First basin scale spatial-temporal characterization of underwater sound in the Mediterranean Sea.

Anthropogenic underwater noise is an emergent pollutant. Despite several worldwide monitoring programs, only few data are available for the Mediterranean Sea, one of the global biodiversity hotspots. The results of the first continuous acoustic programme run at a transnational basin scale in the Mediterranean Sea are here presented. Recordings were done from March 2020 to June 2021, including the COVID-19 lockdown, at nine stations in the Northern Adriatic Sea. Spatial-temporal variations of the underwater sound are described, having one third octave band sound pressure levels (SPLs) from 10 Hz to 20 kHz as metrics. Higher and more variable SPLs, mainly related to vessel traffic, were found close to harbours, whereas Natura 2000 stations experienced lower SPLs. Lower values were recorded during the lockdown in five stations. Median yearly SPLs ranged between 64 and 95 as well as 70 and 100 dB re 1 µPa for 63 and 125 Hz bands, respectively. These values are comparable with those previously found in busy shallow EU basins but higher levels are expected during a business-as-usual period. This is a baseline assessment for a highly impacted and environmental valuable area, that needs to be managed in a new sustainable blue growth strategy.

Addressing underwater noise: Joint efforts and progress on its global governance.

Underwater noise generated by human activities has become a major reason of concern over the past decades as human exploitation of world seas became more intense. A key to reduce human-generated acoustic pressure on aquatic ecosystems depends on an approach based on international cooperation. Over the past years, scientists worldwide worked together to assess trends in underwater noise levels in order to develop mitigation measures that would allow the effective protection of endangered species without reducing the possibilities for a sustainable use of seas. This review focused on international programmes dedicated to underwater noise monitoring, mapping and to programs dedicated to mitigate noise and its effects on marine fauna. All together this review shows the existence of a growing, general, international consensus on the fact that anthropogenic underwater noise should be significantly reduced by setting appropriate mitigation measures and effective regulatory actions.

First assessment of underwater sound levels in the Northern Adriatic Sea at the basin scale.

The protection of marine habitats from human-generated underwater noise is an emerging challenge. Baseline information on sound levels, however, is poorly available, especially in the Mediterranean Sea. To bridge this knowledge gap, the SOUNDSCAPE project ran a basin-scale, cross-national, long-term underwater monitoring in the Northern Adriatic Sea. A network of nine monitoring stations, characterized by different natural conditions and anthropogenic pressures, ensured acoustic data collection from March 2020 to June 2021, including the full lockdown period related to the COVID-19 pandemic. Calibrated stationary recorders featured with an omnidirectional Neptune Sonar D60 Hydrophone recorded continuously 24 h a day (48 kHz sampling rate, 16 bit resolution). Data were analysed to Sound Pressure Levels (SPLs) with a specially developed and validated processing app. Here, we release the dataset composed of 20 and 60 seconds averaged SPLs (one-third octave, base 10) output files and a Python script to postprocess them. This dataset represents a benchmark for scientists and policymakers addressing the risk of noise impacts on marine fauna in the Mediterranean Sea and worldwide.

Characterization of the underwater noise produced by recreational and small fishing boats (<14 m) in the shallow-water of the Cres-Losinj Natura 2000 SCI.

Recreational boats are a dominant source of underwater noise in coastal areas, but reliable boat noise assessment is generally lacking. Here the Underwater Radiated Noise (URN) of seven recreational and small fishing boats moving at two different speeds was measured in the shallow waters of the Cres-Losinj Natura 2000 SCI (Croatia). Measurements were undertaken considering the internationally recognized standards and published guidelines for shallow waters. URN was provided in 1/3 octave band spectra and in narrow band spectra, to highlight the tonal components. Engine power and type rather than the boat length and design result to be more predictive of URN. Highest speeds induce highest noise levels only in a very limited frequency range and different boats with similar speed but different engines show a shift in the spectra. Relevance of the achieved results for the tested area is further discussed.

Determinants of variability in signature whistles of the Mediterranean common bottlenose dolphin.

One of the most studied aspects of animal communication is the acoustic repertoire difference between populations of the same species. While numerous studies have investigated the variability of bottlenose dolphin whistles between populations, very few studies have focused on the signature whistles alone and the factors underlying differentiation of signature whistles are still poorly understood. Here we describe the signature whistles produced by six distinct geographical units of the common bottlenose dolphin (Tursiops truncatus) in the Mediterranean Sea and identify the main determinants of their variability. Particularly, the influence of the region (proxy of genetic distance), the geographic site, and the environmental (sea bottom-related) and demographical (population-related) conditions on the acoustic structure of signature whistles was evaluated. The study provides the first evidence that the genetic structure, which distinguishes the eastern and western Mediterranean bottlenose dolphin populations has no strong influence on the acoustic structure of their signature whistles, and that the geographical isolation between populations only partially affected whistle variability. The environmental conditions of the areas where the whistles developed and the demographic characteristics of the belonging populations strongly influenced signature whistles, in accordance with the "acoustic adaptation hypothesis" and the theory of signature whistle determination mediated by learning.

Influence of foraging context on the whistle structure of the common bottlenose dolphin.

Sounds are particularly important for animals that live in complex social communities. In this study, we assessed the communication calls (whistles) that common bottlenose dolphins emit during their foraging activities in the absence and presence of motor boats and during dolphin depredation on trawlers, in Alghero (Sardinia, Italy) and Cres-Losinj Archipelago (Croatia). The latter behaviour involves foraging on concentrated food sources during very noisy human activity and may require the emission of distinctive whistles. Thus, we investigated if whistle structure, in terms of frequency and time parameters, changes depending on these three foraging contexts. In Sardinia, during foraging in interaction with trawlers, whistles differed from those emitted during the other foraging contexts. Conversely, in Cres-Losinj, significant variations in whistles were found to be related mainly to the presence of motor boats. This study represents the first report on how two dolphin populations adopt different acoustic tactics in the context of similar foraging behaviour. By investigating the effects of opportunistic foraging on acoustic repertoires, we provide new findings on the acoustic adaptation of dolphins to local conditions and contribute to understanding the relationships between dolphins and human activities, such as fishing and boat traffic.

Whistle variation in Mediterranean common bottlenose dolphin: The role of geographical, anthropogenic, social, and behavioral factors.

The studies on the variation of acoustic communication in different species have provided insight that genetics, geographic isolation, and adaptation to ecological and social conditions play important roles in the variability of acoustic signals. The dolphin whistles are communication signals that can vary significantly among and within populations. Although it is known that they are influenced by different environmental and social variables, the factors influencing the variation between populations have received scant attention. In the present study, we investigated the factors associated with the acoustic variability in the whistles of common bottlenose dolphin (Tursiops truncatus), inhabiting two Mediterranean areas (Sardinia and Croatia). We explored which factors, among (a) geographical isolation of populations, (b) different environments in terms of noise and boat presence, and (c) social factors (including group size, behavior, and presence of calves), were associated with whistle characteristics. We first applied a principal component analysis to reduce the number of collinear whistle frequency and temporal characteristics and then generalized linear mixed models on the first two principal components. The study revealed that both geographic distance/isolation and local environment are associated with whistle variations between localities. The prominent differences in the acoustic environments between the two areas, which contributed to the acoustic variability in the first principal component (PC1), were found. The calf's presence and foraging and social behavior were also found to be associated with dolphin whistle variation. The second principal component (PC2) was associated only with locality and group size, showing that longer and more complex tonal sound may facilitate individual recognition and cohesion in social groups. Thus, both social and behavioral context influenced significantly the structure of whistles, and they should be considered when investigating acoustic variability among distant dolphin populations to avoid confounding factors.

Factor associated variations in the home range of a resident Adriatic common bottlenose dolphin population.

This study investigates the influence of the most dominant factors (association patterns, gender, natal philopatry and anthropogenic pressure) on the home range size of the 44 most resident common bottlenose dolphins (Tursiops truncatus) inhabiting the waters of the Cres-Losinj archipelago (north Adriatic Sea, Croatia), a recently declared NATURA 2000 SCI. Results show that variations in home range patterns (MCP, 95% KDE and 50% KDE home range size) among the individual resident dolphins are primarily related to differences in gender and reflect the way in which different genders respond to external stressors. In addition, results confirm the seasonal influence of nautical tourism on both female and male dolphins through changes in their home range sizes. The overall results improve current knowledge of the main anthropogenic threats that should be taken into consideration when developing conservation measures to be applied to this Cres and Losinj SCI.