Fig. 6 Observations (dots) and model estimates (smooth curves)
of variations in porpoise behaviour during the exposure experiments
(phase 1, 2 and 3). Shaded areas along smooth curves show 95%
confidence intervals. The vertical purple lines represent the closest
point of approach (CPA). The shaded light brown areas indicate the
periods where the boat was in motion (i.e. phase 2). * denotes
statistically significant models (p <0.05).
Sound received from an approaching
boat
As the boat moved towards the CPA at 10 knots, the TOL 16 kHz sound
level increased from approximately 90 to 115 dB while the broadband
sound level (0.1 to 150 kHz) increased from 110 to 135 dB. The increase
in sound level was similar at 10 knots (TOL 16k: 27.2–31.6 dB;
Broadband: 26.6–29.4 dB) and 20 knots (TOL 16k: 26.1–29.4 dB;
Broadband: 26.3–30.3 dB) for both sound frequency bands (Fig. 7). The
broadband sound level was 17 dB higher than the TOL 16 kHz band level at
CPA. At a speed of 10 knots, the mean absolute change in noise level per
10 seconds was 3 dB for broadband levels and 5.1 dB for 16 kHz TOL.
Conversely, at 20 knots, the corresponding changes were 5.1 dB for
broadband levels and 5.3 dB for the 16 kHz TOL. In the 10-knot scenario,
the most rapid change in sound levels occurred during the 10 seconds
around CPA, where changes in sounds reached 14.3 dB and 17.9 dB for
broadband and 16 kHz TOL, respectively. In the 20-knot recordings, these
changes were even more pronounced; 22.0 dB per 10 s for broadband and
27.4 dB per 10 s for 16 kHz TOL.
Importantly, porpoises exhibited obvious reactions, including speeding
up and moving away from the boat, when the approaching boat was within
the range of 100–200 m. This coincided with a rapid rise in sound
levels that started occurring from around 200 m with noise levels at
100–105 dB at 16 kHz TOL (Fig. 4 and 7). After the boat had passed, the
sound levels rapidly decreased (Fig. 7).