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The present study describes for the first time the spatial distribution of five macrourid species throughout the Mediterranean Sea and analyses depth, geographical and time-related trends regarding their abundance, biomass and mean fish weight. The data were collected as part of the MEDITS annual bottom trawl survey carried out by several European Mediterranean countries from 1994 to 2015, using the same standardized gear and sampling protocol. The most represented species in terms of abundance and biomass was Coelorinchus caelorhincus. The bathymetric trend was different for each species. The shallowest occurring species was C. caelorhincus, followed by Hymenocephalus italicus and Nezumia sclerorhynchus, while Nezumia aequalis and Trachyrincus scabrus were the deepest. Overall, the mean weight of all the species increased with depth. C. caelorhincus and H. italicus occurred in the entire study area: the first species showed relatively high catches in most areas, while the second was more abundant in the central and easternmost areas. N. aequalis and T. scabrus were mainly reported in the western basin, and N. sclerorhynchus in the central-eastern areas of the Mediterranean. An increasing inter-annual trend in abundance was only detected for C. caelorhincus and N. sclerorhynchus, while variable fluctuations were observed in the other species.

Most studies on European hake focus on the recruitment process and nursery areas, whereas the information is comparatively limited on the ecology of the juvenile stage (ca. second year of life)—the one most exploited by the Mediterranean trawl fisheries. Using information of the MEDITS programme, we provide a spatial and temporal assessment of the influence of body size and growth on hake survival from recruits (age 0) to juveniles (age 1), along with the impact of surface temperature and chlorophyll variability. At a biogeographic scale, size-dependent survival is supported, with areas with higher mean length of recruits and juveniles yielding higher survival. A similar pattern was observed at interannual level in some western Mediterranean areas, also mediated by a density-dependent effect on growth. However, the most recurrent inter-annual pattern was a negative effect of size on survival, which could be attributed to potential ontogenetic changes in catchability and underrepresentation of intra-annual recruitment pulses that are seasonally inaccessible to the MEDITS survey. Results also evidence that survival in the Alboran and Adriatic seas is dependent on the primary production variability, and that Corsica and Sardinia could be potential feeding grounds receiving juveniles from neighbouring areas. The present study reveals the importance of size- and growth-dependent survival in the juvenile stage of European hake in the Mediterranean Sea.

The main characteristics concerning the distribution of two of the most important decapod crustaceans of commercial interest in the Mediterranean Sea, the deep-water rose shrimp, Parapenaeus longirostris, and the Norway lobster, Nephrops norvegicus, are studied in the European Mediterranean waters. The study is based on data collected under the MEDITS trawl surveys from 1994 to 2015 from the Gibraltar Straits to the northeastern Levantine Basin (Cyprus waters). The observed differences can be interpreted as different responses to environmental drivers related to the differing life history traits of the two species. In fact, N. norvegicus is a long-living, benthic burrowing species with low growth and mortality rates, while P. longirostris is an epibenthic, short-living species characterized by higher rates of growth and mortality.

The otolith, found in both inner ears of bony fish, has mainly been used to estimate fish age. Another application that has been developing significantly in recent years, however, is the use of otolith shape as a tool for stock identification. Often, studies have directly used the shape asymmetry between the right and left otoliths. We tested the magnitude of directional asymmetry between the sagittal otoliths (left vs. right) of 2991 individuals according to their catch locations, and we selected species to evaluate whether directional asymmetry may itself be a tool to evaluate stock boundaries. Elliptical Fourier descriptors were used to describe the otolith shape. We used a flatfish, the common sole (Solea solea, n = 2431), from the eastern English Channel and the southern North Sea as well as a roundfish, the bogue (Boops boops, n = 560), from the Mediterranean Sea. Both species showed significant levels of directional asymmetry between the testing locations. The bogue otoliths showed significant asymmetry for only 5 out of 11 locations, with substantial separation between two large areas: the Algerian coast and the western part of the Italian coast. The sole otoliths showed significant asymmetry in the shape analysis (3.84–6.57%), suggesting a substantial separation between two large areas: the English and French parts of the English Channel and the southern North Sea. Consequently, directional bilateral asymmetry in otolith shape is a potential new method for stock identification.

Species diversity is widely recognized as an important trait of ecosystems' functioning and resilience. Understanding the causes of diversity patterns and their interaction with the environmental conditions is essential in order to effectively assess and preserve existing diversity. While diversity patterns of most recurrent groups such as fish are commonly studied, other important taxa such as cephalopods have received less attention. In this work we present spatio-temporal trends of cephalopod diversity across the entire Mediterranean Sea during the last 19 years, analysing data from the annual bottom trawl survey MEDITS conducted by 5 different Mediterranean countries using standardized gears and sampling protocols. The influence of local and regional environmental variability in different Mediterranean regions is analysed applying generalized additive models, using species richness and the Shannon Wiener index as diversity descriptors. While the western basin showed a high diversity, our analyses do not support a steady eastward decrease of diversity as proposed in some previous studies. Instead, high Shannon diversity was also found in the Adriatic and Aegean Seas, and high species richness in the eastern Ionian Sea. Overall diversity did not show any consistent trend over the last two decades. Except in the Adriatic Sea, diversity showed a hump-shaped trend with depth in all regions, being highest between 200-400 m depth. Our results indicate that high Chlorophyll a concentrations and warmer temperatures seem to enhance species diversity, and the influence of these parameters is stronger for richness than for Shannon diversity.

Abstract The bamboo-coral Isidella elongata is a key habitat-forming species in the deep Mediterranean Sea. This alcyonacean is listed as an indicator of Vulnerable Marine Ecosystems (VMEs) and as Critically Endangered due to bottom trawling impacts. In this work, a modeling approach was used to predict and map the habitat suitability of I. elongata in the Mediterranean Sea under current environmental conditions. Occurrence data were modeled as a function of environmental parameters. Using climate change scenarios and fishing effort data, the risk of climate change and fisheries impacts on habitat suitability were estimated, and climate refugia were identified. A drastic loss of habitat is predicted, and climate change scenarios suggest a loss of 60% of suitable habitats by 2100. In the central Mediterranean, climate refugia overlapped with active fishing grounds. This study represents the first attempt to identify hot spots for the protection of soft bottom Vulnerable Marine Ecosystems for the entire Mediterranean Sea, and highlights areas most at risk from trawling. This work is relevant to the objectives of the EU Marine Strategy Framework and Maritime Spatial Planning Directives, the Biodiversity Strategy for 2030 regarding priority areas for conservation.

The Mediterranean Sea is a large marine ecosystem with high heterogeneity in both environmental and ecological characteristics. It presents clear gradients from north to south and west to east. It is also an important area in terms of biodiversity and conservation of vulnerable species, and it suffers from several cumulative human impacts, such as fishing and climate change. Previous studies have characterized spatial and temporal patterns of species distributions and biodiversity indicators. However, a comprehensive analysis combining a wide representation of biodiversity indicators is still missing. In this study, we examined spatial and temporal changes of marine communities along a latitudinal gradient over the continental shelf ecosystems (25–500 m depth) of the Western Mediterranean Sea, from the Gulf of Lion in the north to the Gibraltar Strait in the south. We used information from the MEDITS trawl scientific surveys from 1994 to 2018, and we calculated relevant indicators to investigate spatial and temporal patterns in the region. We selected several indicators measuring alpha (species richness, Shannon diversity index and Pielou evenness index) and beta (decomposing both turnover and nestedness) diversity, as well as previously studied indicators identified to be sensitive to fishing and climate change impacts (biomass-based and trophic-level based metrics). We assessed differences in these indicators for the surveyed community as a whole and for fish, crustaceans and cephalopods, separately, over five regions. Our results show clear latitudinal gradients in some indicators: we observe a reversed pattern between richness (decreasing from south to north) and biomass trends (increasing from south to north) for the demersal community. We also found a generalized increase in β-diversity in most regions with time, and a decline in the trophic level of the surveyed community. In addition, we identify a remarkable increase in several indicators when only considering the cephalopods group, and a general low environmental status for the North Catalan Sea. We discuss our results considering the differences between regions and taxa related to the fishing activity and environmental dynamics that can act at different scales. This in-depth analysis illustrates how to use a selection of indicators that combine the capacity to detect ecological changes from regional to sub-regional scales.

The present work examines the spatio-temporal biomass trends of Mullus barbatus and Mullus surmuletus in the Mediterranean Sea through the analysis of a time series of data coming from the Mediterranean International Trawl Surveys (MEDITS), accomplished annually from 1994 to 2015. The biomass of both species showed clear declining trends below 150 to 200 m depth, which were steeper in the case of M. barbatus. Increases in temporal biomass trends were observed for M. barbatus from 2008 onward in most geographic sub-areas (GSAs), while stability was mostly observed for M. surmuletus. For both species, dynamic factor analysis revealed similarities among neighbouring GSAs and the subsequent cluster analysis identified two major GSA groups corresponding to the eastern and western basins of the Mediterranean. Overall, the results suggested that the combined effects of fishing and environmental conditions determine species abundance variations, but the relative importance of each component may vary among areas.

Trawling pressure and environmental changes may affect the composition of fish assemblages. Our knowledge on large spatio-temporal patterns of demersal fish composition remains incomplete for the Mediterranean Sea. We investigated (1) the spatio-temporal stability of demersal assemblages, (2) the relationships between these assemblages and potential structuring factors (trawling pressure and environmental conditions) in order to assess the dynamic of the assemblage structure at the scale of the northern Mediterranean Sea. We analysed a dataset of 18062 hauls from 10 to 800 m depth performed annually during the last two decades across 17 Geographical Sub-Areas (GSAs) (MEDITS program). A multi-table analysis (STATICO-CoA) evidenced a strong inter-GSAs stability in the organization of assemblages, with specificities for some GSAs. The most stable structuring factors were linked to combined gradients of chlorophyll a, phytoplancton carbon biomass and temperature, inversely correlated with depth, salinity and nutrient gradients (axis 1 of the STATICO-CoA compromise, 93.74% of the total variability). A common pattern linking the distribution of species to these environmental gradients was evidenced for most of the 17 GSAs. Estimate of trawling pressure showed a minor role in the organization of the assemblages for the spatial scale and years investigated (axis 2, 4.67%).

The large-scale distribution pattern of megafauna communities along the Mediterranean middle slope was explored. The study was conducted between 500 and 800 m depth where deep-water fishery occurs. Although community studies carried out deeper than 500 m are partly available for some geographic areas, few large-scale comparative studies have been carried out. Within the framework of the MEDITS survey programme, we compared the megafauna community structure in ten geographical sub-areas (GSAs) along the Mediterranean coasts. Additionally, the spatial distribution of fishing was analysed using vessel monitoring by satellite information. Overall, the community showed a significant difference between sub-areas, with a decreasing eastward pattern in abundance and biomass. Longitude was the main factor explaining variation among sub-areas (by generalized additive models). However, we found a region which did not follow the general pattern. GSA 6 (northern Spain) showed significantly lower abundance and a different composition structure to the adjacent areas. The decrease in community descriptors (i.e. abundance and biomass) in this area is probably a symptom of population changes induced by intense fishery exploitation. Overall, a combination of environmental variables and human-induced impacts appears to influence the bentho-pelagic communities along the slope areas of the Mediterranean.

The present study provides updated information on the occurrence, abundance and biomass distribution patterns and length frequencies of Merluccius merluccius in the Mediterranean Sea, by analysing a time series of data from the Mediterranean International Trawl Surveys (MEDITS) from 1994 to 2015. The highest values of abundance and biomass were observed in the Sardinian Seas. The use of a generalized additive model, in which standardized biomass indices (kg km–2) were analysed as a function of environmental variables, explained how ecological factors could affect the spatio-temporal distribution of European hake biomass in the basin. High biomass levels predicted by the model were observed especially at 200 m depth and between 14°C and 18°C, highlighting the preference of the species for colder waters. A strong reduction of biomass was observed since the year 2009, probably due to the strengthening of the seasonal thermocline that had greatly reduced the availability of food. The general decrease in biomass of several stocks of anchovy and sardine, preys of European hake, might be indirectly connected to the decreasing biomass detected in the present study. The length analysis shows median values lower than 200 mm total length of most of the investigated areas.

Marine litter is one of the main sources of anthropogenic pollution in the marine ecosystem, with plastic representing a global threat. This paper aims to assess the spatial distribution of plastic macro-litter on the seafloor, identifying accumulation hotspots at a northern Mediterranean scale. Density indices (items km–2) from the MEDITS trawl surveys (years 2013-2015) were modelled by generalized additive models using a Delta-type approach and several covariates: latitude, longitude, depth, seafloor slope, surface oceanographic currents and distances from main ports. To set thresholds for the identification of accumulation areas, the percentiles (85th, 90th and 95th) of the plastic spatial density distribution were computed on the raster data. In the northern Mediterranean marine macro-litter was widespread (90.13% of the 1279 surveyed stations), with plastic by far the most recurrent category. The prediction map of the plastic density highlighted accumulation areas (85th, 90th and 95th percentiles of the distribution, respectively, corresponding to 147, 196 and 316 items km–2) in the Gulf of Lions, eastern Corsica, the eastern Adriatic Sea, the Argo-Saronic region and waters around southern Cyprus. Maximum densities were predicted in correspondence to the shallower depths and in proximity to populated areas (distance from the ports). Surface currents and local water circulation with cyclonic and anticyclonic eddies were identified as drivers likely facilitating the sinking to the bottoms of floating debris.

A new approach to recruitment overfishing diagnosis is presented. We hypothesize that condition of recruits should increase when recruitment failures are caused by fishing activity. This would be a consequence of the increase in trophic resource availability, because the population is smaller than that which the ecosystem could support. Temporal series of hake recruit condition were calculated from MEDITS survey data collected in Mediterranean geographical sub-areas (GSAs) 1, 6, 17 and 19 from 1994 to 2015. Multiple linear regressions were used to analyse the relationship between mean annual condition and abundance of recruits and climatic indices in each GSA. Significant correlations were only detected in GSA 6, where 69% of condition variability was explained by the negative correlation with recruit abundance, and with two climatic indices, the Western Mediterranean Oscillation and the standardized air temperature anomaly at surface from the Gulf of Lions. Despite the differences in recruit abundance among GSAs, their mean annual condition oscillated around the same basal value during most of the time series, pointing to density-dependent mortality rates as an important mechanism stabilizing hake recruitment to levels close to the carrying capacity when populations do not suffer recruitment overfishing. This pattern changed when the decreasing recruit abundance trend drove GSA 6 condition values persistently above those of the rest of the GSAs. According to our hypothesis, hake in GSA 6 is in recruitment overfishing.

The high species richness, coupled with high proportion of endemism, makes the Mediterranean one of the world’s ‘biodiversity hotspots’. However, the continuous increase in fisheries in the last few decades has led to the overexploitation of their main commercial stocks. Using fishery-independent data collected under the framework of the MEDITS trawl surveys carried out over the last 20 years, we study the demersal fish diversity pattern in the Mediterranean at a large spatial and temporal scale to determine whether it is being affected by the general fishing overexploitation of the demersal resources. The detected diversity trends are compared with the spatio-temporal variation in bottom trawl fishing effort in the Mediterranean. Our results show a stability and even recovery of demersal fish diversity in the Mediterranean together with higher diversity values on the continental shelves of the Balearic Islands, Sardinia, Sicily and the Aegean Sea. At large temporal and spatial scales, the high diversity of demersal assemblages in the Mediterranean is associated with a reduction in bottom trawl fishing effort. The inclusion of species other than target ones through diversity indices is important in the implementation of an ecosystem-based fisheries management.

The Mediterranean distributions of two species of anglerfish, the blackbellied anglerfish (Lophius budegassa) and the white anglerfish (Lophius piscatorius), were analysed from trawl survey data (MEDITS project – Spain, France, Italy and Greece) from 2006 to 2015 implementing a Delta model approach with residuals autocovariate boosted regression trees. Sea bottom temperature (SBT), sea bottom salinity (SBS), bathymetry, slope of the seabed and distance to the coast were considered possible predictors. The results show that the locations with a higher presence, abundance and biomass of L. budegassa are those with a depth range between 150 to 300 m, with an SBT range between 17.5 and 18.5°C, and SBS of 37-38 PSU. Similarly, L. piscatorius shows a higher probability of presence, abundance and biomass in location with a bathymetry range of 200-400 m, an SBT of 17.5°C to 18.5°C and an SBS of 36.5 to 37.5. Our results identify preference habitats for the anglerfishes in the Mediterranean Sea such as the Aegean Sea, the Gulf of Lions, south and southeast Spain and the northwestern Ionian Sea. In general terms, these findings enhance our understanding of the differences in the spatio-temporal distribution of these two species, providing useful information that can help their fisheries management and conservation.

Thanks to the availability of the MEDITS survey data, a standardized picture of the occurrence and abundance of demersal Chondrichthyes in the northern Mediterranean has been obtained. During the spring-summer period between 2012 and 2015, 41 Chondrichthyes, including 18 sharks (5 orders and 11 families), 22 batoids (3 orders and 4 families) and 1 chimaera, were detected from several geographical sub-areas (GSAs) established by the General Fisheries Commission for the Mediterranean. Batoids had a preferential distribution on the continental shelf (10-200 m depth), while shark species were more frequent on the slope (200-800 m depth). Only three species, the Carcharhiniformes Galeus melastomus and Scyliorhinus canicula and the Torpediniformes Torpedo marmorata were caught in all GSAs studied. On the continental shelf, the Rajidae family was the most abundant, being represented in primis by Raja clavata and then by R. miraletus, R. polystigma and R. asterias. The slope was characterized by the prevalence of G. melastomus in all GSAs, followed by S. canicula, E. spinax and Squalus blainville. Areas under higher fishing pressure, such as the Adriatic Sea and the Spanish coast (with the exception of the Balearic Islands), show a low abundance of chondrichthyans, but other areas with a high level of fishing pressure, such as southwestern Sicily, show a high abundance, suggesting that other environmental drivers work together with fishing pressure to shape their distribution. Results of generalized additive models highlighted that depth is one of the most important environmental drivers influencing the distribution of both batoid and shark species, although temperature also showed a significant influence on their distribution. The approach explored in this work shows the possibility of producing maps modelling the distribution of demersal chondrichthyans in the Mediterranean that are useful for the management and conservation of these species at a regional scale. However, because of the vulnerability of these species to fishing exploitation, fishing pressure should be further incorporated in these models in addition to these environmental drivers.

In this study we investigated the spatio-temporal distribution of gurnards (8 species of Triglidae and one species of Peristediidae) in the northern Mediterranean Sea using 22 years of MEDITS bottom trawl survey data (1994-2015). Gurnards showed significant differences in terms of abundance, dominance and composition among geographical sub-areas and ecoregions, with the highest relative biomass (BIy) being found in Malta, eastern Corsica, the Balearic Islands and the eastern Ionian Sea. The lowest gurnards BIy were observed in the highly exploited areas of the western Mediterranean and the Adriatic Sea, where the largest number of species with a negative linear trend in BIy was also found. The temporal trends in species abundances highlighted a general decrease for the coastal species (C. lucerna, C. lastoviza, C. obscurus) as compared with the species inhabiting the deep continental shelf and slope (T. lyra, P. cataphractum). The results provide for the first time an overview of the spatiotemporal trend in the abundance of gurnards over the wide spatial scale of the northern Mediterranean Sea, also suggesting the possible use of these species as indicators for monitoring the impact of fishing pressure on demersal fish assemblages.

The uncertainty in age estimation by otolith reading may be at the root of the large variability in red mullet (Mullus barbatus) growth models in the Mediterranean. In the MEDITS survey, red mullet age data are produced following the same sampling protocol and otolith reading methodology. However, ageing is assigned using different interpretation schemes, including variations in theoretical birthdate and number of false rings considered, in addition to differences in the experience level of readers. The present work analysed the influence of these variations and the geographical location of sampling on red mullet ageing using a multivariate approach (principal component analysis). Reader experience was the most important parameter correlated with the variability. The number of rings considered false showed a significant effect on the variability in the first age groups but had less influence on the older ones. The effect of the theoretical birthdate was low in all age groups. Geographical location had a significant influence, with longitude showing greater effects than latitude. In light of these results, workshops, exchanges and the adoption of a common ageing protocol based on age validation studies are considered fundamental tools for improving precision in red mullet ageing.

The MEDITS programme started in 1994 in the Mediterranean with the cooperation among research institutes from four countries: France, Greece, Italy and Spain. Over the years, until the advent of the European framework for the collection and management of fisheries data (the Data Collection Framework, DCF), new partners from Slovenia, Croatia, Albania, Montenegro, Malta and Cyprus joined MEDITS. The FAO regional projects facilitated the cooperation with non-European countries. MEDITS applies a common sampling protocol and methodology for sample collection, data storage and data quality checks (RoME routines). For many years, MEDITS represented the most important data source supporting the evaluation of demersal resources by means of population and community indicators, assessment and simulation models based on fishery-independent data. With the consolidation of the DCF, MEDITS routinely provides abundance indices of target species for tuning stock assessment models of intermediate complexity. Over the years, the survey scope has broadened from the population of demersal species to their fish community and ecosystems, and it has faced new challenges, such as the identification of essential fish habitats, providing new scientific insights linked to the Marine Strategy Framework Directive (e.g. biodiversity, trophic webs, allochthonous species and marine macro-litter evaluations) and to the ecosystem approach to fishery and marine spatial planning.

This study analyzes the temporal variability/stability of the spatial distributions of key exploited species in the Gulf of Lions (Northwestern Mediterranean Sea). To do so, we analyzed data from the MEDITS bottom-trawl scientific surveys from 1994 to 2010 at 66 fixed stations and selected 12 key exploited species. We proposed a geostatistical approach to handle zero-inflated and non-stationary distributions and to test for the temporal stability of the spatial structures. Empirical Orthogonal Functions and other descriptors were then applied to investigate the temporal persistence and the characteristics of the spatial patterns. The spatial structure of the distribution (i.e. the pattern of spatial autocorrelation) of the 12 key species studied remained highly stable over the time period sampled. The spatial distributions of all species obtained through kriging also appeared to be stable over time, while each species displayed a specific spatial distribution. Furthermore, adults were generally more densely concentrated than juveniles and occupied areas included in the distribution of juveniles. Despite the strong persistence of spatial distributions, we also observed that the area occupied by each species was correlated to its abundance: the more abundant the species, the larger the occupation area. Such a result tends to support MacCall's basin theory, according to which density-dependence responses would drive the expansion of those 12 key species in the Gulf of Lions. Further analyses showed that these species never saturated their habitats, suggesting that they are below their carrying capacity; an assumption in agreement with the overexploitation of several of these species. Finally, the stability of their spatial distributions over time and their potential ability to diffuse outside their main habitats give support to Marine Protected Areas as a potential pertinent management tool.