Abstract
A total of 147 days spread over 4 years were recorded by a stereophonic sonobuoy set up in the Mediterranean sea, near the coast of Toulon, south of France. These recordings were analyzed in the scope of studying sperm whales (Physeter macrocephalus) and the impact anthropic noises may have on this species. With the use of a novel approach, which combines the use of a stereophonic antenna with a neural network, 226 sperm whales’ passages have been automatically detected in an effective range of 32 km. This dataset was then used to analyze the sperm whales’ abundance, the background noise, the influence of the background noise on the acoustic presence, and the animals’ size. The results show that sperm whales are present all year round in groups of 1–9 individuals, especially during the daytime. The estimated density is 1.69 whales/1000 km^2. Animals were also less frequent during periods with an increased background noise due to ferries. The animal size distribution revealed the recorded sperm whales were distributed in length from about 7 to 15.5 m, and lonely whales are larger, while groups of two are composed of juvenile and mid-sized animals.
Highlights
Background noise analysisTo assess the performance of the detector a Convolutional Neural Network (CNN) as well as to measure the impact of noise on the presence of sperm whales, the amplitudes of different octave bands were computed and analyzed
The objective of this research is to set up an acoustic protocol and relevant methods to determine the number of individuals in an area, to estimate their size, density, and the influence of noise on their attendance
The results of this study are presented in 5 main parts: the sperm whales’ acoustic presence, the Background Noise (BN), the influence of BN on the Acoustic Presence (AP), the animals’ size distribution, and the animal density
Summary
To assess the performance of the detector a Convolutional Neural Network (CNN) as well as to measure the impact of noise on the presence of sperm whales, the amplitudes of different octave bands were computed and analyzed. The average of background noises increased by approximately 3 dB during the ferry periods, while the ferry crossings are only a few kilometers away from the antenna (Fig. 5). The full acquisition chain has been calibrated using Wenz curves[36] to fit the standard noise level of the observed sea state. The wave height h (in m) was computed from the wind speed v87 (in km h− 1 ) following Eq (3) with g the standard acceleration due to gravity[88]
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