Long-Term Monitoring of Dolphin Biosonar Activity in Deep Pelagic Waters of the Mediterranean Sea

Abstract

Dolphins emit short ultrasonic pulses (clicks) to acquire information about the surrounding environment, prey and habitat features. We investigated Delphinidae activity over multiple temporal scales through the detection of their echolocation clicks, using long-term Passive Acoustic Monitoring (PAM). The Istituto Nazionale di Fisica Nucleare operates multidisciplinary seafloor observatories in a deep area of the Central Mediterranean Sea. The Ocean noise Detection Experiment collected data offshore the Gulf of Catania from January 2005 to November 2006, allowing the study of temporal patterns of dolphin activity in this deep pelagic zone for the first time. Nearly 5,500 five-minute recordings acquired over two years were examined using spectrogram analysis and through development and testing of an automatic detection algorithm. Echolocation activity of dolphins was mostly confined to nighttime and crepuscular hours, in contrast with communicative signals (whistles). Seasonal variation, with a peak number of clicks in August, was also evident, but no effect of lunar cycle was observed. Temporal trends in echolocation corresponded to environmental and trophic variability known in the deep pelagic waters of the Ionian Sea. Long-term PAM and the continued development of automatic analysis techniques are essential to advancing the study of pelagic marine mammal distribution and behaviour patterns.

Temporal patterns in acoustic presence and foraging activity of oceanic dolphins at seamounts in the Azores

“…Similar diel cycles in vocal behaviour have been reported in several dolphin species. In general, whistling used in social interactions is higher during daylight hours or remain constant throughout the day, whereas echolocation clicks used primarily for foraging and navigating occur mainly at night [32][33][34][35] . In addition, concurrent observations of behaviour and acoustic recordings indicate that dolphins echolocate at high rates while foraging, exhibit www.nature.com/scientificreports www.nature.com/scientificreports/ low vocal activity while resting, and moderate vocal activity while socializing and travelling 32,36,37 .…”

“…High rates of nighttime foraging and echolocation in dolphins living in pelagic waters have been associated with feeding on vertical migrating micronekton organisms at night 32,34,35,38 . Most micronektonic taxa undergo diel vertical migrations, residing in deeper waters during the day, swimming towards the surface around sunset to feed during night, and returning to deeper waters at sunrise 39 .…”

Monitoring of a Nearshore Small Dolphin Species Using Passive Acoustic Platforms and Supervised Machine Learning Techniques

“…Our improved ability to acquire acoustic data continuously from multiple recording stations over extended periods of time has increased the use of automatic signal identification techniques (Zimmer, 2011) and machine learning approaches (Bianco et al, 2019) for detecting marine mammal vocalizations (Shiu et al, 2020). Several detection and classification algorithms have been developed to identify cetacean sounds, particularly the echolocation clicks of toothed whales (Soldevilla et al, 2008;Roch et al, 2011Roch et al, , 2015Giorli et al, 2016;Caruso et al, 2017;Giorli and Goetz, 2019;Hildebrand et al, 2019). Compared to manual analysis methods (spectrogram analysis), such algorithms have the advantage of being able to quickly and reliably analyze large amounts of data and produce standardized measures of sound characteristics that can be used for statistical analysis (Giorli et al, 2016;Caruso et al, 2017).…”

S-shaped microfiber based diaphragm supported optical microphone

“…In the last decade, a large number of acoustic sensors (i.e. microphones) have been proposed for structural health monitoring of major civil constructions, operational monitoring of machines, recording bioacoustics signals, seismic analysis and many other civil and defence applications [1][2][3][4][5]. Depending on the application, the microphone having the most suitable properties is selected.…”