Odontocetes: TTS growth and Recovery in Harbor Porpoises Exposed to Intermittent and Continuous Signals

The harbor porpoise (Phocoena phocoena) is known to be especially sensitive to sound. For that reason, this species is used by regulators as an “indicator species”, and its onset of Temporary Threshold Shift (TTS) is used both to estimate injury thresholds of Permanent Threshold Shift (PTS) based on the TTS-Sound Exposure Level (SEL) growth rate and as a risk threshold in its own right. The increase in TTS with increasing dose (the ‘growth rate’) is important for estimating permanent hearing threshold shift (PTS) onset levels. An experimental characterization of the growth rate in harbor porpoises should result in a more accurate extrapolation over a smaller amount of TTS, which would lead to a firmer scientific basis for PTS onset level estimations for air gun sounds.

Criteria for TTS onset due to exposure to multiple impulsive pulses currently in use to assess the impact of seismic surveys are based on the cumulative exposure (SELcum) over repeated pulses generated by seismic sources, without taking into account recovery of hearing between successive impulses. Recent TTS studies with harbor porpoises by SEAMARCO for pile driving sounds (inter-pulse interval 1.3 s) have shown that TTS growth due to intermittent sounds is much slower than for continuous sounds, presumably due to recovery of hearing between pulses. Because the inter-pulse interval during a seismic survey is typically in the order of 10 seconds, hearing recover between airgun pulses was expected and assessed in this study.

The main findings from study were 1) that only low levels of TTS 1-4 (~ 5 dB) could occur from exposure to air gun pulses resulting in cumulative sound exposure level (SELcum) up to 191 dB re 1 μPa²s, 2) that the highest TTS was observed for frequencies of 4-8 kHz, even though the one-third-octave (base 10) band SEL for lower frequencies (500 Hz) was 18 dB higher, 3) single exposures did not generate any significant TTS; TTS was only observed after at least 10 pulses, and 4) an adapted version of the modified-power law (MPL) model was the most promising model for harbor porpoise TTS growth that allowed for integrating the effect of hearing recovery between intermittent air gun pulses.

Objectives and methods

The objective of this study was to obtain data to develop a scientifically valid method to predict harbor porpoise TTS growth, recovery, and affected hearing frequencies as a function of cumulative sound exposure level, taking into account the number of airgun pulses, time interval between pulses, and hearing recovery between pulses. Two different approaches for predicting TTS were considered: modified-power law (MPL) model, and a kurtosis-corrected equal energy model.  Because of the limited number of exposures resulting in TTS due to airgun sounds obtained in this study, previous exposures using intermittent and continuous 1 – 2 kHz active sonar exposures were used to evaluate and discuss the potential and limitations of the proposed models instead.

One animal was successfully trained to participate in the TTS studies with airguns using a behavioral protocol to measure TTS. During these experiments, the porpoise was exposed to intermittent exposures of a compact array of 1 to 4 scaled-down airgun(s). The number of air gun pulses ranged between 1 and 1350, with exposure durations up to 90 minutes.

Importance

A comparative analysis of the different exposure conditions showed that frequency-weighted SELcum (Very High Frequency cetacean weighting following Southall et al., 2019) was a good predictor of the amount of TTS, as well as the frequency range in which TTS occurred. In addition, it was found in that peak sound pressure alone was not a good predictor for TTS. The highest peak sound pressure levels for a single airgun exposure (L p,pk = 199 dB re 1 μPa²s) did not generate any significant TTS, and TTS was only observed after at least 10 pulses. The highest peak sound pressures observed in the experiments are at a similar level with the current NMFS (2018) and Southall et al. (2019) threshold for predicting the risk of a permanent threshold shift (PTS) in harbor porpoises.

Links to other research

1. TTS In Odontocetes in Response to Multiple Airgun Impulses.
2. Airgun Effects On Arctic Seals: Auditory Detection, Masking And TTS In Pinnipeds to understand TTS associated with airgun pulses in pinnipeds.

Institutions/PIs

TNO (A.M. von Benda-Beckmann, C.A.F. de Jong, F.P.A. Lam)

SEAMARCO (R.A. Kastelein)

Woods Hole Oceanographic Institution (D. Ketten)