Hearing threshold shifts and recovery in harbor seals (Phoca vitulina) after octave-band noise exposure at 4 kHz
The study pool, showing the harbor seal being tested in position at the listening station, and the seal not being tested with its trainer: (a) top view and (b) side view, both to scale. The platforms, normally used by the seals for hauling out, were made unavailable by means of barriers during the fatiguing noise exposure. In (a), the test seal’s swimming path during a hearing test trial is shown, and x = locations of aerial SPL measurements (10 cm above the water surface).
Mean underwater background noise (n = 3) under test conditions (only researchers involved in the study within 15 m of the pool, water circulation system off, no rain, and wind force Beaufort 4 or below). Measurements were analyzed in 1/3-octave bands and the band levels were converted to PSL (Kinsler et al., 2000). Above ∼10 kHz the background noise is so low that the measured noise is dominated by the electronic noise of the measurement system.
The mean (dB-average; n = 3) spectrum of the OBN centered at 4 kHz which was used as the fatiguing noise. Measurements at 2 m from the projector were analyzed in 1/3-octave bands. The horizontal line indicates the defined bandwidth of the calculated OBN, and shows that the OBN band had the correct width.
The mean SPL distribution of the fatiguing underwater noise in the pool at the highest level projected (mean SPL was 148 dB re 1 μPa; 33 measurement locations at each depth, 99 measurement locations in total; n = 4 measurements at each location).
The SPL distribution of the fatiguing aerial noise 10 cm above the pool surface, based on one measurement per location (25 locations; some data points overlap). The midpoint between the two loudspeakers was taken as the source location (see Fig. 1).
Effect of mean received SPL (124, 136, and 148 dB re 1 μPa) on mean TTS1–4: (a) seal 01, and TTS12–16: (b) seal 02 (in dB, ± standard error; n = 5–7 hearing test sessions) after the seals were exposed to a continuous OBN centered at 4 kHz for 7.5, 15, 30, 60, 120, and 240 min.
Effect of exposure time (7.5, 15, 30, 60, 120, and 240 min) on the mean TTS1–4: (a) seal 01, and TTS12–16: (b) seal 02 (in dB; ±standard error; n = 5–7 hearing test sessions) measured after the seals were exposed to a continuous OBN centered at 4 kHz at mean received SPLs of 124, 136, and 148 dB re 1 μPa.
The observed TTS1–4 (dB) of seal 01 in the present study as a function of the SEL (in dB re 1 ), for fatiguing sounds at 124 (●), 136 (▪), and 148 dB re 1 μPa (▲). Results are shown in order of increasing duration of fatiguing noise (7.5, 15, 30, 60, 120, and 240 min), and only for TTS measured 1–4 min after exposure (so seal 02 is not included). The horizontal dashed line indicates the approximate threshold shift level (∼2.5 dB) above which TTSs were found to be statistically significant in the present study (see Table II).
The prestimulus response rate (based on both signal-present and signal-absent trials, as signal-absent trials are ended with a whistle stimulus) of seals 01 and 02 during BL, PNE periods (all exposure level and duration combinations pooled), and PAE (∼60 min) periods.
The mean TTS (in dB, ± SD; n = 5–7) in harbor seals 01 and 02 after they were exposed to a continuous OBN centered at 4 kHz for various periods. The mean TTS was measured for a narrow-band up-sweep centered at 4 kHz (3.9–4.1 kHz) in four or five time periods for seal 01: 1–4 min (TTS1–4), 4–8 min (TTS4–8), 8–12 min (TTS8–12), 48 min (TTS48), and sometimes 96 min (TTS96) after the noise exposure stopped; and in four or five time periods for seal 02: 12–16 (TTS12–16), 16–20 (TTS16–20), 20–24 (TTS20–24), 60 (TTS60), and sometimes 108 (TTS108) min after exposure stopped. The exact P-values for the Bonferroni post-hoc tests (conducted after the ANCOVAs on hearing thresholds with factor period and covariate session number) are shown in parentheses [where significant; otherwise shown as not significant (NS), or as “—” when post-hoc tests were not conducted because the factor period was not significant in the ANCOVA]. Bonferroni tests are used to compare the hearing thresholds in the test periods with those quantified in the corresponding baseline periods. The control hearing test sessions were conducted after the seals had been exposed for approximately 60 min to the normal (low) ambient noise level in the pool (n = 11). ANCOVAs showed that no TTS took place during control sessions.
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