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Temporary threshold shift to simulated naval continuous active sonar in bottlenose dolphins (Tursiops truncatus)a)
Jason Mulsow1,2, Madilyn R Pardini2, Carolyn E Schlundt3
1United States Navy Marine Mammal Program, Naval Information Warfare Center Pacific Code 56710, 53560 Hull Street, San Diego, California 92152, USA.
The Journal of the Acoustical Society of America
|June 27, 2025
Summary
Bottlenose dolphins exposed to simulated naval sonar experienced temporary threshold shifts (TTS) that increased with sound exposure levels. Higher frequencies showed greater vulnerability, with behavioral TTS observed at lower levels than previously expected.
Area of Science:
- Marine biology
- Bioacoustics
- Auditory science
Background:
- Naval continuous active sonar (CAS) poses a risk to marine mammals.
- Understanding temporary threshold shift (TTS) is crucial for assessing noise impacts on cetaceans.
Purpose of the Study:
- To investigate auditory TTS in bottlenose dolphins exposed to simulated CAS.
- To examine the equal-energy rule for long-duration tonal exposures.
- To determine frequency-specific vulnerabilities to sonar exposure.
Main Methods:
- Behavioral and auditory evoked potential (AEP) methods were used.
- Four bottlenose dolphins were exposed to simulated CAS at varying sound pressure levels and durations.
- Two exposure phases: high-frequency (28 kHz) and mid-frequency (2.8 kHz).
Main Results:
- Behavioral TTS increased exponentially with increasing sound exposure levels (SEL) for both 2.8 kHz and 28 kHz exposures.
- TTS onset SEL was approximately 20 dB lower at 28 kHz compared to 2.8 kHz, indicating higher vulnerability at higher frequencies.
- Auditory evoked potentials (AEPs) did not show clear noise-induced effects during 28 kHz testing, contrary to expectations.
Conclusions:
- Bottlenose dolphins exhibit frequency-specific vulnerability to sonar, with higher frequencies posing a greater risk.
- Behavioral TTS is a sensitive indicator of noise impact, potentially more so than AEPs in certain contexts.
- The equal-energy rule may not fully predict TTS for long-duration tonal exposures across different frequencies.
