Population trend
Trend
Historically, the main stream Doring River has been poorly sampled with very limited quantitative data collected prior to 2000. Extensive surveys were conducted in 2003 and again in 2011 and 2013 (Paxton et al. unpublished). Clanwilliam Sandfish (Labeo seeberi) catch data between main stream sites was variable for all surveys and the species was generally absent or present in low numbers and only adult fish were detected. The Clanwilliam Sandfish subpopulation of the Doring River main stream as a whole can be characterized as relatively rare and heterogeneously distributed, with little to no recruitment taking place. This is supported by quantitative data from 2003, 2011 and 2013 Doring River main stream surveys. Figure 2 (see Supplementary Material) indicates that the Doring main stream system is dominated by predatory alien invasive fish species, predominantly Smallmouth Bass (Micropterus dolomieu), Spotted Bass (Micropterus punctulatus) and Bluegill Sunfish (Lepomis macrochirus). The relative abundance of Clanwilliam Sandfish sampled decreased from 5.8% of all fish sampled in 2003 to 0.9% and 5.1% of the total catch in 2011 and 2013 respectively. In both 2011 and 2013, the number of sites at which the species were caught decreased by more than 50% compared to the 2003 survey. Similarly, the catch per unit effort (CPUE) values decreased from 4.3 in 2003 to 0.6 and 2.3 fish per netting effort in 2011 and 2013 respectively. This illustrates the heterogeneous distribution of this subpopulation, which is likely to be influenced by their migratory and schooling habits, and environmental conditions such as flow and food availability, which would influence their habitat selection and distribution. The ca 50% decrease in presence/absence and CPUE over the last decade in the Doring section of the system suggests that the population is likely to be decreasing and becoming more fragmented in the Doring River main stream.
The available data suggest that the persistence of the current Clanwilliam Sandfish population in the Doring River may be due to the population now being predominantly comprised of old, large fish which are beyond the prey size class of the predatory alien invasive species. This is evident from the size class distribution of the different species sampled during the 2013 Doring River main stream survey (Figure 3; see Supplementary Material). No indigenous fish species smaller than 400 mm (for example no juveniles or sub-adults) were recorded, indicating that there is no or minimal recruitment taking place. In addition to the Doring River main stream, Clanwilliam Sandfish presence has been reported in a number of Doring River tributaries, namely the Biedouw, Tra-Tra, Matjies, Kransgat, Oorlogskloof-Koebee and Gif Rivers (Figure 1; see Supplementary Material). These tributaries were surveyed in 2012, 2013 and 2014. All tributary subpopulations are confined to very limited stretches of river where they are protected from alien invasive fish species by natural barriers such as small waterfalls (Van der Walt 2014). With the exception of the Oorlogskloof, all tributary subpopulations consist of very low numbers of adult individuals and are not considered viable. The Oorlogskloof Nature Reserve is the last remaining viable and recruiting subpopulation, and it is likely that this is the primary source of adult Clanwilliam Sandfish for the Doring River main stream, with only a minimal contribution to subpopulation growth from spawning in other tributaries. The Oorlogskloof Nature Reserve subpopulation is therefore critical in terms of the survival of this highly threatened species.