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Rationale (Changed due to Same category but change in criteria)

The Twee River Redfin (Pseudobarbus erubescens) is endemic to the Twee River catchment and its distributions in its tributaries has declined since 1987. It is likely that the population decline started with the advent of intensive agriculture in the catchment in the 1960s and 1970s. These declines have been accelerated by the introduction of several species of non-native fish, and it is possible that at least a 50% decline has occurred since 1987, but this is speculative. More substantiated records are needed to apply criterion A. The area of occupancy (AOO) is 44 km² according to the IUCN protocol using a 2x2 km² grid overlay and its extent of occurrence (EOO) is 82 km². Although it is known from two subpopulations, one from the interconnected rivers in the Twee River System and the second from an introduced subpopulation to the Tuinskloof Dam, these subpopulations are both small and isolated hence they are considered severely fragmented. Criterion B applies because of the very small size of the species distribution range, continuous decline as a result of increasing levels of threat (invasive alien fish, habitat degradation) and the fragmented nature of the population. It therefore qualifies as Critically Endangered under B1ab(i,ii,iii,v).

Distribution

Endemic to the Twee River catchment and its tributaries, part of the Olifants-Doring River System (ODRS) in the Western Cape Province of South Africa (Skelton 1974). It is restricted to a single quaternary catchment and is found in the Heks, Suurvlei and Middeldeur tributaries, and the Twee River to just before its confluence with the Leeu River (Impson et al. 2007). There is also an introduced subpopulation in the Tuinskloof Dam (Bills 2011, Jordaan et al. 2016). Its distribution range has contracted substantially in the Suurvlei, Middelvlei and Twee rivers due to alien fish invasion and habitat degradation (Marriot 1998, Impson et al. 2007).

Habitat View more ecological information

Population trend

Trend

This highly restricted species is only extant in two subpopulations. The first is from a single quaternary catchment in the connected Heks, Suurvlei, Middeldeur and Twee rivers (referred to as the Twee subpopulation from hence forth). The majority of this subpopulation has been reduced by invasive fish and there is very little suitable habitat where alien fishes are absent (e.g., upper Suurvlei, Heks and Middeldeur rivers). Twee River Redfin (Pseudobarbus erubescens) is absent or uncommon elsewhere in its former range because of the impacts of two extra-limital species, namely Clanwilliam Yellowfish (Labeobarbus seeberi) (predator) and Cape Kurper (Sandelia capensis) (predator and competitor). The most recent estimate for this subpopulation is 8,400 fish of which 4,100 are adults (Marriot 1998). Marr et al. (2009) did a follow-up survey using the sampling sites from Marriot’s study but results are not directly comparable due to different sampling methodology (snorkelling vs fyke nets). Marr et al. (2009) reported that while the distribution of Twee River Redfin did not change compared to Marriot’s study, there was evidence that the species is becoming less abundant and more localised in the lower Middeldeur and upper Twee Rivers.The second subpopulation originated as an experimental translocation in 2005 of 48 fish from the main Twee subpopulation to the Tuinskloof Dam, an impoundment that still falls within the Twee River catchment (Bills 2011). The fate of these fish were investigated in November 2015 and three fyke net efforts yielded 2838 fish (Jordaan et al. 2016). Length distribution data of a subsample of fish from the dam demonstrated an established population containing juveniles and multiple length cohorts of spawning-capable fish. Twee River Redfin can thus be considered fully established in this impoundment. The 2,838 fish sampled in this initial survey represent approximately 30% of the estimated population of this species in the wild based on the population estimate of Marriot (1998). As sampling effort was low (three fyke net efforts), population size in the 10 ha impoundment is likely to be considerably larger and may even exceed the source population in numbers. The conservation significance of this subpopulation from a genetic diversity perspective requires evaluation as the small founder population may have resulted in a genetic bottleneck causing low genetic diversity within the impoundment population versus the diversity in the Twee subpopulation (Jordaan et al. 2016). Two additional dams have since been stocked but these have not yet been surveyed to determine establishment. While the Tuinskloof Dam subpopulation is relatively large, the Twee River subpopulation is considered to be small compared with other redfin populations in the Western Cape. Both subpopulations are also isolated and the species is considered to have a severely fragmented population.

Threats

The biggest threat to this species is the presence of non-native fishes and several species have been introduced into the Twee River catchment. The first introduction was Cape Kurper, alien to the Olifants River System, but indigenous to most rivers of the Western Cape. A local farmer introduced Cape Kurper to an off-stream dam in the early 1980s in a misguided attempt to use an “indigenous” fish for mosquito control (Hamman et al. 1984). The species is now common and widespread in the Suurvlei River where it competes with Twee River Redfin for resources and adult Cape Kurper are also a likely predator on juvenile Twee River Redfin (Marriot 1998). Marr et al. (2009) reported that Cape Kurper has reached the full extent of its invasion in the Twee System and was the most widespread and abundant species in the Twee River.
In the 1980s Clanwilliam Yellowfish (Labeobarbus capensis) was introduced above its natural range above three waterfall barriers into the upper and middle Twee River by the Cape Department of Nature Conservation in a misguided attempt to create a sanctuary for an "indigenous" fish (Impson et al. 2007). This species is now common and widespread, particularly in the lower Twee River. Rainbow Trout (Oncorhynchus mykiss) and Bluegill Sunfish (Lepomis macrochirus) were illegally stocked into the catchment in the 1990s, and have invaded the Twee River, where they appear to be present in low numbers as a result of high summer water temperatures (26°C, a deterrent to trout) and strong winter flows (a deterrent to Bluegill Sunfish).
The establishment of a viable population of Twee River Redfin in an off stream impoundment (Tuinskloof Dam) is highly relevant for the future survival of the species, but as the impoundment is on privately owned land with limited access control, the introduction of non-native fishes such as Bass species (Micropterus spp.) is a significant risk. This risk is also applicable to other non-native species and other dams in the catchment as is illustrated with the illegal introduction of bass into the Tandfontein Dam. This impoundment supplies irrigation water to local farmers and spills into a downstream dam where there is a risk of bass invasion during high flows into the Middeldeur River, the site where there is still a viable section of the Twee subpopulation.
The other major threat is habitat degradation caused primarily by intensive farming of deciduous and citrus fruit. There is over-abstraction of water during the dry summer months and orchards are planted close to the river, resulting in a high likelihood of agrochemical pollution through runoff, spray drift and irrigation return flows. The impacts of pesticides on the Twee System has not been studied to date, but a range of organochlorines, organophosphates, carbamates, pyrethroids and copper-based fungicides are routinely used. The majority of these chemicals have high toxicities to aquatic fauna and, given the intermittent nature of pesticide application, it is reasonable to expect deleterious effects on resident aquatic fauna such as fish and aquatic invertebrates.

Conservation

The distribution range of Twee River Redfin falls almost entirely within privately owned land with the exception of some individuals occurring within the Hexberg Nature Reserve. The species is listed as Endangered by the provincial Nature Conservation Ordinance, preventing capture. The Twee River System has been identified as a priority freshwater environment for fish conservation (Impson et al. 1999) and is listed as a fish sanctuary in the National Freshwater Ecosystem Priority Areas (NFEPA) project (Nel et al. 2011). Conservation staff and researchers have made riparian landowners in the Twee catchment aware of the uniqueness of the river and its native fish fauna. Funding by the WWF Nedbank Green Trust enabled the implementation of the Twee River catchment rehabilitation project in 2014 which focused on alien plant clearing and creating awareness on improved agricultural practices with regard to deciduous and citrus fruit production. The removal of Cape Kurper from the Suurvlei River through the use of rotenone and the construction of a barrier weir to prevent re-invasion has been proposed by CapeNature (Marr et al. 2012). There are however concerns that agrochemical pollution and poor water quality in the river will prevent the recolonization of the treatment zone by Twee River Redfin from the upstream population and as a result this project has not been implemented yet.

Impson et al. (2007) recommended that suitable dams in the catchment be stocked with the Twee River Redfin to create additional refuges for the species. Jordaan et al. (2016) reported the successful establishment of Twee River Redfin in an impoundment following the experimental translocation of 48 Twee River Redfin to the Tuinskloof Dam in 2005. A total of 48 fish were stocked initially and a survey of the dam in November 2015 yielded 2838 fish from three fyke net efforts (see population section for details).

Twee River Redfin from the the highly restricted Suurvlei locality, (part of the Twee subpopulation) have been translocated above a small waterfall on the upper Suurvlei River in 2005 with the aim to increase the number of individuals at this site. A conservation plan for indigenous fish of the Twee River System is being developed by CapeNature. Captive breeding studies at the University of Johannesburg were undertaken for Twee River Redfin but were unsuccessful (O’Brien and Husted 2011).

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