The taxonomic status of chub subpopulations inhabiting the Sado River system in Portugal is currently unclear, hence they are omitted from this assessment. Formerly identified as Squalius pyrenaicus, they are in fact closely-affiliated to S. tartessicus Doadrio, Sousa-Santo & Perea 2023 and may represent a distinct taxon (Perea et al. 2021, Doadrio et al. 2023).

The Tagus Chub 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 is believed to exceed 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 suspected that an ongoing reduction based on field observations, declining habitat quality and the effects of introduced taxa may approach or meet the thresholds for Vulnerable or Endangered under Criterion A (≥ 30% or ≥ 50% over the past 12 years = three generations).

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

This species is native to the Tagus (es. Tajo, pt. Tejo) River system in Portugal and Spain, plus a number of smaller catchments located around the Tagus estuary.

Its presence in the Sado River, Portugal, is considered uncertain for the purposes of this assessment (see 'Taxonomic Notes').

This species' population size is unknown, but is understood to exceed the minimum threshold for Red List criteria (< 10,000 mature individuals). The current population trend has not been quantified, but an ongoing reduction which may have exceeded 30% within the past three generations is suspected as a result of a continued decline in habitat quality. The number of subpopulations is unclear.

There exists documented evidence that its abundance has decreased significantly since the mid-20^th century. This pattern is suspected to be ongoing in some parts of its range, e.g., the Tagus River system, based on field observations, declining habitat quality and the effects of introduced taxa.

This species tends to be absent from large and deep lowland river stretches, but is otherwise considered to be a habitat generalist since it occurs in rivers and streams at different altitudes and with a diverse range of flow regimes. Field observations indicate that adult individuals may exhibit a preference for pools and glides.

Many of its habitats are characterised by seasonal variations in discharge, and can be significantly dewatered during summer. Some individuals survive these drought periods in remnant pools which function as refugia. It has also colonised some fluvial accumulation lakes created by dams at locations where there is access to suitable spawning habitat (but see 'Threats').

It feeds largely on invertebrates, but also consumes detritus and smaller fishes.

Maximum lifespan is at least seven years, and individuals become sexually mature from age 2-3+. The annual reproductive period extends from April to July, with the precise timing dependant on location and environmental conditions such as water temperature. It is characterised by mature adults moving short distances to preferred spawning sites comprising beds of gravel or other coarse substrata in shallow, fast-flowing water.

At some locations this species hybridises extensively with the congener Squalius alburnoides, which comprises an allopolyploid complex reliant on sympatric bisexual Squalius species for the production of diploid individuals.

This species is threatened by river regulation and other forms of anthropogenic habitat degradation. In particular, the construction of large dams plus smaller 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. Hydroelectric schemes 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, plausibly impairing the hatching and survival rates of eggs and larvae.

The quality of available habitat has been further diminished by bank stabilisation, channelisation and other efforts to enhance flood protection or exploit water resources for irrigated agriculture. Unregulated water abstraction may constitute a particular threat to this species' upstream spawning and nursery sites, some of which 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. Pollution can be particularly impactful when discharge is reduced during the summer, especially in temporal rivers where native fishes are often confined to small refugia (see 'Habitat and Ecology').

Non-native aquatic species established within the Tagus Chub's range include Common Carp (Cyprinus domestic strain), Largemouth Bass (Micropterus salmoides), Pumpkinseed (Lepomis gibbosus), Northern Pike (Esox lucius), Eurasian Pikeperch (Sander lucioperca), Topmouth Gudgeon (Pseudorasbora parva), Eastern Mosquitofish (Gambusia holbrooki) and Red Swamp Crayfish (Procambarus clarkii), all of which are considered invasive and can exert detrimental pressures on native freshwater fauna through increased competition, predation, habitat degradation or transmission of pathogens.

The ongoing spread of non-native Common Bleak (Alburnus alburnus) throughout most of this species' range is understood to represent a particular threat, since it is able to hybridise with the Tagus Chub and other native Squalius species.

The negative impact of some threats could plausibly be exacerbated by increasingly rapid climate change, which is already driving extended periods of drought in the Iberian Peninsula.

This species is not used or traded.

This species is included in Appendix III of the Bern Convention (as Leuciscus pyrenaicus), and is nationally-protected in Portugal.

It was assessed as Vulnerable in the Atlas and Red Book of the Freshwater Fishes of Spain (2002), and Vulnerable in the Portugese Red Book of Freshwater and Diadromous Fishes (2023).

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

It has been included in several studies investigating the efficiency of fishways in both Portugal and Spain, and some subpopulations may have benefitted from an increasing tendency to install or improve such structures on existing barriers within its range.

Ex situ reproduction efforts have been implemented for one Portugese subpopulation (Colares River) since 2007, but the current status of this scheme is unclear.

A deeper understanding of this species' demographics (population size and trend), current distribution and life history, particularly the identification of key spawning sites, would likely prove useful in the development of future management efforts.