Molecular studies (e.g., Ketmaier et al. 2008, Kotlík et al. 2008, Živaljević et al. 2017) indicate that this taxon is conspecific with the Black Sea Roach (Rutilus frisii), but no comparative morphological analysis has yet been published.

A putatively conspecific but as yet unidentified congener inhabits the upper Tisza River, a major left-bank tributary of the Danube River, in Hungary and Slovakia (Kottelat and Freyhof 2007). It is omitted from this assessment, pending clarification of its taxonomic status.

The Pearlfish has a restricted range (extent of occurrence (EOO) c. 5,618 km²), which meets the threshold for the Vulnerable category under Criterion B1 (EOO < 20,000 km²). It occurs at six locations but there is no evidence of continuing decline or extreme fluctuations. Therefore, this species is assessed as Near Threatened because it is close to qualifying for Vulnerable under Criterion B.

This species is endemic to the upper Danube River system in Central Europe, where the precise limits of both its current and native ranges are unclear. 

Historical records suggests that it formerly inhabited the Danube main stem and its major right-bank tributaries, from the Lech River in southern Germany to at least the Devin Gate near Bratislava, Slovakia. However, archaeological evidence indicates that it was once present downstream to the Đerdap (Iron Gate) gorge at the border between Serbia and Romania, suggesting that it may have occurred throughout the Danube system.

Its confirmed modern distribution comprises a series of fluvial periapline lakes including Lake Chiem (de. Chiemsee) in Bavaria, Germany, and lakes Mond (de. Mondsee), Atter (de. Attersee), Wolfgang (de. Wolfgangsee) and Traun (de. Traunsee) in the Salzkammergut region, Austria. It is also present in the Traun River, which drains the Salzkammergut lakes and merges with the Danube south of the city of Linz.

A sporadic series of records obtained since the mid-1980s pertain to the Danube main stem, from around its confluence with the affluent Inn River at the Austria-Germany border, downstream to Vienna. These reports have been attributed to vagrancy from the Traun River, or taken as evidence that reproductive subpopulations are established throughout the Austrian part of the Danube.

For example, since 2000 a number of adult individuals have been caught on rod-and-line below the Jochenstein hydroelectric plant, which is located on a stretch isolated by impassable barriers. Subadults have also been reported downstream of the Inn confluence and the Enns River. In 2007/2008, a large number of individuals including nuptial adults were observed moving upstream through a fishway on the Traun River main stem at Wels, but it could not be determined if these were migrating from the Danube or lower Traun. Individuals have also been observed in the Ager River, which is the outflow of Lake Atter.

Therefore, it is plausible that this species' current distribution has been shaped by the extensive engineering works that have taken place in the upper Danube catchment since the 19^th century, and that a series of reproductive subpopulations are currently isolated from one another by the presence of multiple barriers and impoundments. Alternatively, the fact that most recent reports pertain only to a small number of individuals may be indicative that they are vagrants from the lacustrine subpopulations. Apart from those associated with the Salzkammergut lakes, no spawning sites have yet been located anywhere in the Danube or its tributaries, and no targeted studies have taken place. As a result of this uncertainty, only the confirmed reproductive subpopulations are included in this assessment.

Records obtained from the Danube main stem in Hungary and Slovakia during the 20^th century have been attributed to vagrant individuals, but may warrant further investigation.

This species' population size is unknown, but it is likely to exceed 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.

Its abundance and range are understood to have declined significantly since the 19^th century, albeit there is a lack of reliable baseline data to provide a meaningful comparison.

It was extirpated from Lake Chiem during the early 1990s, but intensive restocking has since taken place and a reproductively viable subpopulation is currently established (see 'Conservation'). The current status of this project is unclear, but nuptial adult individuals were reported in 2009, and there exist anecdotal reports of spawning activity in the outflowing Alz River. There are historical records from the Salzach River, which merges with the Inn River close to the Alz, but this subpopulation is presumably extinct.

The size of the Salzkammergut lakes subpopulations decreased markedly after the mid-20^th century, and reports published around the turn of the century suggest that it was extirpated from Lake Traun. However, a small extant subpopulation was subsequently discovered, and all four subpopulations may currently be increasing (see 'Conservation').

The number and status of putative fluvial subpopulations is less certain, and further research is strongly recommended.

All five of the lakes to which this species is native are deep (maximum depth 68-191 metres), naturally oligotrophic, perialpine systems. Lake Chiem is largely fed by the Prien and Tiroler Achen rivers and is drained by the Alz River, a tributary of the Inn River which is the third largest affluent of the Danube by discharge. Lake Traun lies on the Traun River main stem and is linked to Lake Wolfgang via the Ischl River. Lake Atter is connected to the adjacent Lake Mond via the short Seeache River and is drained by the Ager River, which flows into the Traun at the town of Lambach.

There is little information available regarding its habitat use, but field observations indicate that lacustrine individuals predominantly inhabit the littoral zone. Its behaviour in rivers is largely unknown, with the majority of recent observations having occurred in the vicinity of artificial barriers.

This species' diet is understood to comprise molluscs and other benthic macroinvertebrates, supplemented by periphyton and plant material during the summer. In lakes, it reportedly aggregates to prey opportunistically on emergent mayflies (Ephemeroptera) during the summer, and the spawn of Arctic Charr (Salvelinus alpinus) in winter. Emerging evidence suggests that it feeds extensively on the non-native Ponto-Caspian molluscs Dreissena polymorpha (Zebra Mussel) and Dreissena bugensis (Quagga Mussel), which are invasive throughout its range.

The maximum recorded age is 15 years, and individuals mature at age 4+. This species is potamodromous, and the annual reproductive period extends from mid-April to May. The precise timing of spawning behaviour is dependent on river discharge and water temperature. It is characterised by mature adults migrating short (< 30 km) distances to specific sites, which usually comprise well-washed gravel beds in shallow (< 0.5 metres depth) riffles. Fluvial subpopulations tend to travel upstream where they may enter tributaries, while lacustrine subpopulations spawn in both upstream (inflowing) and downstream (outflowing) rivers, depending on location. The bulk of migration is understood to take place at night, while spawning activity mostly occurs during twilight.

This species is a fractional, polygamous spawner, and nuptial male individuals develop conspicuous epidermal breeding tubercles on the head and body. Adult females produce 32,000-35,000 eggs per kilogramme of body weight. Early-stage juveniles are believed to remain in the vicinity of spawning sites for several months before migrating to lakes or favourable upstream sites prior to their first winter.

In the Lake Mond basin, spawning sites are located in the affluent rivers Zeller Ache (which is blocked by an impassable barrier 670 metres upstream) and Fuschler Ache (to around 800 m upstream), plus the mouth of the Drachen lagoon. In the Lake Atter basin, the primary spawning site is the Seeache River, but some individuals also spawn in the ouflowing Ager River and in the littoral zone of the lake itself. In Lake Wolfgang, spawning takes place to around 2.5 km downstream in the outflowing Ischl River. In Lake Traun, it used to spawn in both inflowing and outflowing stretches of the Traun River at Ebensee (recorded to Lauffen some 23 km upstream, and in the Ischl almost to Wolfgangsee) and Gmunden, respectively. In Lake Chiem, the only known spawning site is in the Alz River.

Little is known of its life history in the Danube main stem, where spawning sites are likely to be located in the lower reaches of tributaries if reproduction is taking place (see 'Geographic Range').

This species is understood to be threatened 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. The construction of dams, sills, weirs and other barriers throughout its range has severely altered natural flow and sedimentation regimes, blocked migration routes, fragmented subpopulations, and reduced the extent of suitable habitat for all life stages. The availability of appropriate spawning sites is likely to have decreased significantly, and most of its current spawning migrations are understood to be much shorter than in the past.

Hydroelectric dams have created unnatural fluctuations in discharge and water temperature (hydropeaking and thermopeaking) which bring about artificial dewatering of downstream river stretches and loss of stable nursery habitat for juveniles. 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 some spawning sites, which may impair the hatching and survival rates of eggs and larvae.

The quality of habitat has been further diminished by bank stabilisation, channelisation and other efforts to enhance flood protection or exploit water resources, which has reduced the extent and quality of spawning sites through a loss of the shallow gravel banks required for egg deposition. Some sites may also have been damaged by the industrial extraction of riverine gravel and other sediments for urban development.

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. In particular, Lake Chiem and the Salzkammergut lakes all suffered from anthropogenic eutrophication due to discharge of untreated urban wastewater, although they have largely recovered today (see 'Conservation').

The primary cause of its extirpation from Lake Chiem is uncertain, but it is believed to have been driven by a combination of factors including a decline in water quality and overpredation and/or resource competition due to intensive stocking with European Eel (Anguilla anguilla). The construction of a hydroelectric power plant and weir at Altenmarkt an der Alz (around 5 kilometres downstream of the lake) during the 1960s may have interfered with recruitment by preventing the upstream migration of juvenile individuals from the Alz River to the lake.

The decline of the Lake Traun subpopulation is believed to have been driven by construction of a hydroelectric power plant on the Traun River at Gmunden during the late 1960s, which is assumed to have degraded or blocked access to the main spawning sites. Moreover, Lake Traun was polluted by the discharge of waste materials from local salt and soda works from the early 20^th century until production ended in 2005. However, no negative impact on aquatic species inhabiting the lake has ever been detected, and this has been attributed to rapid washing-out of pollutants due to the lake's relatively short retention time.

Additional threats include the degradation of littoral habitats for urban development, and future shifts in the temperature regime of the perialpine lakes due to climate change.

This species was formerly harvested on a commercial basis, with records dating back to at least the 1400s in Lake Chiem, but it is no longer used or traded due to its protected status (see 'Conservation'). During the mid-20^th century, an average of 200 kg per year was harvested from Lake Chiem, most of which was transferred to the convent on the island of Frauenchiemsee, or sold at the Munich fish market.

In the Salzkammergut lakes, it was never viewed as particularly valuable and was usually targetted only during the annual spawning runs (see 'Habitat and Ecology'). Prior to the First World War, individuals were occasionally traded at markets in Linz or Vienna and it was consumed smoked, steamed or fried.

It is still landed as bycatch in the Austrian lakes and possibly also in the Danube main stem. For example, bycatch in Lake Mond comprised an average of 388 kg from 1998-2004. In Lake Atter, more than 1,200 individuals were landed in 2005. In both of these lakes, any individuals caught must be transferred to the local fisheries authority for scientific research, and fishers are financially compensated for doing so.

It is occasionally caught by recreational anglers, and was formerly a popular game fish.

This species is included in Annexes II and V of the European Union Habitats Directive (originally as Rutilus frisii meidingeri and Rutilus friesii meidingeri, respectively). It is nationally-protected in Austria.

Lakes Atter and Mond together comprise a single protected area that is included in the European Union's Natura 2000 network (site AT3117000). A series of specific conservation measures have been carried out at this site since the mid-2000s, including restoration of longitudinal connectivity in the Seeache River through the installation of fishways permitting both up- and downstream migration, the removal of small barriers, installation of passable ramps in a channelised stretch of the Zellerache River, and implementation of a targeted monitoring system.

Artificial incubation and rearing has been taking place at the Instituts für Gewässerökologie, Fischereibiologie und Seenkunde (BAW-IGF) in Scharfling, Austria, since the late 20^th century. Some juvenile individuals from this scheme were used for restocking and subpopulation reinforcement in Lake Traun.

Lake Chiem is a protected landscape at the national scale in Germany. Annual restocking activities started in 1995 and comprised the release of 40,000-70,000 age 0+ individuals obtained from the BAW-IGF rearing program.

Investment in wastewater treatment facilities and other policy-led measures since the 1970-1980s has driven an improvement in water quality of Lake Chiem and the Salzkammergut lakes, all of which had returned to their natural oligotrophic states by the early 2000s.

This species is also likely to be present within the boundaries of some Natura 2000 sites located along the courses of the Traun River, Danube main stem, and possibly elsewhere. Fluvial subpopulations may have benefitted somewhat from improvements in habitat and water quality associated with implementation of the European Union Water Framework Directive 2000/60/EC.

A deeper understanding of this species' demographics (population size and trend), current distribution and life history, with a particular focus on fluvial subpopulations, would likely prove useful in the development of future management efforts.