The first degree of importance of the Danube Delta’s exceptional value is based on the unique and complex matrix of habitats, with a mix of freshwater, eurihaline and salt water bodies, floodplains, and sand dunes [120,121,122].

About 80% of the Delta’s territory consists of areas that are permanently covered by water, 3% of the area is naturally dry, and the remaining 17% is temporarily flooded [123].

The main foundations of the area’s habitat diversity are the specific hydrological regime characteristics, which encompass fluctuations of water levels, water circulation in the inner Delta, and its residence time in the various water bodies. The duration, height, time period in the course of a year, and frequency of floods all play major roles in the area’s complex of habitats, and influence their distribution along ecological gradients and their biota. In some areas, the natural ecological gradients are overlain with human-induced changes that modify habitat conditions and alter biotic communities [115,116,120].

The complex framework and interrelated dynamic of salt, eurihaline, freshwater, nutrients, suspended solids, soils/sediments, pollutants, etc., generate and maintain the delta’s specific habitats and their accompanying biotal features, many of them being included in Annex I of the EU Habitats Directive or the annexes of the Bird Directive [122,124,125]. We see typical floodplain habitats along the main and secondary larger branches, smaller streams, characteristic lake habitats with surrounding reed vegetation, aquatic macrophyte communities in the open area, silting-up processes with growing reed beds and developing reeds, as well as sedge-peat deposits and lakes connected among them with first, second, or third category branches, small and narrow channels, and flood depressions, sometimes isolated within the reeds [122]. The water courses are bordered by natural river bank levees, with gallery-like floodplain forests on and above mean water level. The marine zone of the delta, and the transition area between the marine and fluvial, are characterized by beach barriers and fan-like sand dune complexes, with a multifaceted mosaic of different, specific habitats, adjacent to the sea and along the coast, where saline areas are present. In fact, this mosaic of habitats always changes, not only in the aspect but also in the way of being linked or more or less isolated, and the mode of functioning, the ecotone characteristics being more present here in space and time that in many other numerous complexes of ecosystems. The number and variety of plant species reveal the very high diversity of habitat and microhabitat conditions, which also characterizes almost all the main invertebrates and vertebrate taxa [120,126,127], with fish being one of the most diverse and abundant groups of vertebrates present [46,47,49].

The likely effects of sea level rise on the delta can be determined rationally [121]; the denial of reality is never a wise option, and this event should be prevented and mitigated based on integrated research approaches to highlight potential management measures.

As a significant factor, the historical factual reality should be stressed, namely, that humans have taken mostly reasonable advantage of the natural resources and services that thrive within this area, with its high diversity of habitats and biocoenoses [104,120].

In the 20th century planetary context in which dramatic various human impacts became significant, it is predictable that, in addition, both marine and fluvial processes influenced by climate modifications will induce further changes in the Danube Delta, their significance being correlated with the magnitude and speed of Black Sea level rise in the future, jeopardizing the opportunities to protect and sustain this natural region.

The highly complex Danube River–Danube Delta–Black Sea geoecosystem has nurtured a dynamic, rich, and unique ichthyofauna. The extinctions of fish species in ancient times were related to evolutionary processes, while in recent times (since ca. 155 years ago), fish species extinctions have been related more to direct anthropogenic impacts [128].

The complex and balanced Danube Delta fish species fauna structure, with 139 species, representing over 70% of Romania’s (238.397 km²) ichthyofauna, includes three classes, 20 Orders, and 45 families; 44.8% are freshwater species, 14.8% are migratory freshwater/marine and/or brackish water species, and 40.4% are marine species [129]. These reveal the role of the structural and functional connectivity of the Danube Delta for the Danube River on the one hand and for the North West Black Sea on the other. The key role of the Danube Delta is evidenced by the fact that 57.26% of the Danube River–Danube Delta–Black Sea fish species require two or even three characteristic areas of those habitats, which provide conditions for shelter, reproduction, feeding, wintering, migration, colonization, recolonization, etc. The role of this delta as a complex and dynamic matrix of habitats offers the chance for Danube River and Black Sea fish to recover from an ecological point of view when one or both of them are negatively affected by external factors. All three of these ichthyosubsystems are part of the Lower Danube River–Danube Delta–North Western Black Sea ichthyosystem, which evolved in time and space interdependently, and which facilitated their flexibility and adaptation in an integrated way [2].

In the last century, compared to the data of Antipa and Bănărescu [46,47,104,130,131,132,133,134,135,136,137,138], the habitat heterogeneity, fish native diversity, and stock abundance of fish species of economic and conservation value of the Danube River–Danube Delta–Black Sea have notably decreased [49,55,139,140,141,142]. Currently, there is no significant indication that this trend will slow or cease in the near future, because the fish fauna and their habitats have over the long term been significantly altered, qualitatively and quantitatively, by a variety of human interventions, and new threats are imminent.

The first fish species category at risk under the scenario of relatively fast rising sea levels in the Black Sea, based on ecological characteristics and needs, include some species that live only in the Danube Delta, others for which optimum specific habitats are located in the Danube Delta, and those not present or with significantly less presence upstream and/or downstream of the Danube River in the Black Sea. These species are Carassius carassius (Linnaeus, 1758), Petroleuciscus borysthenicus (Kessler, 1859), Rutilus frisii (Nordmann, 1840), Cobitis tanaitica (Băcescu and Mayer, 1969), Cobitis megaspila (Nalbant, 1993), Pungitius platygaster (Kessler, 1895), Bentophiloides brauneri (Beling and Iljin, 1927), and Knipowitschia cameliae (Nalbant and Oţel, 1995). Among them, only P. borysthenicus has increased in abundance in the last century; Carassius c. and Pungitius p. have an accentuated decreasing trend in abundance; and the situation of R. frisii, Cobitis tanaitica, C. megaspila, Bentophiloides b., and Knipowitschia c. is worse, their presence nowadays being considered questionable or locally extinct [47,49,130,131,139,140,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166].

Carassius carassius (Linnaeus, 1758) (Actinopteri, Cypriniformes, Cyprinidae, Cyprininae), Crucian carp, a freshwater, brackish fish species, is a key part of the EU species conservation strategy (e.g., the Habitat Directive, the Water Framework Directive, and the EU Biodiversity Strategy for 2030) [130,131]. Adults are found in still, shallow ponds, lakes rich in vegetation, and the slow-moving parts of large rivers, and are usually restricted to densely vegetated backwaters and oxbows. Reproduction is in May–July in shallow water with dense vegetation. In spite of the fact that this species was considered resilient to diverse threats (anthropogenic and natural) [139,140], there is an accentuated gradual and continuing trend of population decrease in this species [47,143,144,145,146], mainly through numerous threats of anthropogenic origin [147,148,149]. Due to the fact that the causes of crucian carp decline across its range include this species’ ecological needs related to habitat characteristics linked to the consequent disappearance of floodplain lakes and backwaters [150,151,152], the flooding of the Danube Delta by the sea could drastically affect the Crucian carp, which were already in accelerated decline by 1960 in the studied area and elsewhere [153]. Here, it should be noted that a main problem for Carassius carassius is hybridization with Carassius gibelio (Prussian carp). In the potential context described, Carassius gibelio will have additional advantages in contrast to Carassius carassius, such as access to the closed water bodies present in the northern/Ukrainian part of the Danube Delta that are protected from spring floods by dams and where Carassius carassius can still live now [154].

Petroleuciscus borysthenicus (Kessler, 1859), (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae), Dnieper chub, is a freshwater, brackish fish species, which prefers shallow areas with slow to no current, inhabiting lowland rivers, limans (lagoons or estuarine features of a type prominent on Black Sea coasts), lakes, deltas, and backwaters [155,156]. This fish prefers to stay in warm water, with temperatures up to 30–32 °C, on sand, sand-muddy, or muddy bottoms, and in shallow places with a slow current along the banks, in backwaters, in small lakes, and in similar calm water sites. It can tolerate only slightly brackish water and low oxygen concentrations [153,157,158]. Sharp declines in some populations have been registered due to habitat changes [159]; from this perspective, the possible flooding of the Danube Delta would obviously affect this fish species.

Rutilus frisii (Nordmann, 1840) (Actinopteri, Cypriniformes, Leuciscidae, Leucisninae), Black Sea roach, is a freshwater to brackish fish species [47,160] protected under the Bern Convention [49]. It has been registered periodically in the north/Ukrainian Chilia/Kiliya part of the Danube Delta, and only once [161] in other areas of the Danube Delta, and one can regard it as a species coming from the north-east, possibly from the Nistru River delta, and which in some particular conditions also entered into the Razim area. With such a restricted area of distribution and questionable long-term presence, any increases in sea level may induce its extinction due to the destruction of its present habitat in the studied area.

Cobitis tanaitica (Băcescu and Mayer, 1969) (Actinopteri, Cypriniformes, Cobitidae), spined loach, is a freshwater fish distributed in the Danube Delta, and any increase in the level of the Black Sea may induce its extinction due to the destruction of its present habitat.

Cobitis megaspila (Nalbant, 1993) (Actinopteri, Cypriniformes, Cobitidae), Danubian spined loach, is a lenitic or slow-flowing freshwater species of rather muddy substrates [49,153,162]. With a restricted area of distribution in the Danube Delta, any increased level of the sea may induce its extinction due to the destruction of its habitat.

Pungitius platygaster (Kessler, 1895) (Actinopteri, Performes/Gasterosteoidei, Gasterosteide), Ukrainian stickleback, is a fish species found in the study area mainly in freshwater, which appeared in some littoral lakes only after they became brackish/freshwater [47]; in other geographical areas, it also inhabits marine habitats [47,159,163]. Any rise in sea level may affect its populations in the Danube Delta due to the pressure of habitat changes.

Benthophiloides brauneri (Beling and Iljin, 1927) (Actinopteri, Gobiiformes, Gobiidae, Gobiinae), beardless tadpole goby, is a freshwater, brackish fish species, rare in the studied Lower Danube area, and sensitive to changes in habitat quality [47,153,164,165]. Increases in the level of the sea may affect the quality of its habitats and change their characteristics to affect the population of this species in the delta.

Knipowitschia cameliae (Nalbant and Oţel, 1995) (Actinopteri, Gobiiformes, Gobiidae, Gobionellinae), Danube Delta dwarf goby, has been found in the Danube Delta area only in brackish sea–delta littoral ponds [166]. For this endemic, critically endangered species [166], with such a restricted area of distribution [153], any increase in the level of the sea may induce its extinction due to the diminishing of its characteristic habitat quality, and even the destruction of its present suitable habitat.

It is clear for this first category of risk that if the Danube Delta habitats were to be affected by sea level rise, the species would be at high risk due to habitat loss, distribution and populations, even possible leading to local or total extinction.

The second category of risk related to fish species, based on their ecological features and needs in a Black Sea level rise scenario, include the fish living in the Danube Delta and Upstream Danube River, and not or only accidentally present or rare in the near Black Sea. These species are Acipenser güldenstaedti (Brandt and Ratzeburg, 1833), Acipenser stellatus (Linnaeus, 1758), Acipenser ruthenus (Linnaeus, 1758), Huso huso (Linnaeus, 1758), Esox lucius (Linnaeus 1758), Abramis ballerus (Linnaeus, 1758), Abramis sapa (Pallas 1814), Vimba vimba (Linnaeus, 1758), Alburnus alburnus (Linnaeus, 1758), Barbus barbus (Linnaeus, 1758), Leuciscus aspius (Linnaeus, 1758), Cyprinus carpio (Linnaeus, 1758), Chondrostoma nasus (Linnaeus, 1758), Hypophthalmichthys molitrix (Valenciennes, 1844), Hypophthalmichthys nobilis (Richardson, 1845), Ctenopharyngodon idella (Valenciennes, 1844), Pseudorasbora parva (Temminck and Schlegel, 1846), Romanogobio albipinnatus vladykovi (Fang 1943), Romanogobio antipai (Bănărescu 1953), Leucaspius delineatus (Heckel, 1843), Squalius cephalus (Linnaeus, 1758), Leuciscus idus (Linnaeus, 1758), Rutilus rutilus (Linnaeus, 1758), Scardinius erythrophthalmus (Linnaeus, 1758), Pelecus cultratus (Linnaeus, 1758), Rhodeus amarus (Bloch, 1782), Tinca tinca (Linnaeus, 1758), Cobitis elongatoides (Băcescu and Mayer, 1969), Sabanejewia bulgarica (Drensky, 1928), Misgurnus fossilis (Linnaeus, 1758), Silurus glanis Linnaeus, 1758, Ameiurus melas (Rafinesque 1820), Ameiurus nebulosus (Lesueur 1819), Lota lota (Linnaeus, 1758), Sander volgensis (Gmelin, 1788); Gymnocephalus baloni (Holcik and Hensel 1974), Gymnocephalus cernua (Linnaeus, 1758), Gymnocephalus schraetser (Linnaeus, 1758), Percarina demidoffi (Nordmenn 1840), Zingel streber (Siebold, 1863), Zingel zingel (Linnaeus, 1766), Benthophilus stellatus (Sauvage, 1874), Knipowitschia caucasica (Berg, 1916), Babka gymnotrachelus (Kessler, 1857), Ponticola kessleri (Günther, 1861), Lepomis gibbosus (Linnaeus, 1758), and Umbra krameri (Walbaun, 1792). Among them, only Esox lucius, Scardinius erythrophtalmus, Rutilus rutilus, Alburnus alburnus, Hypophthalmichthys molitrix, Hypophthalmichthys nobilis, Ctenopharyngodon idella, Pseudorasbora parva, Pelecus cultratus, and Lepomis gibbosus have seen an increase in distribution and abundance, and some of them are invasive in the upper Danube lately; Acipenser güldenstaedti, Acipenser stellatus, Acipenser ruthenus, Huso huso, Abramis ballerus, Abramis sapa, Vimba vimba, Leuciscus aspius, Cyprinus carpio, Romanogobio albipinnatus vladykovi, Romanogobio antipai, Squalius cephalus, Leuciscus idus, Tinca tinca, Lota lota, Gasterosteus aculeatus, and Zingel streber have shown a falling trend in distribution and abundance; and the situation of Barbus barbus, Chondrostoma nasus, Leucaspius delineatus, Cobitis elongatoides, Sabanejewia bulgarica, Sander volgensis, Gymnocephalus baloni, Gymnocephalus schraetser, Benthophilus stellatus, Babka gymnotrachelus, and Ponticola kessleri is worse, with their questionable presence, determined as locally extinct, or there being no data available [47,49,148,153,159,167,168,169,170,171,172,173,174,175,176].

Acipenser güldenstaedti (Brandt and Ratzeburg, 1833) (Chondrostei, Acipenseriformes, Acipenseridae, Acipenserinae), Danube sturgeon, is a marine, freshwater, brackish species [167] that has shown an accentuated decreasing trend in abundance in recent decades [49]. In the case of the delta being flooded by the sea, the shrinking or disappearance of this salt–freshwater–salt transitional zone during the migration period will be an important issue for the species, given that this species is Critically Endangered, being under the protection of the Habitats Directive, CITES, CMS, IUCN Red List, etc.

Acipenser stellatus (Pallas, 1771) (Chondrostei, Acipenseriformes, Acipenseridae, Acipenserinae), Starry sturgeon, is a marine, freshwater, brackish species [167] with a high decreasing trend in abundance in recent decades [49]. In a case of the delta flooding by the sea, the shrinking or disappearance of this salt–freshwater–salt transitional zone during the migration period will be an important issue for the species, given that this species is Critically Endangered, being under the protection of the Bern Convention, Habitats Directive, CITES, CMS, IUCN Red List, etc.

Acipenser ruthenus (Linnaeus, 1758) (Actinopteri, Acipenseriformes, Acipenseridae, Acipenserinae), sterlet sturgeon, is a vulnerable [168], freshwater, brackish, demersal species [168,169], protected under the Bern Convention, Habitats Directive, CITES and CMS, with an accentuated deterioration and shrinking area of distribution in the lower Danube basin [153]. The decrease in Danube Delta habitats due to a rise in the level of the sea may induce a continuous decrease in its population size and distribution.

Huso huso (Linnaeus, 1758) (Chondrostei, Acipenseriformes, Acipenseridae, Acipenserinae), beluga sturgeon, is a marine, freshwater, brackish species [167] that has shown an accentuated decreasing trend in abundance in recent decades [49]. In the case of the delta being flooded by the sea, the shrinking or disappearance of this salt–freshwater–salt transitional zone during the migration period will be an important issue for the species given that this species is Vulnerable, being under the protection of the Bern Convention, Habitats Directive, CITES, CMS, IUCN Red List, etc.

Esox lucius (Linnaeus, 1758) (Esociformes, Esocidae), pike, is a freshwater to brackish species [169], with a decreasing trend of biomass seen in recent decades [49]. In a case of the Danube Delta flooding by the Black Sea, only the local delta population would suffer, which would not be a problem at the regional distribution level for this species (a common fish in the Danube Basin’s large and medium rivers and lakes, and even some other smaller water bodies) [47].

Abramis ballerus (Linnaeus, 1758) (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae), blue bream, is a freshwater, brackish species [169], protected under the Bern Convention, accidentally found in the Danube Delta [49] where specific habitat disappearance due to sea level rise would have a relatively low impact on this species’ area of distribution.

Abramis sapa (Pallas, 1814) (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae), white-eye bream, is a freshwater, brackish species [170] protected under the Bern Convention. It has shown a decreasing trend in its area of distribution [49], and the disappearance of its Danube Delta habitats could have a significant impact on its range.

Vimba vimba (Linnaeus, 1758) (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae), vimba bream, is a freshwater, brackish species [168] protected under the Bern Convention. With a decreasing trend in the Danube Delta area [49], the diminishing or disappearance of the delta᾿s habitats could further negatively influence its range in the Danubian area.

Alburnus alburnus (Linnaeus, 1758) (Cypriniformes, Leuciscidae, Leuciscinae), bleak, is a freshwater, brackish species [167]. Due to the fact that it is an abundant species in the Danube Basin, and in general over its distribution range [47,49], in the event of the Danube Delta being flooded by the Black Sea, only the local delta population would be affected, and flooding would not a problem for this species as a whole.

Barbus barbus (Linnaeus, 1758) (Cypriniformes, Cyprinidae, Barbinae), barbel, is a freshwater species [167]. Due to the fact that it is an abundant species in large rivers of the Danube Basin and generally over its distribution range [47,49], with a sporadic presence in the Danube Delta, the flooding of this area by the Black Sea would affect only the local delta population, which is not a problem for this species in general.

Leuciscus aspius (Linnaeus, 1758) (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae, asp), is a freshwater, brackish species protected by the Bern Convention, Habitats Directive, IUCN Red List. [167]. It is one of the common raptor fish species in the Danube River and its deltas [49]; sea level rise would affect only the local populations.

Cyprinus carpio (Linnaeus, 1758) (Actinopteri, Cypriniformes, Cyprinidae, Cyprininae) [171], common carp, is a freshwater, brackish species [167] that is relatively abundant but has lately shown a trend of relative general reduction in the study area; sea level rise would affect only the local populations of the delta.

Chondrostoma nasus (Linnaeus, 1758) (Cypriniformes, Leuciscidae, Leuciscinae), common nase, is a freshwater species [167]. Due to it being quite widespread in the large rivers of the Danube Basin, and in general within its distribution range [47,49], with a rare presence in the Danube Delta, this area’s flooding by the Black Sea would eliminate only the local delta population, which is not a problem for this species as a whole.

Hypophthalmichthys molitrix (Valenciennes, 1844) (Cypriniformes, Xenocyprididae), silver carp, is a freshwater, brackish species [167]. Due to its presence in the majority of large rivers of the Danube Basin [49], with a rare presence in the Danube Delta, this area’s flooding by the Black Sea would significantly affect only the local delta population, which is not a problem for this species in general.

Hypophthalmichthys nobilis (Richardson, 1845) (Cypriniformes, Xenocyprididae), bighead carp, is a freshwater, brackish species [167]. It is fairly rare in the Danube Basin, including in the delta; if the delta populations were to be affected, given that its natural reproduction was not proven here until recently [49], a regional impact on the Danube River can be considered, along with the elimination of the delta populations.

Ctenopharyngodon idella (Valenciennes, 1844) (Cypriniformes, Xenocyprididae), grass carp, is a freshwater, brackish species [167]. It is relatively rare in the Danube Basin, including in the Danube Delta, and if the delta’s populations were to be affected, in the context in which its natural reproduction was not proven here until recently [49], a regional impact on the Danube River can be considered, as well as the elimination of the delta populations.

Pseudorasbora parva (Temminck and Schlegel, 1846) (Cypriniformes, Gobionidae), topmouth gudgeon, is a freshwater, brackish, benthopelagic species [172] that is rather abundant in the Danube Basin [49], including the Danube Delta. Its populations would be considerably affected by sea flooding, but would not impact the species as a whole.

Romanogobio albipinnatus vladykovi (Fang, 1943) (Actinopteri, Cypriniformes, Gobionidae), white-finned gudgeon, is a freshwater species [159,173] protected under the Bern Convention and Habitats Directive. In comparison with other areas of its geographical range, this species is frequent and abundant in the Danube Delta [49]; the diminution or disappearance of delta habitats could negatively influence this species.

Romanogobio antipai (Bănărescu 1953) (Actinopteri, Cypriniformes, Gobionidae) [174], Danube Delta gudgeon, is a freshwater species [159] protected under the Bern Convention, Habitats Directive, IUCN Red List, etc. Its presence in the Danube Delta is very scarce, with an accentuated decreasing trend in recent years [49], and any habitat modification induced by sea level rise could eliminate this species from the Danube Delta area, to which it is endemic.

Leucaspius delineatus (Heckel, 1843), (Actinopteri, Cypriniformes, Leuciscidae), sunbleak, is a freshwater, brackish species [167], protected under the Bern Convention. It is present in the numerous stagnant lowland and hilly water bodies, is frequent in some parts of the Danube Delta, and any habitat modification induced by sea level rise could eliminate this species from the Danube Delta area.

Squalius cephalus (Linnaeus, 1758) (Actinopteri, Cypriniformes, Leuciscidae), chub, a freshwater, brackish species [169], occurs in low numbers in the Danube Delta [49]; any drastic habitat changes following sea level rise could exclude the species from this area.

Leuciscus idus (Linnaeus, 1758) (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae), orfe, is a freshwater, brackish species [175]. Its presence in the Danube Delta is sporadic [49], and habitat modification induced by sea level rise could eliminate this species from the Danube Delta area.

Rutilus rutilus (Linnaeus, 1758) (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae), roach, is a freshwater, brackish species [169], and is one of the most common species in the Danube Delta [49]; the disappearance of delta habitats would eliminate a high biomass of this fish.

Tinca tinca (Linnaeus, 1758) (Actinopteri, Cypriniformes, Tincidae), tench, has been found in the lower Danube Basin in recent years [176], and any habitat modification induced by sea level rise could affect the distribution of this species in the Danube Delta.

Lota lota (Linnaeus, 1758) (Actinopteri, Gadiformes, Lotidae), burbot, is a freshwater, brackish species [159]. Its presence has decreased in the lower Danube Basin in recent years [49], and any habitat modification induced by sea level rise could affect this species’ distribution in the Danube Delta.

Gasterosteus aculeatus (Linnaeus, 1758) (Actinopteri, Perciformes/Gasterosteoidei, Gasterosteidae) is a marine, freshwater, brackish species [169]. In the past, this fish was known all along the Black Sea coast in the Danube Delta area, but it is now known only in the freshwater and brackish water habitats of the Delta [49]; sea level rise could affect this species’ distribution and abundance in the study area.

Zingel streber (Siebold, 1863) (Actinopteri, Perciformes/Percoidei, Percidae, Luciopercinae), streber, is a freshwater species [170] very rarely found in the Danube [49]; sea level rise could affect the distribution and abundance of this species in the study area.

Knipowitschia caucasica (Berg, 1916) (Actinopteri, Gobiiformes, Gobiidae, Gobionellinae), Caucasian dwarf goby, is a marine, freshwater, brackish fish [172]. It has vanished in the lower Danube River area, and its relatively high frequency and abundance in the Danube Delta [49] could be significantly affected by any sea level rise.

Scardinius erythrophthalmus (Linnaeus, 1758) (Actinopteri, Cypriniformes, Leuciscidae, Leuciscinae), rudd, is a freshwater, brackish species [169] common in the Danube Delta [167]; the loss of delta habitats would eliminate a high biomass of this fish.

If the specific Danube Delta habitats were to be significantly affected by sea level rise, this second category of fish species would be at a low/medium ecological risk of decreased population abundance and dispersion.

The third category of risk of fish species, based on their ecological characteristics and needs, in a relatively fast Black Sea level rise scenario, includes fish living in the Danube Delta and Black Sea and not or only accidentally in the upstream Danube River. These species are Syngnathus schmidti (Popov, 1927), Syngnathus tenuirostris (Rathke, 1837), Syngnathus variegatus (Pallas, 1814), Syngnathus typhle (Linnaeus, 1758), Mesogobius batrachocephalus (Pallas, 1814), Ponticola eurycephalus (Kessler, 1874), Ponticola syrman (Nordmann, 1840), Pomatoschistus marmoratus (Risso, 1810), Zosterisessor ophiocephalus (Pallas, 1814), Aidablennius sphinx (Valenciennes, 1836), Parablennius sanguinolentus (Pallas, 1814), Parablennius tentacularis (Brűnnich, 1768), Callionymus risso (Lesueur, 1814), Chelon auratus (Risso, 1810), Chelon saliens (Risso, 1810), Planiliza haematocheilus (Temminck and Schlegel, 1845), Mugil cephalus (Linnaeus, 1758), Pomatomus saltatrix (Linnaeus, 1766), Sciaena umbra (Linnaeus 1758), Umbrina cirrosa (Linnaeus, 1758), Scomber scombrus (Linnaeus, 1758), Atherina boyeri (Risso, 1810), Salmo labrax (Pallas, 1814), Gasterosteus aculeatus (Linnaeus, 1758), Trachurus mediterraneus (Aleev, 1956), Mullus barbatus (Essipov, 1927), Dicentrarchus labrax (Linnaeus, 1758), A. boyeri (Risso, 1810), Atherina hepsetus (Linnaeus, 1758), Platichthys flesus luscus (Pallas, 1814), and Anguilla anguilla (Linnaeus, 1758). Among them, only Parablennius sanguinolentus, P. tentacularis, Ponticola syrman, and Planiliza haematocheilus have seen increases in abundance and distribution in the last century; Ponticola eurycephalus, Pomatoschistus marmoratus, Zosterisessor ophiocephalus, Aidablennius sphinx, Callionymus risso, Chelon auratus, Chelon saliens, Mugil cephalus, Pomatomus saltatrix, Scomber scombrus, Trachurus mediterraneus, Mullus barbatus, A. boyeri, Salmo labrax, and Platichthys flesus luscus have experienced a decreasing trend of abundance and distribution; and the situation for Syngnathus schmidti, Syngnathus tenuirostris, S. variegates, S. typhle, Mesogobius batrachocephalus, Sciaena umbra, Umbrina cirrosa, Dicentrarchus labrax, A. hepsetus, Anguilla anguilla is worse, their presence being considered as questionable or locally extinct at present.

Ponticola eurycephalus (Kessler, 1874) (Actinopteri, Gobiiformes, Gobiidae, Gobiinae), mushroom goby, is a marine, brackish species, which in freshwater is found only in the Danube Delta [49,156,169]. Sea level rise could affect these unique freshwater populations by destroying their specific habitats.

Pomatoschistus marmoratus (Risso, 1810) (Actinopteri, Gobiiformes, Gobiidae, Gobiionellinae), marbled goby, is a marine, brackish species [177] that can be found inshore, over sand, and enters mainly brackish and hyper-saline waters [178]; sea level rise would not affect it.

Zosterisessor ophiocephalus (Pallas, 1814) (Actinopteri, Gobiiformes, Gobiidae, Gobiinae), grass goby, is a marine, brackish species occasionally recorded in freshwater [165,169,179]; sea level rise would not affect this species within the study area.

Aidablennius sphinx (Valenciennes, 1836) (Actinopteri, Blenniiformes, Bleniidae, Salariinae) is a marine species found in the shallow, rocky, litoral zone [180]; sea level rise would not affect this species within the study area.

Callionymus risso (Lesueur, 1814) (Actinopteri, Callionymiformes, Callionymidae) is a marine species found on sandy bottoms in shallow coastal water [181]; sea level rise would not affect this species.

Chelon auratus (Risso, 1810) (Actinopteri, Mugiliformes, Mugilidae), golden grey mullet, is a marine, freshwater, brackish species [182]. It can be found only sporadically in non-marine habitats, so sea level rise would not affect this local fish population.

Chelon saliens (Risso, 1810) (Actinopteri, Mugiliformes, Mugilidae), leaping mullet, is a marine, brackish species living in coastal waters [182]. In recent years it has been found only in marine habitats [49], so sea level rise would not affect this local fish population.

Mugil cephalus (Linnaeus, 1758) (Actinopteri, Mugiliformes, Mugilidae), flathead grey mullet, is a marine, freshwater, brackish species, the adults of which are found in coastal waters [183,184]. In recent years, due to the reduction in brackish habitats, it has been found only in marine habitats [49], so sea level rise would not affect this local fish population.

Pomatomus saltatrix (Linnaeus, 1766) (Actinopteri, Scombriformes, Pomatomidae), bluefish, is a vulnerable [166], marine, brackish species [169,185]. Sea level rise would not affect this local fish population.

Scomber scombrus (Linnaeus, 1758) (Actinopteri, Scombriformes, Scombridae, Scombrinae), mackerel, is a marine, brackish species [169,186]. Due to the fact that it has not been found in recent years in the delta [49], sea level rise would not affect this fish.

Syngnathus schmidti (Popov, 1928) (Actinopteri, Syngnathiformes, Syngnathidae, Syngnathinae), Schmidt’s pipefish, is a brackish species [187,188]. Due to the fact that it has not been found in recent years in the delta [49], sea level rise would not affect it.

Syngnathus tenuirostris (Rathke, 1837) (Actinopteri, Syngnathiformes, Syngnathidae, Syngnathinae), narrow-snouted pipefish, is a marine species [188]. Due to the fact that it has been found extremely rarely in recent years in the Danube Delta area (49), sea level rise would not affect this fish.

Syngnathus variegatus (Pallas, 1814) (Actinopteri, Syngnathiformes, Syngnathidae, Syngnathinae), thick-snouted pipefish, is a marine species [188]. It has not been found in recent years in the Danube Delta [49], so sea level rise would not affect this fish.

Syngnathus typhle (Linnaeus, 1758) (Actinopteri, Syngnathiformes, Syngnathidae, Syngnathinae), broad-nosed pipefish, is a marine, brackish species [189]. It has been found extremely rarely in recent years in the Danube Delta area [49], so sea level rise would not affect this fish.

Mesogobius batrachocephalus (Pallas, 1814) (Actinopteri, Gobiiformes, Gobiidae, Gobiinae), toad goby, is a brackish species [177]. Due to the fact that it has not been found in recent years in the Danube Delta [49], sea level rise would not affect this fish.

Umbrina cirrosa (Linnaeus, 1758) (Actinopteri, Eupercaria, Sciaenidae), shi drum, is a vulnerable [174], marine species [178]. Due to the fact that it has not been found in recent years in the Danube Delta [49], sea level rise would not affect this fish.

Atherina boyeri (Risso, 1810) (Actinopteri, Atheriniformes, Atherinidae, Atherininae), sand smelt, is a marine, freshwater, brackish species [178]. Sea level rise would affect this fish locally.

If the specific Danube Delta ecosystems were to be affected by sea level rise, this third category of species would be at low or no ecological risk of a reduction in population abundance and distribution.