The limits of applicability of the sound exposure level (SEL) metric to temporal threshold shifts (TTS) in beluga whales, Delphinapterus leucas

Abstract

The influence of fatiguing sound level and duration on post-exposure temporary threshold shift (TTS) was investigated in two beluga whales (Delphinapterus leucas). The fatiguing sound was half-octave noise with a center frequency of 22.5 kHz. TTS was measured at a test frequency of 32 kHz. Thresholds were measured by recording rhythmic evoked potentials (the envelope following response) to a test series of short (eight cycles) tone pips with a pip rate of 1000 s −1 . TTS increased approximately proportionally to the dB measure of both sound pressure (sound pressure level, SPL) and duration of the fatiguing noise, as a product of these two variables. In particular, when the noise parameters varied in a manner that maintained the product of squared sound pressure and time (sound exposure level, SEL, which is equivalent to the overall noise energy) at a constant level, TTS was not constant. Keeping SEL constant, the highest TTS appeared at an intermediate ratio of SPL to sound duration and decreased at both higher and lower ratios. Multiplication (SPL multiplied by log duration) better described the experimental data than an equal-energy (equal SEL) model. The use of SEL as a sole universal metric may result in an implausible assessment of the impact of a fatiguing sound on hearing thresholds in odontocetes, including under-evaluation of potential risks.

Conditioned frequency-dependent hearing sensitivity reduction in the bottlenose dolphin (Tursiops truncatus)

“…This sort of intermittent exposure is much less effective than continuous exposure at producing effects like TTS (Finneran et al, 2010) due to partial recovery during the long time periods between exposures. TTS recovery during just a few minutes after low-SEL exposures has been demonstrated directly (Popov et al, 2013;Popov et al, 2014). So the absence of long-term TTS is very reasonable, and the hearing dampening effect investigated in the present study may be assumed therefore to be a manifestation of the conditioning-based control of hearing sensitivity.…”

“…1 µPa and duration of 5 s has SEL as low as 172 dB re. 1 µPa 2 s. It is much lower than the SEL producing TTS in the majority of odontocete studies Nachtigall et al, 2003;Finneran et al, 2007;Popov et al, 2011;Popov et al, 2013;Popov et al, 2014). The total exposure during a whole session (54 trials, see Materials and methods) was 189 dB re.…”

Evaluating effects of dredging-induced underwater sound on aquatic species : a literature review

“…It assumes that sounds of equal SELcum produce equal risk for hearing loss (i.e., short-duration sounds with a higher energy level would have a similar risk as a longer duration sound with a lower energy). Henderson et al (1991) showed this to be the case in humans and terrestrial mammals, but the EEH does not always accurately describe the exposure situations for marine mammals due to the inherent complexity of predicting threshold shifts (e.g., Kastak et al 2007;Mooney et al 2009a;Mooney et al 2009b;Finneran et al 2010a;Finneran et al 2010b;Finneran and Schlundt 2010;Kastelein et al 2012;Kastelein et al 2013b;Kastelein et al 2014;Popov et al 2014).…”

Mapping probability of shipping sound exposure level

“…3 Effects of noise on marine life are more related to values exceeding threshold limits, combined with the durations and recurrences of exposures and silences, than to mean or total energy. 8,9 The probability distribution function (pdf) of short-time SEL is needed to map where SEL are above or below threshold values with the corresponding percentage of time. Iterative a) Author to whom correspondence should be addressed.…”