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Anthropogenic underwater noise is an emergent pollutant and several monitoring programs have started worldwide. However, only few data are available so far for the Mediterranean Sea, one of the global biodiversity hotspots. To fill this knowledge gap, this study presents the results of the first continuous acoustic monitoring run at a transnational basin scale in the Mediterranean Sea between March 2020 and June 2021, including the COVID-19 lockdown, at nine stations in the Northern Adriatic Sea. Median yearly sound pressure levels (SPLs) ranged between 64 and 95 as well as 70 and 100 dB re 1µPa for 63 and 125 Hz third octave bands, respectively. Although these values are comparable with those previously found in busy shallow EU basins, higher levels are likely to be found during a business-as-usual period. Higher and more variable SPLs, mainly related to vessel traffic were found in stations close to important harbours, whereas stations inside or close to Natura 2000 sites, experienced lower SPLs, and lower values were recorded during the Covid-19 lockdown in four stations. This represent the first baseline acoustic assessment for a highly impacted and valuable area that needs to be taken into account in a new sustainable blue growth strategy.

Small pelagic fishes play a crucial role in pelagic food webs as both top-down and bottom-up controllers. Their role as predators could also exert a wasp-waist flow control. The Adriatic basin is one of the largest areas of occurrence of small pelagic shared stocks and the most exploited basin of the Mediterranean Sea. Here, we analyzed the ontogenetic and spatial variations in the stable isotope composition of three small pelagic fishes (i.e., Engraulis encrasicolus, Sardina pilchardus, and Sprattus sprattus) collected throughout the western side of the basin and relate such differences to both resource availability and environmental variables. Medium-sized specimens of the three species mostly fed on small zooplankton, while adult sprats relied on large copepods and those of sardine and anchovy seemed to shift on phytoplankton, confirming the high trophic plasticity of these two dominants small pelagic species. Resource partitioning occurred at each size and in each sub-area, also favored by the high productivity and in turn high resource availability of the basin, especially in the northern part. These small pelagic fish were located at different, but very close, trophic position with other small pelagic species, and thus competition might occur in the future driven by modified feeding conditions as consequence of increasing sea temperatures.

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.

This research represents the first approach to the study of the shelf zooplankton community at a Mediterranean scale, using acoustic standardized data collected in June-July in 10 different geographical sub-areas (GSAs) established by the General Fisheries Commission for the Mediterranean (GFCM) within the framework of the MEDiterranean International Acoustic Survey (MEDIAS). The analysis of the zooplankton layers based on their acoustic characteristics has revealed the potential of these surveys for the study of zooplankton at a Mediterranean basin scale and, also, the need to collect biological samples to interpret the acoustic records in terms of species. The fish population’s direct assessment is established in the MEDIAS framework, but the integration of zooplankton community data would constitute a qualitative step for the understanding of the fluctuations of fish populations and therefore to achieve the objective of an ecosystem-based management. Results have revealed the ubiquity of the zooplankton layers and its ability to form layers detectable by scientific echosounders throughout the Mediterranean at the common fish assessment frequency (38 kHz). In addition, the use of two frequencies (38 and 120 kHz) has allowed to apply the dB difference method, observing changes in the difference of the Mean Volume Backscattering Strength (∆MVBS) at 38 and 120 kHz frequencies, which would be related to changes in the composition of the zooplankton community.

The Mediterranean is one of the most overfished seas of the world where mesopredators are severely threatened. The trophic strategies of four pelagic species that inhabit the Adriatic Sea (Scomber spp. and Trachurus spp.) were investigated through an integrated approach of stomach contents and stable isotopes analyses. Our study demonstrated that Scomber colias feeds mainly on strictly pelagic prey, with fish larvae as a secondary prey in the Southern Adriatic Sea, while S. scombrus feeds on prey belonging to higher trophic levels. Smaller specimens of Trachurus mediterraneus have a diet mainly based on pelagic prey, while larger fishes rely on prey such as benthic decapods, showing an ontogenetic shift in the diet of the species. Trachurus trachurus shows a preference for offshore and deeper areas and a diet such as that of its congeneric, but no clear ontogenetic shift was observed. This spatial segregation allows the co-existence of these two species of Trachurus. Scomber colias mainly inhabits southern areas and S. scombrus shows a preference for the northern sectors. This latitudinal gradient avoids the overlap of their trophic niches. Bayesian mixing models confirmed that the trophic niches of these species only partially overlap in the middle of the trophic web.

Size at first sexual maturity (L50) represents an important life-history trait that needs to be considered in the development of management measures as it provides fundamental information for avoiding the exploitation of younger individuals. L50 is known to display variability due to fishing pressure, geographical gradients, and environmental features. In this study, to investigate L50 variability among areas in the Mediterranean and Black Seas, maturity ogives of anchovies (Engraulis encrasicolus) were estimated by considering samples collected during the anchovy spawning period in the framework of the MEDiterranean International Acoustic Survey (MEDIAS) program. Anchovy size and sexual maturity data from several geographical subareas (GSAs), i.e., northern Spain, Gulf of Lion, Tyrrhenian Sea, Strait of Sicily, Adriatic Sea and Black Sea, were gathered according to a standard methodological protocol. Maturity ogives were estimated by a logistic regression considering total length, condition factor, sex and GSA. The obtained results showed a significant effect of the condition factor, in that fish in better condition reached maturity earlier, and the results also indicated differences in L50 values among the areas and between the sexes, with males reaching maturity at lower lengths than females. Even though the obtained L50 estimates are relative to the spawning period only, the variability observed at the Mediterranean basin scale highlights the importance of explicitly considering specific habitat characteristics when providing management advice based on an ecosystem approach for fisheries.