The Southern Straight-Mouth Nase is native to the southwestern Iberian Peninsula where it inhabits perennial lowland river channels and the lower reaches of tributaries. It 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. The population size is believed to exceed 10,000 individuals, therefore it does not approach the thresholds for Criteria C or D, and 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, anthropogenic habitat degradation and the effects of introduced taxa may approach or meet the threshold for Vulnerable under Criterion A (≥ 30% over the past 10 years). As a result of this data uncertainty, Least Concern and Vulnerable are equally plausible Red List categories for the present assessment, therefore this species is assessed as Near Threatened.

This species is native to the southwestern Iberian Peninsula, where it inhabits the Guadiana, Odiel and Guadalquivir river catchments, plus a series of Spanish coastal systems extending eastward from the Guadalete River in Cádiz province to the Vélez River in Málaga province.

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 reached 30% within the past ten years is suspected as a result of a continued decline in habitat quality and the effect of invasive species. The number of subpopulations is unclear.

There exists documented evidence of a significant population size reduction since the mid-20^th century. This pattern is suspected be ongoing in some parts of its range, e.g., downstream of the Alqueva Dam on the Guadiana River in Portugal, and in the Guadalquivir River system in Spain, based on field observations, declining habitat quality and the effects of introduced taxa.

This gregarious, potamodromous species inhabits perennial lowland river channels and the lower reaches of tributaries. It has colonised some artificial accumulation lakes at locations where there is access to suitable upstream spawning habitat (but see 'Threats').

It feeds largely on periphyton which it scrapes from the surface of submerged rocks and other surfaces using specialised mouthparts, but also consumes detritus and benthic invertebrates.

The annual reproductive period begins in April, with the precise timing dependant on location and environmental conditions such as water temperature. It is characterised by the upstream migration of mature adults to spawning sites comprising beds of gravel or other coarse substrata in shallow, fast-flowing water. Nuptial male individuals develop numerous small tubercles on the head and body. Juveniles and some subadults reportedly remain in upstream habitats throughout the year, where they exibit a preference for riffles and runs with submerged cover. However, many rivers and streams within this species' range are characterised by significant seasonal variations in flow, and the upper reaches are markedly dewatered or completely dry during summer. Some subadult individuals survive these drought periods in remnant pools which function as refugia.

A number of studies have demonstrated that it naturally hybridises with the Iberian Arched-Mouth Nase (Iberochondrostoma lemmingii) at some locations.

This species' decline is understood to have been driven by river regulation and other forms of anthropogenic habitat degradation, which have resulted in widespread loss of the heterogeneous, interconnected fluvial habitats required to complete its life-cycle. 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 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, plausibly impairing 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 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 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 fish species established within the Southern Straight-Mouth Nase's range include Common Carp (Cyprinus domestic strain), Largemouth Bass (Micropterus salmoides), Pumpkinseed (Lepomis gibbosus) and Topmouth Gudgeon (Pseudorasbora parva), all of which are considered invasive and can exert detrimental pressures on native freshwater fauna through increased competition, predation or habitat degradation. The presence of these taxa is favoured by barrier construction since they broadly prefer lentic conditions.

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 sometimes harvested by commercial fishers in Portugal.

This species is included in Appendix III of the Bern Convention (as Chondrostoma willkommi) and Annex II of the European Union Habitats Directive (originally as Chondrostoma polylepis (including C. willkommi)). It is nationally-protected in Portugal.

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

It is present 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. It is listed as a target species in some of the latter.

It has been included in a number of 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.

A deeper understanding of this species' population trend, current distribution and life history, particularly the identification of key spawning sites, would likely prove useful in the development of future management efforts. Given the extent of its range, such efforts may be best coordinated at local or regional scales.