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).