Molecular analyses have identified three major genetic lineages, which inhabit the Danube main stem and Drava tributary system, plus the Sava and Tisza rivers, respectively. Subpopulations occupying the Dniester River are closely-related to the first of these lineages (Marić et al. 2017).

At the finer scale, at least 14 genetically-differentiated subpopulations exist. Some of these, e.g., those inhabiting the Danube and Dniester deltas, are relatively large and believed to comprise thousands of individuals, but most are much smaller (Takács et al. 2015, Marić et al. 2017, 2019, Bănăduc et al. 2022).

Global and European regional assessment: Vulnerable (VU)
EU 27 regional assessment: Vulnerable (VU)

The European Mudminnow is endemic to the Danube and Dniester river systems in Central and southeastern Europe. The species has experienced an inferred population size reduction of at least 30% within the past 10 years (three generations = six years), based on field observations, reduced area of occupancy (AOO), declining habitat quality, and the effects of introduced taxa. Therefore, this species is assessed as Vulnerable under Criterion A (A2ce), both globally and for the EU 27 member states.

This species is endemic to the Danube and Dniester river systems in Central and southeastern Europe.

In the Danube catchment, its range extends downstream from the vicinity of Vienna (Austria) to the river's mouth, while in the Dniester it is restricted to the lower reaches and delta.

According to current understanding, it remains extant at more than 100 locations spread between 23 principal areas, comprising: Austria east of Vienna and around the borders with Slovakia and Hungary; the Danube main stem in Hungary; the Lake Balaton basin in Hungary; the Danube main stem near its confluence with the Drava River in northwestern Serbia ("Lugomir" channel network); the Drava River (including the affluent Mur River) in the border region of Slovenia, Croatia and Hungary; the Drava River around the city of Virovitica in Croatia; the Sava River in Croatia, from east of Zagreb to the border with Bosnia and Herzegovina; the Matura River, a tributary of the Sava in Bosnia and Herzegovina; the confluence of the Drina and Sava rivers at the border between Bosnia and Herzegovina and Serbia; the lower Sava River and Pančevački Rit floodplain in Serbia; the upper Tisza River around the border between Slovakia, Hungary and Ukraine; the Tisza River floodplain west of Mezőkövesd in Hungary; the Tisza River system east of Budapest in Hungary (Tápió River); the Tizsa River system in eastern Hungary and northwestern Romania (Barcău and Ier rivers); the Timiș River in Romania; the Kraljevac nature reserve and lower Great Morava River in Serbia; the Danube main stem close to the Iron Gates II dam, at the border between Serbia and Romania; the lower Jiu River in Romania; the lower Olt River in Romania; the lower Vedea River in Romania; the lower Argeș, Mostiștea, and middle Ialomița river systems in Romania, plus Lake Srebarna in Bulgaria; the Danube Delta region in Romania and Ukraine, including the adjacent Sasyk Lagoon basin; the Prut River close to Iași in Romania; the Dniester River delta in Moldova and Ukraine.

A number of previously unreported locations in Bosnia and Herzegovina, Croatia, Hungary, Serbia and Romania have been discovered since 2010, and the possibility that it is present elsewhere cannot be discounted.

Outside of the native range, it has been introduced and is established at a single site in northern Germany.

This species' population size is uncertain, but is believed to exceed the minimum threshold for Red List criteria (< 10,000 mature individuals). The present population trend has not been quantified, and the number of subpopulations is unclear. However, the overall population size has reduced significantly since the mid-19^th century and it has been extirpated from many former locations, and today occupies an estimated 15% of its original range. A continuing decline in the number of mature individuals is therefore inferred.

At the national level, its abundance is currently inferred to be relatively stable in Austria, Croatia, Bosnia and Herzegovina, Republic of Serbia, Republic of Moldova and Ukraine (see 'Conservation'). A subpopulation located close to the city of Virovitica in Croatia is possibly expanding due to the construction of a canal network that provides suitable habitat conditions.

Nevertheless, steep declines continue to be noted in other parts of its range, particularly the Tisza River system in Hungary, where it has been extirpated from 29 of 63 (46%) known locations since 2010, and the rate of decline in the upper part of the basin is estimated at c. 95% since 2015. In Slovakia, a nationwide decline of 30-50% has been estimated for the period 2012-2022, including its possible extirpation from eastern parts of the country, while in northwestern Romania six of 11 (55%) known subpopulations have been extirpated since 2008. In all of these areas, but particularly the upper Tisza River, the conservation status of the remaining subpopulations is reported to be precarious (see 'Threats').

Based on the above, the global population is suspected to have declined by at least 30% over the course of the past ten years (three generations = c. six years). This is not a precise value, but a best estimate based on the consensus view of multiple experts.

This species exhibits a somewhat narrow suite of ecological demands, being a near-exclusive inhabitant of lentic backwaters, disconnected side channels, oxbows and other depressions in floodplains and alongside the margins of major lowland rivers. These largely alluvial habitats are typified by slow-moving to stagnant water, narrow channel width, abundant floating, submerged and riparian vegetation, layers of leaf litter, soft substrata, and in most cases isolation from associated rivers and other water bodies except during flood periods.

A small number of subpopulations inhabit swampy peatlands or minor affluent streams with abundant vegetation. It has also colonised many artificial habitats, particularly irrigation canals and gravel pits. These are of major importance, and today support the largest remaining subpopulations over some parts of its range.

The circulatory system and swim bladder are modified in comparison to most other bony fishes, allowing it to breathe atmospheric air to an extent and thus survive in anoxic and hypoxic conditions. These adaptations permit the European Mudminnow to colonise relatively harsh environments that are unsuitable for most other fishes, although it is often found in association with the European Weatherfish (Misgurnus fossilis), which has similar ecological requirements. However, these attributes are accompanied by poor interspecific competition and predator avoidance capacities, and research has demonstrated that it occurs at significantly lower densities in well-connected habitats which hold diverse fish communities.

The results of field studies also suggest that the European Mudminnow has a low level of tolerance to eutrophication, which may be indicative of an affinity to low-nutrient, groundwater-fed environments.

Its diet comprises a broad range of aquatic invertebrates, plus minor quantities of plant material and occasionally smaller fishes. The maximum recorded lifespan is 5+, and this species is characterised by early maturity, with both sexes able to reproduce at age 1+. The annual reproductive period extends from March to June, and is triggered when water temperatures reach 12-18°C. Spawning sites are selected by ripe female individuals, and can comprise submerged roots, vegetation, or a shallow depression excavated from the substrate and lined with plant material. Several males may attempt to spawn with a single female. Post-spawning, the female is solely responsible for broodcare, both defending and tending the eggs until they hatch. Individual females are believed to spawn on a single occasion each year, during which they produce c. 1,500-2,700 eggs.

The extent and quality of habitat are estimated to be undergoing continuing decline based on the ongoing threats (see 'Threats').

Historically, this species' decline was driven by large-scale river engineering projects designed to manage the annual flooding cycle of the Danube River and its principal lowland tributaries, which were implemented throughout the system from the mid-19th century onwards. The primary measures included canalisation, bank regulation, levee construction, removal of riparian vegetation, and cutting/straightening of meanders leading to the loss of huge tracts of wetlands and marshes. For example, in the Tisza River system the original floodplain area occupied an estimated area of 15,000 km², but was reduced to 539 km², including more than 4,200 kilometres of levees, and the length of the river itself was shortened by almost 500 kilometres. Moreover, an estimated 97% of wetlands associated with the Danube main stem in Hungary have been lost, and flooding events which formerly lasted for several months no longer occur throughout the system.

During the same period that the river engineering works were implemented, many wetlands were drained for conversion to intensive agriculture or urbanisation and transport infrastructure projects. Vast networks of drainage and irrigation canals were built throughout these areas, leading to increased nutrient loads (particularly nitrogen and phosphorous) plus discharge of untreated domestic wastewater, industrial toxins and runoff from landfills. Many water bodies thus became eutrophic or otherwise polluted, and increased soil erosion caused a reduction in macrophyte cover. These processes are ongoing throughout much of the Danube system.

A second wave of floodplain degradation was driven by the construction of large dams to improve navigation routes and generate hydroelectricity, which began in the early 20^th century. Many wetland habitats which were located upstream of dam sites have been lost due to inundation, while downstream effects include altered flow, sediment and temperature regimes (hydropeaking and thermopeaking) which hamper floodplain renewal and ecosystem stability.

River regulation and damming have thus limited the creation of new oxbows and side branches, while many of those that previously existed have dried out and/or become filled with vegetation over time, further reducing the extent of suitable European Mudminnow habitat

The reduction of flooding events has also significantly curtailed seasonal connectivity between adjacent subpopulations, and this may have negatively affected genetic diversity at the local scale.

In addition, some stretches of the Danube main stem have been deepened due to increased flow rates or development of the Rhine-Danube shipping corridor, leading to a lowering of the groundwater table and increased rates of dewatering in marginal floodplains.

Where the European Mudminnow has colonised agricultural landscapes (see 'Habitats and Ecology), it is often threatened by maintenance practices including dredging, artificial regulation of water levels and cutting of bank vegetation, which often take place during the key reproductive or winter dormancy periods. In other cases, areas of former agricultural land have been abandoned and the water level of drainage channels is no longer maintained.

Climate change-related pressures are expected to increase significantly in the near future, especially in the middle Danube region. Since 2012, parts of Slovakia, Hungary and northwestern Romania, e.g., the Ier and upper Tisza river systems, have experienced a succession of severe droughts, and this has led to artificial channels annually drying out and the possible loss of resident European Mudminnow subpopulations.

In addition, a number of invasive, non-native fish species have been introduced throughout this species’ range since the mid-20^th century, some of which readily colonise lentic habitats. Among the most successful is the Chinese Sleeper (Perccottus glenii), which originates from Eastern Asia but is expanding rapidly in some parts of the Danube system, e.g., the upper Tisza River. This species broadly shares habitat and dietary requirements with the European Mudminnow, and has been implicated in its extirpation from at least one location in Hungary. Studies have demonstrated that it negatively affects mudminnow foraging behaviour through aggressive interactions, while it is also known to exert profound effects on the structure of macroinvertebrate communities and may thus reduce prey availability. Other established non-native fishes which might negatively impact mudminnow subpopulations through predation or resource competition include the Pumpkinseed (Lepomis gibbosus), Black Bullhead (Ameiurus melas), Brown Bullhead (Ameiurus nebulosus) and Topmouth Gudgeon (Pseudorasbora parva).

Illegal harvesting for use as a bait fish by commercial and recreational fishers may represent a localised threat in some parts of Romania.

This species was formerly so abundant that it was used to make animal feed or fertiliser in parts of Central Europe. It is not currently traded, but is sometimes utilised as live bait by anglers and commercial fishers.

This species is included in Appendix II of the Bern Convention and Annex II of the European Union Habitats Directive. It is nationally-protected and included on the National Red List or equivalent documentation in most counties within its range. For example, it has most recently been assessed as Critically Endangered in Austria, Slovakia, Slovenia, Serbia, and Bulgaria, Endangered in Hungary and Vulnerable in Croatia. It is also included in the Red Data Books of Moldova and Ukraine.

It occurs in numerous protected areas, including national parks and a series of sites covered by the European Union's Natura 2000 network. At least 72 Special Areas of Conservation have been designated specifically for the European Mudminnow in EU member states, including 39 in Hungary, 10 in Slovakia, nine in Romania, nine in Croatia, two in Slovenia, two in Austria, and one in Romania.

Conservation of the European Mudminnow has long been of interest to scientists and conservation managers, and in 1995 an international workshop was held to assess its population status and identify management possibilities. Since then, a series of conservation actions have been implemented in several countries.

In Austria, successful conservation actions which took place from 2000-2001 included modification of the floodplain channel where the largest subpopulation occurs, in order to reconnect it with the groundwater and connect formerly isolated pools, plus deepening of existing pools to serve as refuge habitats during periods of drought. From 2002-2005, individuals from this somewhat stable subpopulation were translocated to canals in the Austrian portion of the transboundary Fertö-Hanság National Park, located to the southeast of endorheic Lake Neusiedl (de. Neusiedler See; hu. Fertő). Adult individuals have since been captured at the introduction site on a number of occasions, most recently in 2018.

In Hungary, individuals were also translocated to the Fertö-Hanság National Park in 2005. Elsewhere, an ex situ breeding and introduction program jointly administrated by the Tavirózsa Association of Environmental Protection and Nature Conservation, Hungarian University of Agriculture and Life Sciences (MATE, formerly Szent István University) and Danube-Ipoly National Park Directorate has been ongoing since 2008, and has resulted in the establishment of new subpopulations in a pilot area near the village of Szada. Individuals were collected from locations in the upper Tisza River for ex situ breeding purposes in 2010 and 2021, respectively.

In Slovakia, ex situ breeding efforts were underway by the late 1990s, and captive-bred individuals were subsequently introduced to a number of sites in the lower Morava River system, some of which have since been modified to favour the establishment of macrophytes. In 2007, a series of habitats were deepened and improved on the river island of Veľký Žitný ostrov.
Pilot translocation efforts also took place in Slovenia during the early 1990s, but the outcome of these is uncertain.

In Croatia, a 10-year (2019-2029) management action plan covering the Virovitica subpopulation is underway. Planned measures include non-native species removal, improving connectivity between the seven identified locations, enhancing water quality by reducing fertiliser runoff, and improving canal maintenance techniques.

Moreover, in-depth reports containing recommended management measures have recently been published in several countries e.g., Austria, Slovakia, Croatia, and Ukraine, while a number of journal articles have also included suggestions for appropriate future steps.

These recommendations include: restoration and improved management of floodplains, old river side channels, meanders and oxbows; creation of new refuge habitats and ex situ breeding schemes in areas where such actions have not already taken place; introduction of individuals to closed environments free of the Chinese Sleeper; improved management of irrigation channels through stakeholder discussion, e.g., control of water abstraction, the establishment of minimum accepted water levels at sites where the European Mudminnow is known to occur, changes in management practices and timing; creation of new protected areas and individual management plans for wetlands of interest; removal of non-native fish species; poaching control.

Studies addressing the European Mudminnow's population genetic structure are available and should ideally be considered within the framework of prospective management efforts in order to avoid mixing the identified genetic clusters (see 'Population'). In addition, a deeper understanding of this species' demographics (population size and trend), life history and interactions with non-native fish taxa would likely prove useful for conservation planning. Given the sizeable nature of its range, such efforts may be best coordinated at local or national scales.

Conservation managers should also be aware that schemes designed to reconnect backwater habitats with main river channels, e.g., in Gârla Mare, Romania, may threaten local European Mudminnow subpopulations by facilitating the passage of non-native fish species into floodplain habitats.