This taxon has been regularly treated as a synonym or subspecies of the congeneric Italian Chub (Squalius squalus) since the 1980s (Bianco and Recchia 1983, Kottelat 1997, Bianco 2014, Lucentini et al. 2014). As a result, it was not included in the most recent checklist of Italian freshwater fishes or Red List of Italian Vertebrates (Lorenzoni et al. 2019, Rondinini et al. 2022). In contrast, the majority of recent molecular analyses have concluded that it represents a valid species (Perea et al. 2010, Geiger et al. 2014, Buj et al. 2019). There also exists evidence that individuals from Lake Trasimeno and nearby tributaries in the Tiber (it. Tevere) River system may together represent a distinct genetic lineage (Lucentini et al. 2014).

The Trasimeno Chub has a restricted range (extent of occurrence (EOO) c. 135 km², area of occupancy (AOO) c. 120 km²), which meets the thresholds for the Endangered category under Criterion B1 (EOO < 5,000 km²) and Criterion B2 (AOO < 500 km²). It occupies a single location where the extent and quality of habitat are estimated to be declining

Therefore, this species is assessed as Endangered under Criterion B (B1ab(iii)+2ab(iii)).

This species is endemic to Lake Trasimeno in the region of Umbria, central Italy.

This species' current population size and trend have not been quantified.

It has been considered rare in Lake Trasimeno since at least the early 1990s, but there is a lack of baseline comparative data and its abundance may be naturally low. Furthermore, the extent of its putative introgression with the Italian Chub (Squalius squalus) has not been investigated (see 'Threats' and 'Conservation').

Trasimeno is a shallow (average depth 4.7 metres, maximum depth 6.3 metres) endorheic lake of tectonic origin, and is the fourth-largest freshwater body in Italy by surface area. It has no surface outflow and is recharged only by a few ephemeral streams; therefore, its hydrology is strongly influenced by precipitation which renders it prone to marked intra- and interannual fluctuations in water level, chemistry and turbidity. A significant drought during the 1950s led to the Trasimeno catchment being artificially enlarged to include a few small tributaries draining to the Tiber River system.

The lake is currently classified as meso-eutrophic, and of 'Moderate' ecological status in line with the criteria of the European Union's Water Framework Directive 2000/60/EC.

The littoral zone is characterised by soft substrata and stands of Common Reed (Phragmites australis). Dense macrophyte beds including members of the genera Myriophyllum, Stuckenia and Vallisneria emerge during the summer, particularly along southern shorelines.

The Trasimeno Chub is reportedly most abundant in the pelagic zone. The annual reproductive period presumably extends from late spring (May) to early summer, when nuptial individuals spawn in both natural and artificial affluents of the lake.

This species is plausibly threatened by the effects of non-native aquatic taxa, with at least 15 fish and several invertebrate species established in Lake Trasimeno, most of which have been introduced since the early 20^th century. These include Largemouth Bass (Micropterus salmoides), Pumpkinseed (Lepomis gibbosus), Eurasian Perch (Perca fluviatilis), Common Carp (Cyprinus domestic strain), Grass Carp (Ctenopharyngodon idella), Goldfish (Carassius auratus), Italian Bleak (Alburnus arborella), Topmouth Gudgeon (Pseudorasbora parva), Eastern Mosquitofish (Gambusia holbrooki), Black Bullhead (Ameiurus melas), Red Swamp Crayfish (Procambarus clarkii), Killer Shrimp (Dikerogammarus villosus) and Zebra Mussel (Dreissena polymorpha), all of which are known to exert negative effects on native taxa through habitat alteration, hybridisation, predation, resource competition or transmission of pathogens.

Ongoing declines in some sympatric native species, e.g., Cisalpine Pike (Esox cisalpinus) have been directly attributed to the presence of non-native competitors, while the South European Roach (Sarmarutilus rubilio) and Italian Spined Loach (Cobitis bilineata) have been extirpated from the lake since the 1960s. Fisheries data collected from 1966-2016 demonstrate a significant reduction in the biomass of native species and a concurrent increase in most non-native fishes. An emerging threat related to the stocking of non-native fishes is the presence of potentially zoonotic parasitic nematodes of the genus Eustrongylides, which were first detected in Eurasian Perch during 2015 and have since infected most resident species.

The Italian Chub (Squalius squalus) has been stocked in Lake Trasimeno on multiple occasions since the mid-20^th century, and the origin of these individuals is unclear. Insidious introgressive hybridisation may therefore represent a significant threat to the Trasimeno Chub, but has not been extensively investigated.

Around 70% of the Lake Trasimeno catchment is cultivated, with 28% turned over to intensive irrigated agriculture, and water abstraction is understood to be contributing to gradual shrinking of the lake (see below). It is impacted by diffuse and point source agricultural and domestic pollution, although the trophic level has not varied significantly since the 1960s. A rapid accumulation of salts has been observed since 1989, as a result of domestic wastewater discharge, increased rates of erosion associated with changing land-use practices, and a lack of natural turnover in the lake due to increased evapotranspiration (see below).

The lake is considered to be particularly vulnerable to the effects of climate change. Its surface area has been steadily decreasing since the early 1990s due to reduced precipitation and escalating evapotranspiration, and during dry periods it regularly becomes disconnected from the drainage system designed to maintain the water level. Other documented effects including higher water temperatures, reduced dissolved oxygen concentrations, increased turbidity and a higher rate of salinisation. The extent of marginal reed and macrophyte beds has been depleted (e.g., 20% reduction in macrophyte coverage between 2003-2008), and some tributary channels and streams are increasingly dewatered during the spring, leading to a decline in the extent of suitable spawning and nursery habitat for native fishes. For example, the presence of an artificial barrier in a particular drainage channel utilised by spawning Trasimeno Chub prevents the passage of fishes when the water level is low.

Lake Trasimeno has supported the largest commercial fishery operating on Italian inland waters since the 1920s, with several thousand gillnets and fyke nets operating nightly. Data collated between the mid-1950s and 1980 demonstrate that yield is closely correlated with the lake's water level, with a general decline from the early 1970s followed by a sharper contraction starting in the early 1990s (see 'Threats'). The number of active fishers subsequently fell from 170 in 1990 to 76 by 2016.

The Trasimeno Chub is regarded as bycatch, with the primary target species currently the non-native Big-Scale Sand-Smelt (Atherina boyeri), which comprises > 50% of all landings, Eurasian Perch (Perca fluviatilis), Largemouth Bass (Micropterus salmoides) and Common Carp (Cyprinus domestic strain).

Restocking of several native and non-native fish species, particularly Cisalpine Pike (Esox cisalpinus), Eurasian Tench (Tinca tinca) and Common Carp is handled by a dedicated hatchery located at the southern shore of the lake.

This species is not covered by any national or international conservation measures, and no specific management actions are in place.

Lake Trasimeno is a regional protected area that is also included in the European Union's Natura 2000 network (site IT5210018).

This species' population trend and life history are not currently understood, but the immediate research priority should comprise an unambiguous resolution of its taxonomic status. The extent of introgression with introduced chubs should then be confirmed in order to ascertain its threat status and establish the need for conservation actions. Moreover, the suggestion that chubs inhabiting Lake Trasimeno and some adjacent parts of the Tiber River system comprise a distinct genetic lineage within the genus Squalius (see 'Taxonomic Notes') implies that that their conservation management should possibly be considered independent of their systematic classification.