Behavioral Responses by Captive Bottlenose Dolphins (Tursiops truncatus) to 15- to 50-kHz Tonal Signals

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Abstract: Anthropogenic noise has received much attention as a potential factor affecting the behavior of marine mammals. For behavioral responses to occur, a sound stimulus would have to be a certain number of decibels (dB) above noise levels and the animal’s audiogram. Bottlenose dolphins (Tursiops truncatus) have good hearing sensitivity in the mid- to high-frequency range (15 to 50 kHz). Thus, in this study, two captive bottlenose dolphins housed in a floating pen were subjected to three tonal signals of 15, 20, and 50 kHz with the same signal durations and duty cycles. The effect of each signal was judged by comparing the dolphins’ surfacing locations, number of surfacings, and number of echolocation clicks during test periods with those during baseline periods. The location of the sound source did not change during the study. The results showed that the two dolphins swam away from the sound source and came up to the surface more often, but the dolphins exhibited a slight degree of habituation to the sounds. The two dolphins produced fewer echolocation clicks when the three signals were produced. The average avoidance threshold sound pressure level (SPL) of bottlenose dolphins for the three test signals were approximately 65, 70, and 83 dB above the hearing threshold SPL, respectively.
Key Words: acoustic exposure, behavioral response, bottlenose dolphin, Tursiops truncatus, hearing, anthropogenic noise
Page Numbers: 1-10

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