This taxon was formerly regarded to be conspecific with the Italian Roach (Rutilus pigus), which is native to rivers of the northern Adriatic Sea basin. However, a series of molecular analyses published since the late 2000s have clearly demonstrated that the two species are distinct (Ketmaier et al. 2008, Perea et al. 2010, Geiger et al. 2014, Levin et al. 2017).

Global and European regional assessment: Least Concern (LC)
EU 27 regional assessment: Least Concern (LC)

This species does not approach the range thresholds for Vulnerable under Criterion B (extent of occurrence (EOO) < 20,000 km², area of occupancy (AOO) < 2,000 km²) or D2. Although it may be declining in some parts of its range, there is no evidence that any population size reduction approaches the threshold for Vulnerable under Criterion A (≥ 30% over the past 15 years = three generations). The population size does not approach the thresholds for Vulnerable under Criterion C (< 10,000 mature individuals) or D1, and there exists no quantitative analysis of extinction probability which would permit application of Criterion E.

Therefore, the Cactus Roach does not currently meet the thresholds for any Red List criteria, and it is assessed as Least Concern for both Europe and the EU 27 member states.

This species is endemic to the Danube River system in Central and Southeastern Europe, where it is understood to occur downstream from the state of Baden-Württemberg in southern Germany to the Đerdap (Iron Gate) gorge at the border between Serbia and Romania.

It is present in the majority of larger affluent rivers within this range, but the extent of its distribution in some tributaries is unclear.

This uncertainty is in part driven by the sheer size and volume of the Danube watershed, which hinders exhaustive monitoring, plus a lack of comprehensive baseline data. In addition, it may be somewhat related to natural distribution patterns driven by this species' habitat preferences (see 'Habitat and Ecology'). For example, it is apparently limited to the lower reaches of rivers draining northern slopes of the Alps mountains in Germany and Austria, while its confirmed distribution in the extensive Tisza River watershed comprises a relatively small area in the border region of Hungary, Slovakia and Ukraine. It has been observed at a number of new sites in Ukraine since 2010, but all are within c. 30 km of the previous range limits.

Elsewhere, the lack of clarity is plausibly linked to as yet unquantified declines in area of occupancy which occurred during the 20^th century. For example, historical records from the Morava River indicate that it formerly occurred upstream to the Thaya River, Austria and the city of Olomouc, Czech Republic, which corresponds to several hundred kilometres of linear river length. However, recent reports suggest that it might today be restricted to the lowermost portion of the river (see 'Population' and 'Threats').

This species' population size is unknown, but it far 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.

Overall abundance is understood to have declined significantly since the late 19^th century, particularly in the Upper Danube catchment where it is currently considered to be scarce and may no longer occur in some tributary systems, e.g., parts of the Inn River plus the Salzach, Enns, Traun, and Rába rivers. In the Czech Republic, it was thought to have been extirpated from the Morava River system and thus nationally extinct between the mid-1950s and 2015.

There are indications that the putatively isolated subpopulation present in the upper Tisza River may be undergoing continued decline. In particular, it may have been extirpated from the Someş River in Hungary and Romania since the turn of the century (see 'Threats').

The largest and most stable stocks inhabit the Drava, Sava and Great Morava river systems, although site-scale declines have been reported in all three systems within the past 70 years. In contrast, the size of some subpopulations inhabiting rivers in Slovakia, e.g., the Váh River, is estimated to be increasing.

This gregarious species primarily inhabits relatively deep, flowing reaches of larger river channels.

Its diet is understood to comprise molluscs and other benthic macroinvertebrates, supplemented by periphyton and plant material during the summer.

The maximum recorded age is 14 years, and individuals mature at age 2-3+. It is potamodromous, and the annual reproductive period extends from March to May. The precise timing of spawning behaviour is believed to be dependent on river discharge and water temperatures rising above 10-14°C. It is characterised by mature adults migrating to specific upstream sites, which typically comprise beds of submerged vegetation or well-washed gravel in shallow (< 0.5 metres depth) riffles. There exists very little information regarding the distances migrated or the locations of important spawning sites.

This species is a fractional, polygamous spawner. Nuptial individuals of both sexes develop a conspicuous epigamic colour pattern, while males possess conspicuous epidermal breeding tubercles on the head and body. Females can produce up to 60,000 eggs per year depending on their age, condition and body size, which hatch after an average of 14 days.

This species is 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 since the late 19^th century has severely altered natural flow and sedimentation regimes, blocked migration routes, fragmented subpopulations, and reduced the extent of suitable habitat for all life stages.

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 is likely to have increased the 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, imporove navigation or exploit water resources, which has reduced the extent and quality of spawning sites through disconnection of floodplains and 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 threatened by diffuse and point source agricultural, domestic and industrial pollution, which has reduced the extent and quality of habitat at some locations due to eutrophication or discharge of toxic substances. Although water quality in the Danube watershed has improved somewhat since the turn of the century (see 'Conservation'), the Someş River in Romania and Hungary was extensively contaminated with cyanide and heavy metals following the rupture of a commercial water reservoir in January 2000. This event drove a substantial reduction in fish abundance, and may have caused the resident Cactus Roach subpopulation to go extinct.

This species is utilised in recreational fisheries, particularly in the Drava and Sava river systems. Current landing totals are unclear, but 8-12 tonnes were caught annually by anglers in Slovenia during the 1980s and 1990s.

This species is included (as Rutilus pigus) in Appendix III of the Bern Convention and Annex II of the European Union Habitats Directive.

Under National Red List criteria, it has most recently been assessed as Critically Endangered in Germany, Endangered in Austria and Czech Republic, Vulnerable in Hungary, Montenegro, Slovakia and Ukraine, and Least Concern in Croatia.

It occurs within the boundaries of various protected areas throughout its range, including a number of national parks and sites included in the European Union's Natura 2000 network.

Some subpopulations may have been favoured by improvements in habitat and water quality associated with implementation of the European Union Water Framework Directive 2000/60/EC, such as efforts to restore fluvial connectivity through barrier removal or the creation of fishways.

In Slovenia, it is protected by an annual closed fishing season which extends from March 1 to May 31, a daily bag limit of five individuals and a minimum landing size of 30 cm. The latter measure has been described as inadequate, since 30 cm appears to be the smallest size at which individuals mature. Successful attempts at captive-rearing and restocking of several thousand individuals also took place during 1989-1990, but the impact of this scheme is uncertain.

In Austria, it may benefit from the ongoing European Union co-funded LIFE IP Integrated River Solutions in Austria project (LIFE17 IPE/AT/000006). This extensive campaign, also known as IRIS, is scheduled to run from 2018-2029 and aims to align the goals and actions outlined in Austria's national Water and Flood Risk Management plans while simultaneously considering other uses and requirements of flowing waters, including ecological quality.

During the late 1990s, extensive conservation management actions took place on the stretch of the Danube River between Vienna and Bratislava. Various sections of the floodplain were reconnected to the Danube by lowering check-dams and culverts within the associated network of side-arms. The different projects were funded by the European Union U (e.g., LIFE projects) and from various national sources. The objectives of these projects included improvement of lateral connectivity between the main channel and side-arms, improved water supply to the floodplains, the establishment of lotic habitats and the recolonisation of floodplain side-arms by rheophilic fish species including the Cactus Roach. In general, these projects achieved some structural improvements in the side-arms, but failed to provide suitable habitat for rheophilic fishes due to insufficient discharge and lack of gravel or other coarse substrata.

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