Global and European regional assessment: Endangered (EN)
EU 27 regional assessment: Endangered (EN)

The Rhône Streber is endemic to Europe, where it has a restricted range (area of occupancy (AOO) c. 480 km²), which meets the threshold for the Endangered category under criterion B2 (AOO <500 km²). The population occurs at fewer than five locations, while the extent and quality of habitat, number of locations and number of mature individuals are inferred to be undergoing continued decline as a result of the identified threats.

Therefore, this species is assessed as Endangered under criterion B (B2ab(iii,iv,v)), both globally and for the EU 27 member states.

This species is endemic to the Rhône River system in France and Switzerland, with the exception of the Lake Geneva (fr. Lac Léman) basin (Kottelat and Freyhof 2007, Keith et al. 2020). Since the turn of the 20^th century, it has undergone a continuous decline in both extent of occurrence (EOO) and area of occupancy (AOO); in 1900, it was present throughout most of the system (c. 2,200 kilometres of linear river length), but by 1988 its range was estimated to have contracted by at least 83% (Georget 2019, 2021).

At present, it occurs at five disparate locations in affluents of the Rhône, comprising the Ardèche (including the Beaume and Chassezac tributary systems), Durance (including the Asse, Bléone, Jabron, Sasse, Buëch and Verdon tributary systems, with the latter regarded as a distinct location due to its isolation by the presence of several dams), and Loue (a small affluent of the Doubs River) rivers in France, plus a short stretch of the Doubs River that flows through both French and Swiss territory (Episse et al. 2017, Georget 2021, Périat and Paris 2024).

In 2017, its estimated range was increased from c. 255 to c. 365 kilometres of linear river length as a result of reinforced monitoring efforts, and to a lesser extent recolonisation following conservation actions, particularly in the Ardèche River (Episse et al. 2017). However, by 2024 it was estimated to occur in only c. 240 kilometres of linear river length (see 'Population').

The species experienced a prolonged period of significant decline during the 20^th century (see 'Geographic Range'). The current population size and trend have not been explicitly quantified, but a continued population reduction is suspected based on field observations within the past three generations (= 9 years). Five isolated subpopulations remain extant, and these are treated individually in terms of conservation management.

Among these, the conservation status of that inhabiting the Doubs River is considered to be the most precarious due to its extremely small size and low genetic diversity. This subpopulation is no longer considered to be viable and is in all likelihood on the verge of extirpation, with only a single individual recorded during surveys carried out in 2023 (Périat and Paris 2024). Recent abundance data suggest that the Loue River subpopulation is also declining, with a tenfold decrease in the number of individuals observed between 2015–2020 (Georget 2021, Gil pers. comm.).

The largest extant subpopulation inhabits the Durance River. Field observations at sites in some tributaries where the species was previously absent have increased since 2015, while abundance appears to be rising in the Durance main stem between the Cadarache and Saint-Lazare accumulation dams, and the number of individuals inhabiting the Verdon River is reported to be relatively stable. However, the species may no longer be present in the Jabron and Sasse rivers, where the last confirmed observations date to 2012 and 2009, respectively (Georget 2019, 2021; Gil pers. comm.).

Since 2009, abundance has increased in at least one c. 30 km stretch of the Ardèche River main stem, but there are no recent records from the previously occupied lower section. Moreover, abundance in the Chassezac tributary system is understood to be very low due to a series of ongoing threats (Episse et al. 2017; Georget 2019, 2021; Gil pers. comm.).

A reintroduced subpopulation inhabiting the Drôme River as a result of conservation activities appears to have been extirpated since pilot restocking efforts ended in 2018, and is not considered for the purposes of this assessment (Gil pers. comm., also see 'Conservation').

This strictly benthic species inhabits clean, cool, well-oxygenated river reaches with well-washed substrata comprising mixed coarse gravel, cobbles and boulders. It is typically found in riffles and runs at intermediate depth (0.3–0.8 m) in stretches with moderate water flow (0.05–0.4 m/s). It is territorial and believed to naturally form low density subpopulations. While most individuals are sedentary in nature, some travel substantial distances outside the breeding season (Kottelat and Freyhof 2007, Cavalli et al. 2009, Freyhof et al. 2020, Keith et al. 2020).

Reproduction occurs annually from February to April, and is triggered when water temperatures reach c. 11°C. Males form temporary territories in the deeper parts of riffles, where they are approached by ripe females. Fecundity varies from 300–2,000 eggs, depending on the age and size of the individual female, and the first juveniles are normally seen in June and July (Kottelat and Freyhof 2007, Keith et al. 2020).

This species is short-lived, and most individuals are believed to spawn on only one or two occasions within their 3–4 year lifespans. However, a number of individuals aged 5–7+ have been reported from the Verdon River, where adults also grow noticeably larger than elsewhere. These discrepancies have been linked to variations in prey availability, substrate quality and water temperature between different locations (Cavalli et al. 2003, Monnet et al. 2022).

Studies have demonstrated that although its diet varies to an extent throughout the year, it feeds on a relatively narrow range of epibenthic invertebrates typical of unclogged lotic environments, especially Diptera, Ephemeroptera, Plecoptera and Trichoptera, alongside smaller quantities of gammarid amphipods. Larvae of the Pale Watery Mayfly (Baetis fuscatus) appear to represent the favoured prey except in the Verdon River, where mayflies of the genus Ecdyonurus comprise the major component of the diet (Cavalli et al. 2003, Archambaud‐Suard et al. 2019, Villsen et al. 2022).

The quality of habitat is estimated to be declining in several rivers within the current range (e.g., the Doubs, Loue, Chassezac; see 'Threats').

The primary factors which have driven this species' decline are habitat modification and pollution from agricultural, domestic and industrial sources. Different combinations of these and the other threats mentioned below are prevalent depending on the individual site (Keith et al. 2020, Georget 2021, Périat and Paris 2024).

The Rhône River has been heavily-exploited for energy production, with the first hydroelectric dams built during the 1880s. The ensuing construction of dams and other barriers throughout the system has caused Rhône Streber subpopulations to become fragmented and significantly reduced the extent of suitable habitat by altering water quality and hindering natural flow, sedimentation and water temperature regimes (Freyhof et al. 2020, Olivier et al. 2022, Périat and Paris 2024).

Moreover, the results of experimental trials demonstrate that this species is unable to pass even very low obstructions. Molecular analyses have revealed a loss of genetic diversity in the Ardèche and Verdon river subpopulations that have probably been driven directly by their isolation due to barriers (Laroche and Durand 2004).

A number of other forms of habitat modification are considered to negatively affect this species. In particular, regulation of channels and banks has eradicated entire subpopulations through the loss of vital morphological features such as rapids and riffles, while harvesting of gravel from riverbeds has removed extensive stretches of suitable habitat (Keith et al. 2020, Georget 2021, Périat and Paris 2024).

Certain land management practices and the periodic flushing of artificial dam lakes have resulted in increased soil erosion and siltation, leading to gravel beds becoming clogged and further reducing quality of its preferred habitats (Keith et al. 2020, Georget 2021, Périat and Paris 2024).

In addition, a combination of channelisation, hydropeaking from hydroelectric dams and sediment clogging has driven a decline in the biodiversity of macrobenthic communities and thus interfered with prey availability due to the Rhône Streber's rather specialised dietary requirements (Villsen et al. 2022, Périat and Paris 2024; see 'Habitats and Ecology').

This species is also sensitive to anthropogenic pollution, which in the Rhône system peaked during the latter half of the 20^th century. Improved wastewater treatment has since the 1990s led to a general decrease in the concentration of organic pollutants in the Rhône main stem and its larger tributaries, but some smaller affluents still receive high contamination loads from local industries and touristic centres during periods of low flow. Regular eutrophication events leading to fish kills continue to be reported from the Loue River and appear to be related to nutrient runoff from agriculture (Olivier et al. 2022).

In 2012, more than 200 different pesticides were detected in the Rhône, and most sources exploited for human drinking water were contaminated to some extent. Elevated levels of organochlorine pesticides continue to be discharged from some tributaries such as the Bourbre, Gier and Saône rivers. The presence of heavy metals remains an issue in the Arve, Gier and Saône rivers, where they mostly derive from agriculture including livestock, chemical fertilisers and fungicides. However, the impact of these contaminants on the Rhône Streber has not been investigated in depth (Olivier et al. 2022).

At the subpopulation scale, the non-native and predatory Wels Catfish (Silurus glanis) has been recorded in the Ardèche and lower Loue rivers, while the potentially competitive Round Goby (Neogobius melanostomus) has recently been reported from the Durance River. The Doubs River subpopulation is impacted by a highly pathogenic strain of Saprolegnia sp. cotton mould (Olivier et al. 2022, Doenz pers. comm.).

Water abstraction for irrigated agriculture is considered to be a factor potentially limiting self-sustaining recolonisation of the lower Drôme River, where agreed minimum flow levels below a weir at the town of Crest are reportedly not being respected (Georget 2021).

The Ardèche and Verdon river subpopulations may be threatened by prey trampling during the summer months, when the inhabited river stretches are popular for water-based recreational activities (Ferment and Guillard 2019).

Increasing temperatures due to climate change is likely to drive shifts in the Rhône Streber's range and impact its physiological processes, e.g., by interfering with reproductive timing and subsequent recruitment (Béjean 2019).

This species is not used or traded.

This species is included in Appendix II of the Bern Convention and Annex IV of the European Union Habitats Directive. In France, it has been protected at the national level since 1988 and was assessed as Endangered for the 2019 iteration of the National Red List (UICN Comité français 2019). In Switzerland, it was assessed as Critically Endangered for the 2022 National Red List of Freshwater Fishes and Cyclostomes (Zaugg 2022). It occurs within the boundaries of at least 14 protected areas that are included in the European Union's Natura 2000 European network.

It has been included in two European Union co-funded LIFE projects, which were active from 1998–2001 (LIFE98 NAT/F/005208) and 2004–2010 (LIFE04 NAT/FR/000083), respectively (Episse et al. 2017).

A five-year National Action Plan was in 2011 approved by the French Ministry and National Council for the Protection of Nature, and from 2012–2016 resulted in more than 30 actions to enhance understanding of the Rhône Streber. These were linked to six primary objectives, which included improving knowledge of the species and the nature of anthropogenic impacts, increasing the size of extant subpopulations by defragmenting rivers, habitat preservation and restoration, ensuring its inclusion in public policy and planning legislation, public dissemination, and inclusion of all relevant stakeholders. Research outputs have included an improved understanding of its distribution by combining field observations with environmental DNA, population genetic studies, refinement of ex situ breeding techniques, and dietary analyses using modern DNA genotyping and sequencing methods (Episse et al. 2017, Georget 2019, Roche et al. 2019).

In Switzerland, the Rhône Streber is incorporated within the framework of a national action plan for the Doubs River which was initiated in 2015. The plan includes more than 30 direct, monitoring and research measures aiming to improve the ecological status of the river, and is implemented by a scientific group comprising NGOs alongside cantonal and federal authorities (Hefti et al. 2015, Zaugg et al. 2018, Hefti 2024).

A number of direct conservation actions have also taken place, including the installation of fishways on barriers in the Ardèche, Drôme, Durance, Loue and Doubs river systems, the establishment of guaranteed minimum flows below dams in the Ardèche, Durance and Doubs, restoration of gravel beds, and a revised protocol for dam operation during floods in the Durance. One weir on the Durance River has also been removed completely after it was partially destroyed by floods. As a result of these efforts, the Ardèche subpopulation has expanded both upstream and downstream, and the species is now present throughout much of the middle and lower Durance River, where it was previously limited to a small stretch between the towns of Manosque and L'Escale. A number of additional management actions involving the modification or removal of barriers in the Ardèche River have been blocked due to opposition from local communities, however (Episse et al. 2017, Georget 2019, Roche et al. 2019).

An ex situ breeding project has been based at the Besançon Natural History Museum since 2005, and around 26,000 individuals were released into the Drôme River between 2012 and 2018. Genetic analyses carried out on juveniles collected during later fieldwork have demonstrated that successful reproduction did take place, but the species no longer appears to be present in the river (Béjean 2019, Gil pers. comm.).

A public awareness campaign saw live individuals exhibited to more than 1.45 million people at four popular tourist sites, a documentary filmed in 2015 was screened on French national television, 50,000 information leaflets were distributed, and workshops were offered to tourist firms operating in the species' distribution area. Results of the projects were made available to the public in the 2017 document 'Plan national d’actions en faveur de l’apron du Rhône: Bilan et perspectives' (French National Action Plan in favour of the Rhône apron: Results and Perspectives) (Episse et al. 2017).

In the Verdon and Doubs rivers, extensive consultation with local communities and tour operators has led to improved awareness and a sense of ownership regarding the need to conserve the resident subpopulations (Zaugg et al. 1999, Zaugg and Plomb 2012, Ferment and Guillard 2019).

In France, all of the above has been integrated into an updated ten-year (2020–2030) National Action Plan for conservation management of the Rhône Streber. The primary objectives of the new plan include improving knowledge necessary for its conservation, enhanced monitoring, habitat restoration, possible restocking based on the pilot scheme in the Drôme River, a new public awareness and communication strategy, inclusion of the species in administrative and legislative documents, and ensuring the implementation and sustainability of conservation actions (Georget 2021).

Plans to reduce or eradicate priority hazardous substances in the Rhône River system are underway in both France and Switzerland and include such initiatives as the promotion of organic farming practices. The industrial extraction of gravel and other sediments has also been prohibited in some areas within its range, e.g., the Durance River (Olivier et al. 2022).