Abstract
The underwater hearing sensitivities of two one-year-old female harbor seals were quantified in a pool built for acoustic research, using a behavioral psychoacoustic technique. The animals were trained to respond when they detected an acoustic signal and not to respond when they did not (go/no-go response). Pure tones (0.125-0.25 kHz) and narrow-band FM (tonal) signals (center frequencies 0.5-100 kHz) of 900 ms duration were tested. Detection thresholds at each frequency were quantified using the up-down staircase method. The underwater audiograms (50 % detection thresholds) of the two seals did not differ statistically: both plots showed the typical mammalian U-shape, but with a wide and flat bottom. Maximum sensitivity (54 dB re 1 μPa, rms) occurred at 1 kHz. The range of best hearing (10 dB from the maximum sensitivity) was from 0.5 to 40 kHz (6 octaves). Higher hearing thresholds (indicating poorer sensitivity) were observed below 1 kHz and above 40 kHz. Thresholds below 4 kHz were lower than those previously described for harbor seals, which demonstrates the importance of using quiet facilities, built specifically for acoustic research, for hearing studies in marine mammals. The results suggest that many anthropogenic noise sources are audible to harbor seals at greater ranges than formerly believed.