642 INTRODUCTION Atlantic white-beaked dolphins (Lagenorhynchus albirostris) do not always have a white beak. Frequently their short, approximately 5·cm long nose is dark or mottled grey (Ellis, 1982; Rasmussen and Miller, 2002). The beak of this fast-swimming dolphin is not the most distinctive characteristic that can be readily seen; instead the large white patch below the dark dorsal fin area may be quickly noticed in a surfacing, jumping or bow-riding animal. The dolphins are found in temperate and subarctic waters of the North Atlantic. They are the most common dolphin species all around Iceland and are frequently seen in the summer in Faxaflói Bay off Keflavík, Iceland, where they often ride the bow wave of vessels (Víkingsson and Ólafsdóttir, 2004; Rasmussen et al., 2006). These dolphins are acoustically active, producing both whistles and clicks (Rasmussen and Miller, 2004). Mitson (Mitson, 1990) recorded white-beaked dolphins feeding on sand-eels and reported energy with frequencies as high as 305·kHz, much above the typical upper hearing frequency limit for odontocetes of about 150·kHz (Nachtigall et al., 2000). Whistles may be used for communication and can presumably be heard at distances over 10·km (Rasmussen et al., 2006), but the hearing thresholds of white-beaked dolphins have not been measured. Most odontocete audiograms measured up to this point have been collected using behavioral psychophysical procedures in which the animal is captured, kept within a laboratory setting and then trained to respond to the presence or absence of acoustic stimuli. These sorts of hearing measurements are ideally made within the quiet laboratory tank environments, but are occasionally made at oceanaria, in open sea pens, or in tanks above ground (Nachtigall et al., 2000). While these procedures and settings are ideal for obtaining auditory measurements on marine mammals, this quiet sort of laboratory environment is becoming increasingly difficult to obtain. Training and testing with traditional psychophysical procedures is expensive and time consuming. The use of auditory evoked potential (AEP) procedures, in which the animal’s hearing is measured by passively receiving the animal’s electric potentials from the surface of its skin over its head when in the presence of sound stimuli, provides the opportunity to rapidly test animals outside of normal laboratory circumstances. (Nachtigall et al., 2005). Unlike other large mammals, odontocetes are particularly suited for this quick hearing evaluation because their brain electrical patterns readily follow patterned sound beyond 1000 modulations per second (Dolphin et al., 1995; Supin and Popov, 1995; Mooney et al., 2006). Fortunately, there is good agreement between hearing measurements using the AEP procedure and those collected using traditional behavioral techniques for odontocete cetaceans (Yuen et al., 2005; Houser and Finneran, 2006). Stranded or newly captured dolphins and small whales normally become quite passive after coming onto a beach or being taken in a net. This passivity provides a unique opportunity to test the hearing of cetaceans using AEP in a catch-and-release scenario. As The Journal of Experimental Biology 211, 642-647 Published by The Company of Biologists 2008 doi:10.1242/jeb.014118 Shipboard measurements of the hearing of the white-beaked dolphin Lagenorhynchus albirostris P. E. Nachtigall 1, *, T. A. Mooney 1 , K. A. Taylor 1 , L. A. Miller 2 , M. H. Rasmussen 2,3 , T. Akamatsu 4 , J. Teilmann 5 , M. Linnenschmidt 2 and G. A. Vikingsson 6 1 Marine Mammal Research Program, Hawaii Institute of Marine Biology, University of Hawaii, PO Box 1106 Kailua, HI 96734 USA, 2 Institute of Biology, SDU-Odense, Campusvej 55, DK-5230, Odense M, Denmark, 3 Húsavik University Center, University of Iceland, Garðarsbraut 19, 640 Húsavik, Iceland, 4 National Research Institute of Fisheries Engineering, 7620-7, Hasaki, Kamisu, Ibaraki 314- 0408, Japan, 5 National Environmental Research Institute, Aarhus University, Frederiksborgvej 399, DK-4000 Roskilde, Denmark and 6 Marine Research Institute, Skúlagata 4, PO Box 1390, 121 Reykjavík, Iceland *Author for correspondence (e-mail: nachtiga@hawaii.edu) Accepted 17 December 2007 SUMMARY This is the first report of an underwater audiogram from a dolphin in a capture-and-release scenario. Two bow-riding white-beaked dolphins Lagenorhynchus albirostris (a female and a male) were captured using the hoop-net technique in Faxaflói Bay, Iceland. The dolphins were transferred to a stretcher and hoisted into a plastic research tank on board a small fishing vessel. Two underwater transducers were used to cover the frequency range from 16 to 215·kHz. Two human EEG electrodes mounted in suction cups, one placed near the blow hole and the other on the dorsal fin, picked up bioelectrical responses to acoustic stimuli. Responses to about 1000 sinusoidal amplitude modulated stimuli for each amplitude/frequency combination were averaged and analyzed using a fast Fourier transform to obtain an evoked auditory response. Threshold was defined as the zero crossing of the response using linear regression. Two threshold frequencies at 50·kHz and 64·kHz were obtained from the female. An audiogram ranging from 16 to 181·kHz was obtained from an adult male and showed the typical ʻUʼ shaped curve for odontocetes. The thresholds for both white-beaks were comparable and demonstrated the most sensitive high frequency hearing of any known dolphin and were as sensitive as the harbor porpoise. Key words: AEP, catch-and-release, dolphin, hearing, Iceland, shipboard. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY