Exposure amplitude and repetition affect bottlenose dolphin behavioral responses to simulated mid-frequency sonar signals

• Dolphins show a dose–response relationship to varying levels of sound exposure.
• The probability of response is asymmetrically related to the sound pressure level.
• The severity of the response also increases with sound pressure level.
• Habituation and/or tolerance rapidly occurs to exposures ≤ 160 dB re 1 μPa.
• All dolphins abandoned trained behaviors at exposures of 185 dB re 1 μPa.

Thirty bottlenose dolphins (Tursiops truncatus) participated in a controlled exposure study using an acoustic stimulus that consisted of a simulated tactical sonar signal (1-s duration, 3250–3450 Hz). Each dolphin was trained a behavior requiring it to swim across an enclosure, touch a paddle, and return to the starting location. Ten-trial control and exposure sessions were performed with each dolphin. Dolphins were placed into one of five groups, each of which received a 115, 130, 145, 160, 175 or 185 dB re 1 μPa (rms) sound pressure level (SPL) exposure while crossing the enclosure on exposure trials. A canonical correlation analysis was used to determine the set of behavioral responses most interrelated with the independent variables of exposure level, trial number, and age. Responses that significantly contributed to the canonical model were used to create dose–response functions based on the received SPL. Dose–response functions demonstrated a robust relationship between received SPL and the probability of response and indicated rapid habituation to repetitive exposures with received SPL ≤ 160 dB. No habituation was observed at received SPL ≥ 175 dB re 1 μPa and all dolphins refused to participate in trials when the received SPL = 185 dB re 1 μPa. Although bottlenose dolphins may rapidly habituate to sound exposures below a certain level, particularly if there is food motivation, abandonment of behaviors increases rapidly at received levels ≥ 175 dB re 1 μPa.