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Rationale (Changed due to Same category and criteria)

The Slender Redfin (Pseudobarbus tenuis) is restricted to the Gouritz River System where it occurs mainly in clear headwater streams and tributaries (Skelton 2001). The mainstream reaches of the Gouritz River System have been impacted by alien fishes, excessive water extraction and is affected by various agricultural activities. Given that the Slender Redfin prefers headwater type habitat, it is unlikely to have been severely affected by these impacts. The species is still represented by several subpopulations. The overall population size is unknown, but the species is usually abundant in areas where it occurs. Many populations occur within formally protected areas (nature reserves and mountain catchment areas) where they are safeguarded against the majority of anthropogenic impacts, but some of these refugial or sanctuary areas remain vulnerable to invasion by non-native species from the mainstem river sections. The species is also vulnerable to the impacts of climate change in the future. It has an extent of occurrence (EOO) of 10,262 km² and an area of occupancy (AOO) of 428 km² (calculated using 2x2 km² grid). Presently there are 21 subpopulations and 12 locations which are defined based on the distribution of invasive alien species posing a threat. The species does not currently meet the qualifying thresholds for any threat category, however, it is experiencing decline in quality of habitat and it has a restricted range, and is therefore assessed as Near Threatened B1b(iii)+2b(iii).

Distribution

The Slender Redfin is endemic to the Gouritz River System where it occurs mainly in tributaries (Skelton, 2001). As a result of its habitat preference for headwater streams, it is relatively well represented within the formal protected area network (provincial nature reserves (NRs) and declared mountain catchment areas (MCAs)) of the Western Cape Province. The biggest nature reserve of the region is the Swartberg NR Complex and associated MCA which encompasses the headwaters of the Buffelskloof, Dwars, Kobus, Seweweekspoort, Huis-Waterkloof, Klein Le Roux’s, Nels, Elandspad, Lemoenkloof and Oshoekshang Rivers. All of these rivers currently have intact subpopulations of the Slender Redfin based on 2016 survey data (Jordaan et al. unpublished data). Further surveys are required to determine the presence and location of invasion barriers in these rivers. In addition, the lower reaches of the Groot River within Meiringspoort is also within the Swartberg NR and has an intact subpopulation. This river is however not protected from alien fish invasion as its upstream reaches are outside a formally protected area. Viable Slender Redfin subpopulations also exist within the Gamkaberg NR Complex, which encompasses the headwaters of the Huis, Bos, and Assegaaibos Rivers (Jordaan and Bills unpublished data) as well as the Wilge, Buffelsklip, Marnewicks and Vermaaks Rivers associated with the Kamanassie NR (Jordaan et al. unpublished data). Several viable subpopulations also exist in rivers outside of formally protected areas. These include the Nels, Kruis, Wynands and Grobbelaars Rivers to the south of Swartberg NR, all of which were found to have viable populations based on a recent survey (Jordaan et al. unpublished data). Additional populations outside of protected areas include the Langtou River located near the town of Herbertsdale and the Groot-Doring downstream of Doringrivier Wilderness Area.

Habitat View more ecological information

Population trend

Trend

There are 21 subpopulations within mountain tributaries of the Gourits River System, namely the Holdrif, Wilge, Buffelsklip, Marnewicks-Vermaaks, Aaps-Groot (Meiringspoort), Nels, Meul (upstream of Dysselsdorp), Hoeks-Grobblelaars-Klein Le Roux, Buffelskloof, Dwars, Groot-Doring, Saffraan, Wilgerdal, Kobus-Seweweekspoort, Wynands/Vinknes, Huis, Kruis, Nels (upstream of Calitzdorp Dam), Oshoekshang, Waterkloof-Huis, Lemoenkloof-Elandspad/Mooifontein. These make up 12 locations in total of which 11 consist of subpopulations above invasion barriers namely Langtou, Aaps, Assegaaibos, Kobus (a large subpopulation exists in the upper end of the Kobus above an abstraction weir), Wilgerdal, Wynands/Vinknes (Large Department of Water Affairs (DWA) gauging weir in upper Wynands River), Saffraan, Vermaaks-Marnewicks (lower sections of tributaries dry as a result of water abstraction, thereby isolating subpopulation in upper tributaries), Buffelsklip, Dwars (subpopulation isolated as a result of water abstraction weir), Buffelskloof (subpopulation isolated as a result of water abstraction weir). The rest of the subpopulations form one location as Kruis, Groot-Meiringspoort, Bos, Seweweeks, Nels, Mooifontein (Elandspad), Huis-Waterkloof, Groot Doring, Wilge and Holdrif Nels (upstream of Dysselsdorp), Klein Le Rouxs, Grobbelaars, Hoeks, Meul (Swartberg MCA), Oshoekshang, Lemoenkloof-Elandspad/Mooifontein.The majority of these subpopulations are viable and largely secure against alien fish invasions and many populations occur within formally protected areas, mainly the Groot Swartberg NR Complex, Gamkaberg NR complex, Kamanassie NR and Doringrivier Wilderness area. These areas protect against impacts related to habitat (for example over-abstraction of water, disturbance of riverine habitats and pollution) but cannot completely safeguard against all downstream alien fish invasions. While instream invasion barriers such as small waterfalls and weirs are effective against invasion by Black Bass (Micropterus spp.), other invasive species particularly the Sharptooth Catfish (Clarias gariepinus) are generally more capable of invading headwater streams upstream of instream physical barriers. Rainbow Trout (Oncorhynchus mykiss) can invade upstream of Black Bass barriers and pose a potential risk to the Slender Redfin in headwater habitats. To date, no data exists on population numbers or trends, but the presence and viability of a number of subpopulations associated with the Groot Swartberg NR Complex and Gamkaberg NR complex was confirmed in 2015 and 2016 (Jordaan et al., unpublished data).

Threats

Slender Redfins, as with the majority of other native redfins of the region are susceptible to impacts from invasive alien fish species such as Black Bass, African Sharptooth Catfish, Banded Tilapia (Tilapia sparrmanii), Smallmouth Yellowfish (Labeobarbus aeneus) and Rainbow Trout. However, the preference of Slender Redfin for headwater streams has reduced the potential impact of non-native species which are currently abundant, and appear to prefer, the mainstem sections of the rivers. The exception to this is Rainbow Trout whose preferred habitat overlaps with that of the slender redfin. The Slender Redfin populations are however vulnerable in the lower foothills of the tributaries where no instream barriers exist (e.g. Hoeks, Groot and Grobbelaars Rivers). Other impacts, particularly in the lower foothills include bulldozing and channelisation (which disturbs instream habitat), excessive water abstraction (which may result in complete drying of lower river sections) and pollution also threaten the Slender Redfin, although populations which occur within formally protected areas are relatively secure. The African Sharptooth Catfish is reported to be present in dams in the Gouritz System but the extent of their invasion into the system is unknown. They have been shown to be able to invade headwaters streams (Ellender et al. 2014) but their impacts in these environments, and thus on the Slender Redfin, remains to be scientifically documented. This species was recently recorded in the lower Groot River near De Rust (van der Walt, pers. comm). This is a significant cause for concern given the conservation value of this river.

Uses and trade

All indigenous fish species of the Western Cape Province are listed as Endangered Wild Animals under Schedule 1 of the Provincial Nature Conservation Ordinance for the Western Cape, thereby preventing the collection and trade of the species without a permit. Translocations of indigenous freshwater fish species are controlled by Cape Nature’s Indigenous Fish Utilisation policy (Jordaan et al. 2016).

Conservation

No specific conservation actions are proposed for this species at present, however a large portion of subpopulations are distributed within protected areas. The presence of the African Sharptooth Catfish in the Groot River should be closely monitored.

References

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