental Research and Development Program (SERDP), and NOAA Fisheries Office of Science and
Fig. 5. Field portable system developed at Space and Naval Warfare Systems Center (SSC) San Diego consisting of bioamp (in Pelican case, bottom photo, shown with system running onboard a ship), ruggedized computer with data acquisition hard- ware, and Evoked Response Study Tool (EVREST) software developed by J. J. Finneran for generating sound stimulus signal, recording and storing auditory evoked potentials (AEPs), and analyzing data.
Technology. The results of this study will be forthcoming over the next few years and hopefully will help solve the mys- tery of how these animals react to mid-frequency sonar.
The second research priority recommended by the 2004 Workshop participants was for studies of the anatomy, physi- ology and pathology of beaked whales. Another fundamental aspect of the interaction of beaked whales with mid-frequency
28 Acoustics Today, April 2008