Detection of low-frequency tones and whale predator sounds by the American sand lance Ammodytes americanus.

Auditory evoked potentials (AEP) were used to measure the hearing range and auditory sensitivity of the American sand lance Ammodytes americanus. Responses to amplitude-modulated tone pips indicated that the hearing range extended from 50 to 400 Hz. Sound pressure thresholds were lowest between 200 and 400 Hz. Particle acceleration thresholds showed an improved sensitivity notch at 200 Hz but not substantial differences between frequencies and only a slight improvement in hearing abilities at lower frequencies. The hearing range was similar to Pacific sand lance Ammodytes personatus and variations between species may be due to differences in threshold evaluation methods. AEPs were also recorded in response to pulsed sounds simulating humpback whale Megaptera novaeangliae foraging vocalizations termed megapclicks. Responses were generated with pulses containing significant energy below 400 Hz. No responses were recorded using pulses with peak energy above 400 Hz. These results show that A. americanus can detect the particle motion component of low-frequency tones and pulse sounds, including those similar to the low-frequency components of megapclicks. Ammodytes americanus hearing may be used to detect environmental cues and the pulsed signals of mysticete predators.

Shipboard measurements of the hearing of the white-beaked dolphin Lagenorhynchus albirostris.

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 ;U' 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.