Since the mid-2000s, rudd subpopulations from the Adriatic and Ligurian/Tyrrhenian sea basins of Italy have been referred to as S. hesperidicus Bonaparte 1845 and S. scardafa (Bonaparte 1837), respectively. However, these two taxa are diagnosed only on the basis of minor meristic and molecular characters, and their taxonomy has not been definitively resolved (Ketmaier et al. 2003, 2004, Bianco 2014, Geiger et al. 2014, Lorenzoni et al. 2019).

It has also been suggested that the introduction of S. hesperidicus caused S. scardafa to go extinct throughout its native range during the 20^th century. The only extant S. scardafa subpopulation thus occupies Lake Scanno in Abruzzo and purportedly originates from nearby Lake Fucine, which was artificially drained during the late 1800s (Bianco 2004, 2014).

However, there is no clear evidence that this translocation took place, and it is perhaps unlikely that the introduction of S. hesperidicus to some locations within the range of S. scardafa would have driven the latter to extinction within the space of a few decades (Lorenzoni et al. 2019).

Moreover, the results of a comparative analysis demonstrate that Italian rudd subpopulations exhibit a defined phylogeographic structure, with no significant anthropogenic influence on their native distribution (Stefani et al. 2004). In addition, older literature indicates that there exists natural variation in meristic characters, which have not been fully-investigated (Lorenzoni et al. 2019)

Molecular research also suggests that both S. hesperidicus and S. scardafa are closely-related to S. plotizza, which is native to the Neretva River system in the western Balkan Peninsula. Therefore, additional systematic research is recommended (Geiger et al. 2014).

This species' global population comprises a single introduced subpopulation located outside of its putative native range (see 'Taxonomic Notes'). The intent of the introduction was not documented, but it may plausibly have been related to preservation of fishes which would otherwise have been extirpated by the intentional draining of a nearby lake (see 'Geographic Range'). Application of the Red List categorisation process is thus deemed appropriate, since the introduced subpopulation is geographically close to the natural range of the taxon, produces viable offspring and has been established for at least 150 years.

The Tiber Rudd has an extremely restricted range (area of occupancy (AOO) c. 4 km²), which meets the threshold for the Critically Endangered category under Criterion B2 (AOO < 10 km²). It is present at one location where the quality of habitat is estimated to be declining, and is consequently assessed as Critically Endangered under Criterion B (B2ab(iii)).

This species is putatively native to rivers draining the Ligurian and Tyrrhenian slopes of western Italy, where its range extends from the Magra River system in the regions of Liguria and Tuscany to the Garigliano River in Lazio and Campania.

The only extant subpopulation is non-native and inhabits Lake Scanno in the upper Aterno-Pescara River catchment, Abruzzo, to which it was purportedly introduced from nearby Lake Fucine before it was drained for agricultural purposes between 1862-1878 (but see 'Taxonomic Notes').

This species was putatively extirpated from its entire native range during the 20^th century (see 'Taxonomic Notes').

The current population size and trend have not been quantified, but it is reportedly the most abundant fish species inhabiting Lake Scanno.

Lake Scanno is a karstic, landslide-dammed lake that is understood to have been impounded as a result of seismic activity during the early 10^th century. It is located at c. 922 metres asl, has a maximum depth of 32 metres and develops surface ice during the winter.

This species' biology and ecology have not been extensively-studied, but it reportedly inhabits the littoral and benthic zones of the lake and is often observed in the vicinity of reed and macrophyte beds.

Congeneric species tend to be omnivorous generalists in terms of diet, feeding on a variety of planktonic and benthic invertebrates, higher plant material and insects taken at the water surface.

The annual reproductive period extends from April until June, and it is probably a fractional, polygamous spawner which deposits its eggs among submerged vegetation.

The quality of habitat is estimated to be declining due to the effects of introduced taxa (see 'threats').

A number of non-native fish species have been introduced to Lake Scanno since the mid-2000s in order to stimulate interest in the recreational fishery. These include Eurasian Perch (Perca fluviatilis), Common Carp (Cyprinus domestic strain), Eurasian Tench (Tinca tinca), Largemouth Bass (Micropterus salmoides) and Pumpkinseed (Lepomis gibbosus), plus unidentified pike (Esox sp.), barbel (Barbus sp.), chub (Squalius sp.) and whitefish (Coregonus sp.) species. There are also at least two introduced trout (Salmo spp.) taxa present in the lake.

Although the presence of these species is not reported to have caused any reduction in the Tiber Rudd population size, no temporal abundance trend analyses have been carried out. In addition, regular restocking takes place and the introduction of a non-native congener or related species represents a plausible future threat due to the risk of introgressive hybridisation.

A series of summer fish kills have been observed in Lake Scanno since 2010. These have particularly affected Eurasian Perch and whitefish, and may be related to increased stratification and deoxygenation of the hypolimnion. The cause of these incidents warrants further investigation, since they comprise typical signs of incipient eutrophication. Lake Scanno is an increasingly popular tourist destination, and it is plausible that excessive nutrients are entering the system as a result of recent development.

Lake Scanno experiences periodic drying events during which the water level can fall by five metres or more, and an increase in the severity of these episodes due to climate change may constitute a further plausible threat.

Lake Scanno supports a popular recreational fishery based entirely on introduced fish species (see 'Threats'). The current level of offtake is unclear, but overharvesting does not appear to represent a plausible threat. There are closed fishing seasons, daily bag limits and minimum landing size regulations in place for some taxa, but these do not apply to the Tiber Rudd.

Lake Scanno is a protected area included in the European Union's Natura 2000 network (site IT7110101). The Tiber Rudd is not a target species, and no specific conservation measures are in place.

A deeper understanding of this species' demographics (population size and trend) and life history would likely prove useful in the development of future management efforts. However, confirmation of its taxonomic status should ideally be treated as a research priority in order to inform appropriate decision-making.