Biological Sound vs. Anthropogenic Noise: Assessment of Behavioural Changes in Scyliorhinus canicula Exposed to Boats Noise

Abstract

Despite the growing interest in human-made noise effects on marine wildlife, few studies have investigated the potential role of underwater noise on elasmobranch species. In this study, twelve specimens of small-spotted catshark (Scyliorhinus canicula) were exposed to biological and anthropogenic sounds in order to assess their behavioural changes in response to prey acoustic stimuli and to different amplitude levels of shipping noise. The sharks, individually held in aquariums, were exposed to four experimental acoustic conditions characterized by different spectral (Hz) components and amplitude (dB re 1 µPa) levels. The swimming behaviour and spatial distribution of sharks were observed. The results highlighted significant differences in swimming time and in the spatial use of the aquarium among the experimental conditions. When the amplitude levels of biological sources were higher than those of anthropogenic sources, the sharks' swimming behaviour was concentrated in the bottom sections of the aquarium; when the amplitude levels of anthropogenic sources were higher than biological ones, the specimens increased the time spent swimming. Moreover, their spatial distribution highlighted a tendency to occupy the least noisy sections of the aquarium. In conclusion, this study highlighted that anthropogenic noise is able to affect behaviour of catshark specimens and the impact depends on acoustic amplitude levels.

Keywords: anthropogenic noise; biological sounds; signal/noise ratio; small-spotted catshark.

Figure 1

(a) Spectrogram (FFT 2048, Hanning window, frequency scale linear) of the four acoustic experimental conditions, showing the different spectral features; (b) Spectrogram (FFT 2048, Hanning window, frequency scale linear) of the four acoustic experimental conditions recorded inside the experimental aquarium, showing the different spectral characteristics. The yellow dotted line represents the upper hearing sensitivity threshold limit of elasmobranchs benthic demersal species [25,50]. C, control condition; B, biological condition; B > A, biological > anthropogenic condition; B < A, biological < anthropogenic condition.

Figure 2

Averaged power spectrum of the three audio-created acoustic tracks and the aquarium’s background noise. The power spectral density (PSD) is expressed in dB re 1 μPa2/Hz versus the logarithmic frequency scale expressed in Hz. The yellow dotted line represents the limit below which the sound speaker has a limited frequency response.

Figure 3

Subdivisions of the aquarium in cells and sectors. Section Up, constituted by 1, 2, 3, and 4 up cells. Section Down, constituted by 1, 2, 3, and 4 down cells. Section Left, constituted by 1–2 up and 1–2 down cells. Section Right, constituted by 3–4 up and 3–4 down cells. On the left wall, the location of the underwater sound speaker is presented.

Figure 4

Values (median ± 25th–75th percentiles, whiskers ± 1st–99th percentiles) of the time spent in swimming assessed in S. canicula in the four acoustic experimental conditions. Different letters represent significant differences among the acoustic experimental conditions (p < 0.05).

Figure 5

Cluster heatmap showing the spatial occupancy in terms of time spent in swimming (expressed in seconds) by the sharks among the aquarium cells. Section Up, constituted by 1, 2, 3, and 4 up cells. Section Down, constituted by 1, 2, 3, and 4 down cells. Section Left, constituted by 1–2 up and 1–2 down cells. Section Right, constituted by 3–4 up and 3–4 down cells. On the left wall, the location of the underwater sound speaker is presented. C, control condition; B: biological condition; B > A, biological > anthropogenic condition; B < A, biological < anthropogenic condition.