Justification
The Júcar Nase has a restricted range (area of occupancy (AOO) c. 260 km2), which meets the threshold for the Endangered category under Criterion B2 (AOO < 500 km2). It occurs at seven locations (which exceeds the threshold for Endangered) where the extent and quality of habitat are estimated to be declining.
Therefore, this species is assessed as Vulnerable under Criterion B (B2ab(iii)).
Geographic Range Information
This species is endemic to the Júcar River and adjacent Albufera de València wetland basin in the autonomous communities of Castilla-La Mancha and the Valencian Community, eastern Spain.
Population Information
This species' current population size and trend have not been quantified. It has declined significantly since the mid-20th century, with an estimated area of occupancy (AOO) contraction of c. 80% between 1990 and 2010.
It has been extirpated from the Albufera de València and most of the Júcar River system, including the main stem, and is today confined to a series of increasingly isolated reaches representing c. 5% of its historical range. It is most abundant in the Cabriel River, the major tributary of the Júcar.
Habitat and Ecology Information
This gregarious species typically inhabits relatively small (width < 20 metres), shallow (depth < 1 metre) river stretches with largely intact riparian vegetation.
Within these reaches, it demonstrates a preference for relatively deep pools with slow current plus cover in the form of aquatic vegetation or submerged woody structures.
It also occurs in a handful of small karstic lakes located in the headwaters of the Júcar River system, and may occasionally enter artificial accumulation lakes created by dams.
Most of its habitats are characterised by significant intra- and inter-annual variations in discharge. Some are naturally dewatered in summer, giving rise to an increase in mortality of resident fishes. Some individuals survive for several months per year in isolated, shaded pools that function as refuge habitats.
Its life history has not been studied in depth, but the annual reproductive period extends from February to May. Adults are believed to undertake short upstream migrations to spawning sites comprising riffles with coarse substrata.
This species' diet comprises benthic macroinvertebrates and periphyton.
Threats Information
This species' decline is understood to have been driven by river regulation and other forms of anthropogenic habitat degradation.
In particular, the construction of 17 large dams plus hundreds of weirs and other barriers throughout the Júcar River system 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.
The quality of available habitat has been further diminished by bank stabilisation and channelisation to enhance flood protection or maximise water abstraction for irrigated agriculture.
This species is also threatened by pollution from agricultural, domestic and industrial sources, which has in some parts of the Júcar system reduced the extent and quality of suitable dry-season refuge habitats due to eutrophication or discharge of toxic substances.
The removal of riparian vegetation may reduce the extent of summer refugia pools through increased evaporation, while the elimination of cover, unregulated abstraction of surface water and increasingly long periods of drought due to climate change can cause them to dry out entirely.
The Júcar River has since 1979 been artificially connected to the Tagus (es. Tajo) River, which drains to the Atlantic Ocean, and the adjacent Segura River via the Tagus-Segura Water Transfer system, which comprises a complex series of canals, diversion dams, pipelines and tunnels. The related Iberian Nase (Pseudochondrostoma polylepis) has been introduced to the Júcar system through this network, and possibly outcompetes and/or hybridises with the Júcar Nase. The Iberian Gudgeon (Gobio lozanoi) became established via the same pathway and may compete for similar resources.
A range of other non-native fish and crustacean species, particularly the Pumpkinseed (Lepomis gibbosus), Largemouth Bass (Micropterus salmoides), Northern Pike (Esox lucius), Eurasian Pikeperch (Sander lucioperca) and Red Swamp Crayfish (Procambarus clarkii) might also have contributed to the decline of the Júcar Nase though predation, resource competition, habitat degradation or transmission of pathogens.
The overall negative impact of these threats is highlighted by the current distribution of the Júcar Nase, since it is found exclusively in habitats comprising clear, unpolluted water, well-preserved riparian vegetation and absence or low abundance of non-native species.
Use and Trade Information
This species is not used or traded.
Conservation Actions Information
This species is included (originally as Chondrostoma toxostoma) in Appendix III of the Bern Convention and Annex II of the European Union Habitats Directive.
It is listed as Endangered in the Spanish Catalogue of Threatened Species and Critically Endangered in the Valencian (regional) Catalogue of Threatened Species.
It occurs within the boundaries of several protected areas, and is listed (as Chondrostoma toxostoma) as a target species for at least one reserve that is included in the European Union's Natura 2000 network (site ES0000472). A management plan is in place for this site.
Fishing for this species is prohibited in Castilla-La Mancha.
Annual population monitoring and ex situ breeding efforts have been established in the Valencian Community since the early 2000s. Between 2012-2014 captive-bred individuals were reintroduced to four locations, with reproduction later confirmed at two of these. Subsequent releases at additional locations took place in 2019 and 2020. A pilot ex situ breeding programme was underway in Castilla-La-Mancha as of 2019.
In the upper Cabriel River, native fishes have on at least one occasion been collected by electrofishing and relocated downstream due to their habitat becoming completely dewatered. Some studies have concluded that efforts to improve connectivity in the upper Cabriel should be avoided for the time being, since the presence of a weir near the village of Pajaroncillo is currently preventing upstream dispersal of non-native species.
Suggested conservation actions include the creation of new protected areas targeting the Júcar Nase, enforcement of favourable flow regulation and water management policies, physical removal of non-native fish species, and education programmes for local communities and recreational angling groups in order to avoid further introductions.
It is recommended that future research should focus on understanding this species' life history, which is poorly understood, and the development of a structured conservation action plan.
