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Lagoons and estuaries are transition spaces between earth and sea. Beyond their expanse, geographic repartition, geomorphological, hydroclimatic and ecobiological diversity and biodiversity, they play an important role in regional economies and are some of the ecosystems most vulnerable to global change. Reinforced by numerous references, this book studies Mediterranean lagoonal and estuarine fishes whose diversity largely depends on the composition of neighboring marine and continental ichtyofauna. The authors describe their morphological, biological, ecological and behavioral characteristics by evoking their distinctive features and differences with their marine or freshwater homologues. Their adaptation strategies, elucidated thanks to recent advances in morphology, genetics and molecular biology, are recognized as a major advantage in the context of climate change. This book is for natural environment managers, engineers, teachers, students and researchers.

Marine biodiversity monitoring in the Mediterranean’s increasingly threatened ecosystems is crucial for effective ecosystem conservation and management. Here, we leveraged the Mediterranean International Trawl Survey program (MEDITS) to implement eDNA sampling through the recently tested ‘metaprobe’ procedure and characterize fish assemblages in three separate areas off the Italian coasts: Northern Adriatic Sea (NoAS), Ligurian and Northern Tyrrhenian Sea (LNTS), and Sardinian Sea (SaS). By combining the information from two homologous mitochondrial 12S metabarcodes––i.e., Elas02 and Tele02 targeting elasmobranchs and teleosts, respectively––we identified 108 species, over 60% of which overlapped with those caught by the trawl net. We produced an accurate reconstruction of fish community composition of the examined sites, reflecting differences in species assemblages linked with both geographic area and depth range. Metaprobe eDNA data consistently returned a biodiversity ‘bonus’ mostly consisting of pelagic taxa not captured through bottom trawl surveys, including rare and endangered taxa (e.g., elasmobranchs). Overall, the spatial characterisation of the assemblages across the surveyed areas was better delineated and more robust using eDNA metabarcoding than trawl data. Our results support the operationalisation of the metaprobe as a simple, inexpensive, versatile sampling tool, in association with pre-existing ship surveys, to overcome many of the limitations of marine data collection and strengthen marine management.

Marine Protected Areas (MPAs) are an essential tool for marine biodiversity conservation. Yet, their effectiveness in protecting marine ecosystems from global stressors is debated. Biological invasions are a major driver of global change, causing biodiversity loss and altering ecosystem functioning. Here, we explored the relationships between MPAs and alien/native range-expanding fishes in the Mediterranean Sea, the world’s most invaded sea. We surveyed fish and benthic communities in nine MPAs and adjacent unprotected areas across six countries. In the South and Eastern Mediterranean MPAs, the biomass of alien and native range-expanding fishes often exceeded 50% of the total fish biomass. Conversely, in the North and Western Mediterranean, alien fishes were absent. A negative relationship was found between native and alien species richness. Average and minimum sea surface temperature (SST) over six consecutive years were positively correlated with the total biomass of alien species; no alien fishes were recorded below 20.5°C average SST and 13.8°C minimum SST. We also found a negative relationship between alien fishes’ biomass and the distance from the Suez Canal, which is the main pathway for the introduction of alien fish in the Mediterranean Sea. The biomass of alien and native range-expanding fishes was found to be higher in the South and Eastern Mediterranean MPAs than in adjacent unprotected areas. The association of barrens (rocky reefs deprived of vegetation) and invasive herbivores was observed at all eastern sites, regardless of protection status. Currently, the level of fishing pressure exerted on alien and native range-expanding fishes seems to be the most influential factor determining the lower biomass of invasive fishes at unprotected sites compared to MPAs. Our findings suggest that complementary management actions, such as species-targeted removals, should be taken in MPAs to effectively control invasive fish populations.

Aim: A large body of knowledge exists on individual anthropogenic threats that have an impact on marine biodiversity in the Mediterranean Sea, although we know little about how these threats accumulate and interact to affect marine species and ecosystems. In this context, we aimed to identify the main areas where the interaction between marine biodiversity and threats is more pronounced and to assess their spatial overlap with current marine protected areas in the Mediterranean. Location: Mediterranean Sea. Methods: We first identified areas of high biodiversity of marine mammals, marine turtles, seabirds, fishes and commercial or well-documented invertebrates. We mapped potential areas of high threat where multiple threats are occurring simultaneously. Finally we quantified the areas of conservation concern for biodiversity by looking at the spatial overlap between high biodiversity and high cumulative threats, and we assessed the overlap with protected areas. Results: Our results show that areas with high marine biodiversity in the Mediterranean Sea are mainly located along the central and north shores, with lower values in the south-eastern regions. Areas of potential high cumulative threats are widespread in both the western and eastern basins, with fewer areas located in the south-eastern region. The interaction between areas of high biodiversity and threats for invertebrates, fishes and large animals in general (including large fishes, marine mammals, marine turtles and seabirds) is concentrated in the coastal areas of Spain, Gulf of Lions, north-eastern Ligurian Sea, Adriatic Sea, Aegean Sea, south-eastern Turkey and regions surrounding the Nile Delta and north-west African coasts. Areas of concern are larger for marine mammal and seabird species. Main conclusions: These areas may represent good candidates for further research, management and protection activities, since there is only a maximum 2% overlap between existing marine protected areas (which cover 5% of the Mediterranean Sea) and our predicted areas of conservation concern for biodiversity.

A striking example of climate‐mediated range shifts in marine systems is the intrusion of tropical species into temperate areas world‐wide, but we know very little about the ecological consequences of these range expansions. In the Mediterranean Sea, the range expansion of tropical rabbitfishes that first entered the basin via the Suez Canal provides a good example of how tropical herbivorous fish can impact the structure of rocky bottoms in temperate seas. Two species of rabbitfishes have now become a dominant component of total fish biomass in the southernmost part of the eastern Mediterranean. Experimental evidence shows these species can profoundly transform benthic communities, turning algal forests into ‘barrens’, but the specific mechanisms that facilitate this shift have not been established. We surveyed ˜1000 km of coastline in the eastern Mediterranean and identified two clearly distinct areas, a warmer group of regions with abundant tropical rabbitfish and a colder group of regions where these consumers were absent/ extremely rare. In regions with abundant rabbitfish, canopy algae were 65% less abundant, and there was a 60% reduction of overall benthic biomass (algae and invertebrates) and a 40% decrease in total species richness. Video‐recorded feeding experiments showed that the extensive barrens characteristic of regions with abundant rabbitfish were not due to greater rates of herbivory by these tropical consumers, but rather by functional differences among the herbivores. Temperate herbivorous fish displayed the greatest macroalgae consumption rates overall, but they fed exclusively on established adult macroalgae. In contrast, in regions with abundant rabbitfishes, these consumers fed complementarily on both established macroalgae and on the epilithic algal matrix, which typically contains macroalgal recruits. Synthesis. Range‐shifting tropical rabbitfish can severely reduce the biomass and biodiversity of temperate reefs at a scale of hundreds of kilometres. A shift from macroalgal dominance to barrens is mediated by the addition of functionally diverse herbivores that characterize tropical reefs. This work highlights the importance of assessing the functional traits of range‐shifting species to determine potential mechanisms of impact on ecological communities.

Important changes have been observed in recent decades in small pelagic fish (SPF) populations of the NW Mediterranean Sea: declines in biomass and landings of European anchovy and sardine, and a geographical expansion of round sardinella. These changes have been linked to environmental factors directly influencing annual recruitment and growth. The role of climate change in affecting the composition of plankton has also been suggested to explain declines in SPF, while other causes could be the recovery of predators, competition with other pelagic organisms that prey on early life phases of SPF (i.e. gelatinous zooplankton), interspecific competition for food, or impacts from fisheries harvest. To test the role of these potential pressures, we developed qualitative mathematical models of a NW Mediterranean pelagic food web. We used analyses of sign directed graphs and Bayesian belief networks to compare alternative hypotheses about how SPF species may have responded to combinations of different pressures. Data documenting changes in SPF populations were used to test predicted directions of change from signed digraph models. An increase in sea surface temperature (SST) that had either a positive impact on round sardinella or on gelatinous zooplankton abundance was the pressure that alone provided the most plausible insights into observed changes. A combination of various pressures, including an increase in SST, an increase of exploitation and changes to zooplankton also delivered results matching current observations. Predators of SPF were identified as the most informative monitoring variable to discern between likely causes of perturbations to populations of SPF.

The use of underwater video techniques has expanded rapidly in ecological studies and is particularly desirable in protected areas since the method does not impact the habitat or remove fish. The Mediterranean Sea is a biodiversity hotspot under high anthropogenic pressure and consequently, non-destructive and non-extractive techniques for fish monitoring are advantageous. Here, we review 110 publications that used underwater video in fish-related studies in the Mediterranean basin. The most common technique used in the Mediterranean Sea was Remotely Operated Vehicles (ROV) (33%), followed by Remote Underwater Video (RUV) systems (20%), Diver Operated Video (DOV) systems (20%) and Baited Remote Underwater Video (BRUV) systems (19%). Approximately one third of the studies used an additional sampling method, such as fisheries-based or molecular methods with the aim to compare the surveying effectiveness or produce complementary data. The most frequent objectives of the reviewed studies were related to fish community structure, i.e., focusing on community wide metrics such as abundance and biodiversity, or behavioral analyses, while the most commonly studied environments were those of the western Mediterranean and shallow waters, usually involving sandy or rocky reef habitats. Sampling protocols differed widely among studies with transect lengths, soak times and baits all varying. Future research should focus on the least studied parts of the region, such as the eastern and southern Mediterranean Sea and deep-sea habitats. Finally, the development of standardized sampling protocols is recommended to ensure that data are comparable among studies.

Besides providing key ecosystem services, including improved water quality, coastal protection, and carbon sequestration, seagrasses are highly productive habitat-forming species essential in supporting fish diversity at the global level. In the Mediterranean Sea, seagrasses represent the main component of the sublittoral marine environment hosting a huge number of fish species that use this habitat for reproduction, foraging and/or refuge from predation. However, a complete synthesis of fish species observed in different seagrass habitats is still lacking at the whole Mediterranean basin scale, so hindering a thorough understanding of the main mechanisms involved in determining fish diversity patterns. We performed a systematic review by implementing a semi-automated, threshold-based filtering pipeline that allowed building up a dataset concerning all fish species reported in native Mediterranean seagrasses, including specific functional traits known to be involved with the potential use of seagrasses by fish. These data allowed to carry on a narrative synthesis on fish diversity in seagrass habitats, providing support to several assumptions repeatedly stated in literature but so far sustained mainly by local and fragmented data. Our findings suggested the onset of a general pattern in the occurrence of species, mostly based on life history and driven by body size and feeding habits. We evidenced unexpected knowledge gaps on the role of habitat heterogeneity and fish life stages in determining the presence and the potential use of seagrasses by species. In depth studies are therefore needed to better understand the mechanisms behind the structuring of fish communities, fundamental for the maintenance of marine biodiversity.

One of the most degraded states of the Mediterranean rocky infralittoral ecosystem is a barren composed solely of bare rock and patches of crustose coralline algae. Barrens are typically created by the grazing action of large sea urchin populations. In 2008 we observed extensive areas almost devoid of erect algae, where sea urchins were rare, on the Mediterranean coast of Turkey. To determine the origin of those urchin-less 'barrens', we conducted a fish exclusion experiment. We found that, in the absence of fish grazing, a well-developed algal assemblage grew within three months. Underwater fish censuses and observations suggest that two alien herbivorous fish from the Red Sea (Siganus luridus and S. rivulatus) are responsible for the creation and maintenance of these benthic communities with extremely low biomass. The shift from well-developed native algal assemblages to 'barrens' implies a dramatic decline in biogenic habitat complexity, biodiversity and biomass. A targeted Siganus fishery could help restore the macroalgal beds of the rocky infralittoral on the Turkish coast.