Continental shelves and coastal areas have been called “Large Marine Ecosystems”, occupying coastal ocean space around the margins of the continents. In contrast to the open sea areas, they have a high biodiversity of organisms and habitats and a great primary productivity, hosting the main upwelling areas. Moreover, they constitute areas of recruitment and distribution of a multitude of commercial species and produce 80 per cent of the world’s annual marine fish catch (Sherman 2015). Due to its strategic interest, it is subjected to a multitude of impacts and threats, both climate and anthropogenic: accelerated warming from climate change, acidification, overfishing, loss of habitats, pollution, eutrophication...(He and Silliman 2019)
The Mediterranean Sea has been shown as an area of important recent changes. Most fish stocks have decreased (Machias et al. 2008; Vasilakopoulos et al. 2014; Van Beveren et al. 2016; Colloca et al. 2017; FAO 2020) and while overexploitation is a fact, the synergy with environmental changes probably represents a big part of the explanation. In certain cases, other more ecological reasons have been proposed for declining small pelagic fish populations. For instance, several studies have shown that the variations observed in sardines and anchovies in the Gulf of Lion (smaller size, lower body condition, and changes in age structure) might result from changes in the plankton community (reviewed in Saraux et al. 2019). Further, the size and condition of sardine and anchovy decreased in most areas of the Mediterranean Sea (Brosset et al. 2015), which could reflect a modification in the available resources for small pelagic, namely changes in the zooplankton abundance, composition or distribution. Given the extremely high dependence of small pelagic species development on the zooplankton community, the integration of its study in scientific surveys such as MEDiterranean International Acoustic Survey (MEDIAS) could be crucial to explain the fluctuations in distribution or biological parameters of the fish populations under study.
The pelagic habitat is the largest biome on Earth, key for temperature regulation, oxygen, and food production. Its physical and biological components and processes vary spatially and temporally depending on multiple drivers. Understanding this variability, processes and interactions is fundamental to identify the drivers of changes and properly assess pelagic habitats and communities (Magliozzi et al. 2021). Traditionally, the study of the pelagic environment has been approached regionally, making intercomparison and benchmarking difficult. This fact is especially relevant in the Mediterranean Sea, where there is a lack of knowledge related to the spatial coverage of the pelagic community studies, which makes the conclusions difficult to be extrapolated to the entire basin (Varkitzi et al. 2018). Macroscale studies are essential to understand the peculiarities of each area in a common frame, to determine general reference points and to achieve the basin scale management. In this context, the MEDIAS survey, whose main strengths are: (i) the standardization of data collection and processing across the Mediterranean (same methodology, same time of year), (ii) the common historical series and (iii) and its temporary continuity (monitoring), can constitute an essential tool to describe and analyze the pelagic ecosystem at basin scale and contribute to its integrated management.
In the framework of fisheries acoustics, the goal target usually is the precise estimation of fish abundance, so that plankton echotraces have been considered as reverberation or undesired elements, systematically removed from the echograms (Simmonds and MacLennan 2005). However, as fisheries acoustics are changing toward an ecosystem based management, zooplankton has become a key component in ecology assessment models (Möllmann et al. 2014). Although fisheries acoustic data can still be considered as underused (Trenkel and Berger 2013), they present a high potential for the study of the pelagic ecosystem as a whole, integrating concurrent information both on pelagic fish and zooplankton assemblages. The joint treatment of the pelagic fish data collected in the MEDIAS has made it possible to reveal differences in the growth and maturity of anchovies in the Mediterranean Sea (Ferreri et al. 2021), to detect displacements of small pelagic species forced by climatic changes (De Felice et al. 2021), to determine the influence of the nycthemeral migration of pelagic fish on their abundance determination (Bonanno et al. 2021), to estimate the correlation between the small pelagic body size variation with temperature (Hattab et al. 2021) or to assess the effects of sampling intensity and biomass levels on the precision of acoustic surveys (Barra et al. 2021). Nonetheless, the study of the zooplankton community has not been jointly addressed to date.
Acoustic methods can be an adequate tool for monitoring and studying the zooplankton community, complementary to traditional zooplankton sampling methods. Acoustic instruments permit remote and quasi-continuous sampling of fish and plankton throughout the water column and consequently offer a spatio-temporal resolution similar to that of physical oceanographic data, therefore they have been proposed to generate new ecosystem indicators (Trenkel et al. 2011) in the frame of marine ecosystem acoustics (Godo et al. 2014). However, they require a large investment on infrastructure and technology for groundtruthing echotraces i.e. identify or verify the identity of the acoustic targets (Simmonds and MacLennan 2005; Chu 2011). Furthermore, modelling the acoustic properties of zooplankton is a complex process, making it difficult to infer the specific composition of zooplankton from acoustic data alone (Stanton et al. 1998; Warren and Wiebe 2008; Jech et al. 2015). In the other hand, traditional methods determine the exact zooplankton taxonomic composition and seasonal variations in short time intervals and small spatial scales (Fernández de Puelles et al. 2007; Fernández de Puelles and Molinero 2008; García-Comas et al. 2011). While they constitute a systematic, standardized and robust methodology to study the zooplankton communities, they remain very time consuming, despite great strides being made to develop automatic processes (e.g. zooscan), almost prohibiting a spatio-temporal monitoring at high resolution (Pitois et al. 2018). Most long-term zooplankton time-series are indeed sampled at a single location (e.g. Fernández de Puelles et al. 2007; Fernández de Puelles and Molinero 2008; García-Comas et al. 2011). Uniting the strengths of each method, is a challenge that could be addressed in the future to understand marine ecosystem in the Mediterranean Sea and MEDIAS surveys can play a strategic role in its achievement.
Plankton organisms are extremely abundant in the water column often forming dense layers, so when they are insonified, their echoes overlap to form the diffuse cloud-like marks frequently seen on echograms (Simmonds and MacLennan 2005). Moreover, due to the wide variety of shapes, structures and sizes of plankton, the acoustic identification of planktonic targets to species or even genera level is difficult (Martin et al. 1996; Stanton et al. 1996). The evolution of underwater acoustics technology from single frequency (narrow band) to multiple frequency systems provided scientists with additional capability to characterize or classify scattering targets (Chu 2011). Since then, numerous studies on the distribution (Lebourges-Dhaussy et al. 2009, 2014; Lezama-Ochoa et al. 2014), migration (Mutlu 2007; Benoit-Bird et al. 2013; Benoit-Bird and McManus 2014) and biological composition (Lavery et al. 2007; Mutlu 2007) of zooplankton have been carried out using acoustic methods all over the world. Nevertheless, studies regarding zooplankton scattering layers are scarce in the Mediterranean Sea, mainly focusing on mesopelagic fish (Peña et al. 2014) or fish larvae (Bonanno et al. 2006; Ventero et al. 2021) or displaying a limited spatial coverage (Kačić, 1973; Mutlu, 2005; Ventero, 2016).
The purpose of this study is to contribute to the plankton knowledge at a Mediterranean Sea scale. The general framework is the first jointly characterization of the plankton acoustic layers present on the continental shelf of ten GSAs along the Mediterranean Sea. The main objectives were (i) to describe and compare the acoustic structures associated to the Mediterranean plankton layers at two frequencies (38 and 120 kHz), with emphasis on the influence of the enrichment process of each study area, (ii) to apply the dB different method (Madureira et al. 1993) in order to infer the most likely zooplankton acoustic category (Madureira et al. 1993; Stanton et al. 1996) that form the plankton layers and, (iii) to establish a reference point for evaluating future zooplankton community changes in the Mediterranean Sea.