Red List of South African Species

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Endangered (EN)
B1ab(i,ii,iii,iv,v)+2ab(i,ii,iii,iv,v)
Assessors: Albert Chakona Martine Jordaan Wilbert Kadye Riaan Van der Walt
Contributors: Dimitri Geelhand Tobias Musschoot Gert Boden
Facilitators: Dewidine Van Der Colff
Reviewers: Jörg Freyhof

Rationale

Pseudobarbus skeltoni had a broad distribution range in the past that encompassed the mainstem sections of the Breede and Riviersonderend rivers as well as other major tributaries of the upper Breede River, such as the Wit River (Chakona and Swartz 2013). Extensive surveys and comprehensive genetic analyses of all representative subpopulations of Pseudobarbus skeltoni in the Breede River revealed that this species now persists in three isolated localities; the upper Riviersonderend River (a major tributary of the Breede River), the Krom and Tierkloof rivers (both tributaries of the upper Breede River). After extensive and comprehensive surveys conducted across the Breede River and adjacent river systems between 2008 and 2015, these three subpopulations most probably represent the remaining natural range of this species (Chakona and Swartz 2013). 

The upper Riviersonderend River, the type locality is the remaining stronghold of this species where it has been recorded from about 2 km section of this river (Kadye et al. 2016). The Krom and Tierkloof Rivers subpopulations are unlikely to be viable in the long-term because they persist in very small sections of the rivers (less than 500 m). These subpopulations are both small and susceptible to invasion by Rainbow Trout (Oncorhynchus mykiss) and Bass species (Micropterus spp.) that are abundant in the mainstem Molenaars River. 

The distribution range of Pseudobarbus skeltoni is severely fragmented (small and isolated subpopulations) across its current distribution range, the species has an extent of occurrence (EOO) of 132 km2 (calculated using a minimum convex polygon around known localities) and an area of occupancy (AOO) of 12 km2 (calculated using a 2x2 km2 grid). There is continuing decline in EOO, AOO and habitat quality due to invasion by non-native piscivorous fish, intensive agricultural development, over-abstraction and pollution, bulldozing, channelisation of rivers and sedimentation. There is also a decline in subpopulation numbers and number of mature individuals. The species is assessed as Endangered under criteria B1ab(i,ii,iii,iv,v)+2ab(i,ii,iii,iv,v). The AOO and EOO sizes are close to the threshold for Critically Endangered and it’s important to note that the Krom river subpopulation is considered non-viable as the impact of invasive alien fish are severe. This subpopulation could be lost in the near future, hence this species needs to be monitored closely. 

Distribution

The species is endemic to the Breede River System where it is confined to three localities; the upper Riviersonderend, Krom and Tierkloof rivers (all tributaries of the upper Breede River) (Chakona and Swartz 2013). Museum records at the South African Institute for Aquatic Biodiversity (SAIAB) indicate that this species historically occurred in the lower Wit River (Bainskloof). Recent surveys in 2014, however, did not record redfins in the lower Wit, suggesting that this subpopulation may have been extirpated due to the presence of non-native alien invasive piscivorous fish (African Sharptooth Catfish (Clarias gariepinus) and Bass species (Micropterus spp)) which are now dominant in this section of the river (Chakona pers. obs., Lowe et al. 2008). Given the extensive surveys and comprehensive genetic analyses of redfins across the Breede River and adjacent river systems (Chakona et al. 2013), the upper Riviersonderend, Krom and Tierkloof rivers subpopulations most-likely represent the remaining natural range of Pseudobarbus skeltoni sp. nov (Chakona and Swartz 2013).

Population trend

Trend

The species is represented by only three fragmented subpopulations which persist precariously in the upper Riviersonderend, Krom and Tierkloof rivers. Remaining subpopulations represent less than 5% of the species’ inferred historical range. The long-term persistence of the Krom and Tierkloof rivers subpopulations is uncertain because they occur within very small stretches of the rivers and are both likely to be represented by small population sizes. Ongoing decline of the Krom River subpopulation is likely because the river has been invaded by Rainbow Trout (Oncorhynchus mykiss). Similarly, there are no barriers to protect the Tierkloof River subpopulation from invasion by Bass species (Micropterus spp.) and Rainbow Trout from the mainstem Molenaars River where these species have become established. The upper Riviersonderend River, the type locality, is the remaining stronghold of this species where the largest individuals were recorded from about 2 km section of this river (Kadye et al. 2016) and in preceding surveys in 2009, there was 87 large adults and more than 200 subadults were captured in two fyke nets and released alive (Chakona and Swartz 2013). This subpopulation, which represents the remaining stronghold of P. skeltoni, is however threatened by potential invasion by non-native piscivorous fish, particularly African Sharptooth Catfish (Clarias gariepinus) and Bass species (Micropterus spp.) which are dominant in Theewaterskloof Dam.


 

Threats

Invasion of the mainstem sections of the Breede, Riviersonderend, Wit and Molenaars rivers by non-native piscivorous fishes has severely reduced the historical distribution range of Pseudobarbus skeltoni. Remaining subpopulations represent less than 5% of the species’ inferred historical range. The future survival of the upper Riviersonderend subpopulation is threatened by possible invasion by non-native piscivorous fishes because there are no barriers to prevent upstream migration of Bass species (Micropterus spp.) and African Sharptooth Catfish (Clarias gariepinus) from Theewaterskloof Dam. There are serious concerns that if immediate measures are not taken, it will not be feasible to eradicate non-native piscivorous fish from the upper Riviersonderend River should this section become invaded because of the presence of deep pools which would provide ideal habitat for non-native species to proliferate. The Krom River subpopulation has already been invaded by Rainbow Trout (Oncorhynchus mykiss), whereas the Tierkloof River subpopulation is not protected from potential invasion by non-native species from the mainstem Molenaars. Other severe threats are intensive agricultural development, culminating in over-abstracted and polluted rivers, bulldozing, channelisation of rivers and sedimentation.

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. It is envisaged that the listing will be amended to include Pseudobarbus skeltoni and other newly described species of the region. Translocations of indigenous freshwater fish species are controlled by Cape Nature’s Indigenous Fish Utilisation policy (Jordaan et al. 2016).

Conservation

Despite all subpopulations of Pseudobarbus skeltoni occurring within formally protected areas (the upper Riviersonderend River subpopulation occurs within Hottentots Holland Nature Reserve and the Krom and Tierkloof population occur within the Limietberg Nature Reserve), the protection offered by these reserves is ineffective against invasion and impacts from non-native fish species. The non-viability of the Krom River subpopulation can probably be attributed to the presence of an established Rainbow Trout population downstream. This trout population is managed for long-term sustainability to promote the utilisation of Rainbow Trout within the reserve by anglers. This conflicts with the conservation objectives of protected areas (Weyl et al. 2015). Management of non-native fishes and building of weirs to protect remnant populations are immediate measures that are required to prevent the imminent loss of Pseudobarbus skeltoni. Feasibility of rehabilitating and constructing a barrier in the lower Wit River where the species once occurred should be explored as an option for possible reintroduction.

References 

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,]
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