Temporary hearing threshold shift in harbor seals (Phoca vitulina) due to a one-sixth-octave noise band centered at 32 kHz

Abstract

Two female harbor seals were exposed for 60 min to a continuous one-sixth-octave noise band centered at 32 kHz at sound pressure levels of 92 to 152 dB re 1 μPa, resulting in sound exposure levels (SELs) of 128 to 188 dB re 1 μPa2s. This was part of a larger project determining frequency-dependent susceptibility to temporary threshold shift (TTS) in harbor seals over their entire hearing range. After exposure, TTSs were quantified at 32, 45, and 63 kHz with a psychoacoustic technique. At 32 kHz, only small TTSs (up to 5.9 dB) were measured 1–4 min (TTS1–4) after exposure, and recovery was within 1 h. The higher the SEL, the higher the TTS induced at 45 kHz. Below ∼176 dB re 1 μPa2s, the maximum TTS1–4 was at 32 kHz; above ∼176 dB re 1 μPa2s, the maximum TTS1–4 (up to 33.8 dB) was at 45 kHz. During one particular session, a seal was inadvertently exposed to an SEL of ∼191 dB re 1 μPa2s and at 45 kHz, her TTS1–4 was >45 dB; her hearing recovered over 4 days. Harbor seals appear to be equally susceptible to TTS caused by sounds in the 2.5–32 kHz range.

Temporary Hearing Threshold Shift in California Sea Lions (Zalophus californianus) Due to a Noise Band Centered at 32 kHz

“…At all three hearing test frequencies for F01, significant TTS occurred already at an SEL of 180 dB re 1 µPa 2 s (SPL 144 dB re 1 µPa for 1 h). A similar pattern of lower (or similar) 6 dB TTS onset SELs after exposure to a fatiguing sound at 32 kHz than to one at 16 kHz was found in harbor seals (Kastelein et al, 2020a): the TTS onset SEL of two harbor seals to a onesixth octave NB at 32 kHz was between 176 and 181 dB re 1 µPa 2 s (SPLs of 140 and 145 dB re 1 µPa for 1 h) at a hearing frequency of 45 kHz (Kastelein et al, 2020a). TTS onset SELs at these high fatiguing sound frequencies may be low because the hearing frequencies at which the highest TTSs occurred are close to the upper frequencies of the California sea lions' and harbor seals' audiograms, where hearing becomes much less sensitive.…”

“…The low TTS onset levels seen at the very highest frequencies within the hearing range of California sea lions (32 kHz), as well as in harbor seals (32 and 40 kHz; Kastelein et al, 2020a) and Yangtze finless porpoises (testing 32, 64, and 128 kHz; the lower the frequency of the fatiguing sound at the same SEL, the higher the initial TTS; Popov et al, 2011), show that TTS onset levels are not as closely related (especially at the lower and higher frequencies) to the unmasked hearing thresholds (audiograms) as was assumed, based on very few available data by Southall et al (2019).…”

“…Both within and between species, there is variation in the causes and effects of TTS and in recovery times. Fatiguing sounds of different amplitudes and durations result in varying levels of reduced hearing sensitivity at different frequencies, and recovery times also vary (Popov et al, 2014;Finneran, 2015;Kastelein et al, 2020aKastelein et al, , 2020bKastelein et al, , 2021aKastelein et al, , 2021b. The present study is one of five in a comprehensive research project on TTS in California sea lions.…”

Exposure of individual harbour seals (Phoca vitulina) and waters surrounding protected habitats to acoustic deterrent noise from aquaculture

“…Measurements of the auditory systems of harbour seals have noted that their highest underwater hearing sensitivity occurs between 50 Hz and 86 kHz (Mohl, 1968; Terhune, 1988; Kastelein et al, 2009a; Kastelein et al, 2009b; Cunningham & Reichmuth, 2016), encompassing the frequency ranges at which the majority of ADDs operate (0.5–40 kHz) (Lepper et al, 2014; Todd et al, 2021). A number of captive studies have documented both TTS and PTS from exposures to noise between these frequencies (0.5–40 kHz) (Kastak et al, 2005; Kastelein et al, 2012; Kastelein et al, 2013b; Kastelein, Helder‐Hoek & Gransier, 2019; Reichmuth et al, 2019; Kastelein et al, 2020a; Kastelein et al, 2020b). However, although models have predicted this noise source as a risk of auditory impairment to seals (Götz & Janik, 2013; Lepper et al, 2014), the potential for auditory impairment in non‐target seals at sea and in waters around protected sites has yet to be quantified.…”