Nase subpopulations inhabiting the Drin River system in North Macedonia, Albania, Montenegro and the Republic of Kosovo, including the Lake Ohrid and Lake Skadar basins, plus coastal rivers of Albania south to the Vjosa (el. Αώος/Aoös) River system are omitted from this assessment because molecular analyses have demonstrated that they might represent a distinct taxon for which the name Chondrostoma ohridanum Karaman, 1924 is available (Geiger et al. 2014, Shumka et al. 2023).

The Common Nase does not approach the range thresholds for Vulnerable under Criterion B1 (extent of occurrence (EOO) < 20,000 km²) or D2, and Criterion B2 is precluded by its uncertain area of occupancy (AOO). The population size far exceeds 10,000 mature individuals, and hence does not approach the thresholds for Criteria C or D. There exists no quantitative analysis which would permit application of Criterion E.

Although no explicit population trend data exists, it is possible that a suspected ongoing reduction based on field observations and declining habitat quality may approach or meet the threshold for Vulnerable under Criterion A2 (≥ 30% over the past 21 years = 3 generations).

As a result of this data uncertainty, Least Concern and Vulnerable are equally plausible Red List categories for the present assessment, and this species is assessed as Near Threatened.

The majority of this species' native range comprises major European river systems draining to the North, Baltic and Black sea basins.

In the North Sea basin its range extends from the Meuse River to the Elbe River, stretching eastward to the Neman River in the Baltic Sea basin.

It is restricted to the northwestern portion of the Black Sea basin, from the Danube River northward to the Dnieper River.

An isolated but putatively native subpopulation inhabits the Simav River in the southern Sea of Marmara basin, Republic of Türkiye.

It has been widely introduced outside of this range in France, where it is established in the Seine, Loire, Hérault and Rhône river systems. An additional non-native subpopulation inhabits the Soča (It. Isonzo) River in western Slovenia and northeastern Italy. The primary vectors for these expansions were artificial shipping canals connecting the Rhine River system to the Seine, Loire and Rhône rivers, plus the recreational angling sector.

This species' population size is unknown, but it significantly exceeds the minimum threshold for Red List criteria (< 10,000 mature individuals). The current population trend has not been quantified, and the number of subpopulations is unclear.

Historical records and field studies demonstrate that abundance has declined significantly throughout much of its range (e.g., Austria, Switzerland, Belgium, Germany, Poland, Czech Republic) since the early 20^th century.

Although it remains widespread, only small remnant subpopulations exist at many locations where it was formerly abundant.

This pattern is suspected to be ongoing, based on field observations and declining habitat quality. Moreover, recent (post-2015) subpopulation declines or local extirpations have been reported in Austria, Switzerland and southern Germany.

This gregarious, rheophilic species inhabits deep, relatively fast-flowing reaches of lower and middle river courses with coarse substrata. Outside of the annual spawning season, adult individuals inhabit well-defined home ranges, tending to occupy stretches of no more than a few kilometres in length.

A few subpopulations enter perialpine lakes, e.g., Lake Constance (de. Bodensee), and in some cases it has successfully colonised artificial accumulation lakes provided there is access to suitable spawning habitat in affluent rivers or streams.

It is a specialised grazer of periphyton, which it scrapes from the surface of submerged rocks and other surfaces using specialised mouthparts. It is the only obligate herbivorous fish species within its native range, and its consumption of benthic algae is believed to function as an important top-down regulator of primary production in fluvial systems. In addition, it is itself an important food source for the threatened Danube Salmon (Hucho hucho).

It is considered invasive throughout its non-native range, particularly in areas where it hybridises and/or competes with the related French Nase (Parachondrostoma toxostoma) and South European Nase (Protochondrostoma genei).

This species is relatively long-lived (up to 20 years) and slow-maturing (c. age 3-7+). Older individuals can weigh in excess of 1.5 kilogrammes, and mean fecundity is 15,000-20,000 eggs per kilogramme of body weight.

The annual reproductive period is brief and occurs between March and May, depending on location, when mean daily water temperatures rise above c. 10°C. It is characterised by sometimes considerable upstream migrations to specific spawning sites comprising well-washed gravel banks in shallow, fast-flowing water. Whereas some subpopulations migrate within larger river channels, others move into tributaries to spawn.

In larger rivers, there may be several runs within a single season, with older individuals migrating earlier. In some cases, males migrate several weeks prior to females. Whereas males are reproductively active for a period of several weeks per year, each female is ripe for a period of a few days.

Reproductive males develop a conspicuous epigamic colour pattern. Both sexes develop epidermal breeding tubercles on the head, body and fins, which may help protect the fish from physical injuries during the spawning process.

Males form dense aggregations at spawning sites, where they aggressively compete to maintain an optimal position, while females occupy deeper downstream pools nearby. Ripe females enter the spawning sites individually and are immediately mated by a group of males, returning to the downstream stretch once they have released their eggs. If not completely spent, this behaviour can be repeated several times.

The adhesive eggs are deposited mainly into the interstitial zone, which functions as an incubation and development space during the early life stages.

Hatched larvae move deeper into the substrate where they remain until the yolk sac is absorbed, at which point they emerge as pelagic fry and move into habitats with low water movement where they largely feed on rotifers. With increasing size they gradually move into areas with stronger currents and their diet shifts to zooplankton, drifting invertebrates and terrestrial insects.

The quality and structure of the interstitial space habitat in terms of looseness, permeability and oxygen supply is considered to be crucial for successful development.

The recruitment of young Common Nase is considered to be a useful indicator of the ecological and environmental integrity of river systems.

This species' decline has primarily been driven by river regulation and other forms of habitat degradation, which have resulted in widespread loss of the heterogeneous, interconnected fluvial habitats required to complete its life-cycle.

In particular, the construction of dams, weirs and other barriers has altered natural flow and sedimentation regimes, blocked migration routes, fragmented subpopulations, and reduced the extent of suitable habitat for all life stages. The quality of available habitat has been further diminished by bank stabilisation, channelisation and other efforts to enhance flood protection or river transportation links.

Hydroelectric dams have created regular fluctuations in discharge and water temperature (hydropeaking and thermopeaking) which cause dewatering of spawning sites and loss of stable nursery habitat for juveniles, plus downstream displacement and stranding of individual fish.

Furthermore, the combined effect of hydropeaking, dam flushing operations, changes in land use, and the removal of riparian vegetation has increased accumulation of fine sediments at spawning sites, impairing the hatching and survival rates of eggs and larvae.

The industrial extraction of riverine gravel and other sediments for urban development has further reduced the extent of available spawning sites.

This species is also likely to have declined due to widespread agricultural, domestic and industrial pollution during the 20^th century, some of which persists today.

Increasing nutrient levels have driven eutrophication throughout its range, which is likely to have reduced food availability by altering composition of the periphyton layer (e.g., a transition from diatoms to filamentous algae). Increased algal growth due to eutrophication has also reduced the quality of some spawning sites.

Certain herbicides, pesticides and industrial pollutants may cause direct mortality or impair reproduction through sperm DNA damage when present in sufficient concentrations.

Fragmentation or isolation of subpopulations due to barrier construction and management efforts based on restocking (see 'Conservation') also threaten some subpopulations, due to the potential for inbreeding and a loss of genetic diversity.

This species is harvested commercially and marketed for human consumption at the local scale in parts of its range, e.g., Hungary, Romania. It is usually lumped together with related species in fisheries data, hence precise landing figures are usually unavailable.

Declining abundance in combination with a general transition from commercial to recreational inland fisheries has seen landings cease almost entirely in some countries, e.g., Czechia, Slovakia.

It is targeted by recreational anglers across both its native and non-native ranges, and is sometimes consumed in areas where catch and release fishing is not practised.

Significant numbers of unreported landings are likely to occur annually in both commercial and recreational fisheries, but overexploitation is unlikely to represent a plausible threat.

It is also present in the ornamental fish trade, with captive-bred juvenile individuals sometimes sold for home aquaria or garden ponds.

The Common Nase has been described as both a flagship and indicator species for the conservation of European rivers, and is included in Appendix III of the Bern Convention.

At the national scale, it has most recently been assessed as Critically Endangered in Switzerland, Endangered in Belgium and Czech Republic, and Near Threatened in Austria and Germany. It is officially protected in a number of countries, and closed fishing seasons timed to coincide with the annual reproductive period are established within some parts of its range.

It occurs within the boundaries of numerous protected areas, some of which are included in the European Union's Natura 2000 network. In some European Union member states it is likely to have benefitted from efforts to improve the ecological status of rivers within the framework of the European Water Framework Directive 2000/60/EC.

The latter legislation was implemented in 2003 and has alongside a general decline in heavy industry resulted in improving water quality across much of its range. Pollution has in many areas been reduced, while management actions focussed on freshwater fishes have included measures to improve habitat quality, e.g., restoration of spawning sites or stream banks, or connectivity, e.g., dam-removal projects or installation of nature-like fishways to re-establish migration routes.

In some cases these procedures have specifically targeted Common Nase at the local or national scales, and have been supplemented by short-term ex situ methods to reinforce declining subpopulations or repopulate locations in the wake of extirpation events. Such projects have typically involved translocation of adult individuals to new locations or supportive rearing followed by restocking with captive-reared juveniles and larvae.

Supportive rearing techniques typically involve the stripping of eggs and milt from small numbers of wild adult individuals during the reproductive season, then growing the offspring to a certain size in hatchery ponds or other facilities before releasing them. In some regions these efforts have been ongoing for decades, and they are frequently led by local angling associations.

However, both translocation and supportive rearing can negatively impact wild subpopulations through genetic drift, loss of genetic diversity, reduced effective population size, outbreeding depression and reduced fitness, and research suggest that alternative methods such as collecting fertilised eggs from the wild may prove more successful.

Restocking may also have little long-term impact if habitat conditions remain unsuitable, therefore an integrated management approach has been widely recommended.

In particular, the management of spawning grounds has been highlighted as a critical element of successful re-establishment, and should include erosion protection measures, substrate restoration and re-establishment of dynamic flow processes to provide functional interstitial habitat for the proper development of early life stages.

Improving the quality and connectivity of both adult and juvenile habitats followed by a period of monitoring should therefore be prioritised ahead of restocking in order to mitigate any further depletion of genetic diversity.

The taxonomy of subpopulations inhabiting some lakes and rivers of the Western Balkans is also in need of resolution (see 'Taxonomic Notes').