Inter-seasonal persistence and size-structuring of two minnow species within headwater streams in the Eastern Cape, South Africa

Abstract

Summary This study examined temporal variation in population dynamics and size structuring of two cyprinid minnows, Pseudobarbus afer and Barbus anoplus, in relation to their proximate physical habitats. Population estimates were determined using three‐pass depletion sampling during both summer and winter. The habitats were characterised by seasonal variation in all physico‐chemical conditions and spatial variation in substrata compositions. Whereas significant differences in population size were noted between seasons for B. anoplus, no differences were found between seasons for density and capture probability for either species. An increase in boulders was associated with increase in population size and density for P. afer; for B. anoplus, increased percentages of bedrock and bank vegetation were associated with an increase in population size and probability of capture, respectively. According to Canonical Correspondence Analysis, size structuring in P. afer was explained predominantly by seasonality, with smaller length classes associated with the seasonal variable of summer, while larger length classes were associated with pH that was higher in winter. By comparison, for B. anoplus, the habitat variables – bank vegetation and bedrock – accounted for much of the explained variance for size structuring. Recruitment appeared to be the major driver of size structuring for the two species; refugia, especially boulders and bank vegetation, also appeared to be important. Overall, the two species were adapted to the headwater streams that were generally variable in environmental conditions. Potential invasions by non‐native invasive fishes that occur within the mainstream habitats threaten these two species. Efforts should continue to protect these minnows from such invasions by constructing barriers to upstream migration of non‐native fishes into these headwater habitats.

Innate responses to conspecific and heterospecific alarm cues in the endangered eastern cape redfin Pseudobarbus afer

“…This is one of the first demonstrations of this phenomenon for African fishes, one other concerning the aquaculture of African sharptooth catfish Clarias gariepinus (Burchell 1822) (van de Nieuwegiessen et al ., ), although presumably many other African fishes possess such chemicals. The main response was to dart to shelter, which corresponds with their preferred habitat type (Kadye & Booth, , ) and the normal behaviour shown in this study. However, contrary to predictions, redfins reduced the time they spent motionless in response to alarm cues.…”

“…The consequently reduced distribution of P. afer sensu stricto suggests its conservation status may be underestimated. Redfins would therefore benefit from conservation of existing populations (Kadye & Booth, , ; Chakona & Skelton, ) and assisted reintroduction. However, it is also under threat from invasion by novel predators (Ellender et al ., ; Ellender & Weyl, ; Kadye & Booth, ).…”

“…First, we examined normal redfin behaviour and hence their expected threat response behaviour. As a major requirement of redfin habitat is sufficient shelter options (Kadye & Booth, , ), we predicted that normal redfin behaviour will be based around available shelter. Second, we assessed whether redfins possess and respond to conspecific alarm signals.…”

“…However, as we are concerned with innate responses, we used an allopatric, taxonomically related heterospecific cyprinid minnow, the chubbyhead barb Enteromius anoplus (Weber 1897). Chubbyhead barbs inhabit similar headwater streams in the Eastern Cape but do not co‐occur with redfins (Kadye & Booth, , ). Both of these small fishes are likely to be at comparable predation risk so cues produced by chubbyheads should be relevant to redfins and are expected to produce a response (Dalesman et al ., ; Seppӓnen et al ., ; Goodale et al ., ; Manassa et al ., ).…”

Environmental niche patterns of native and non‐native fishes within an invaded African river system

“…Studies elsewhere have shown that lower reaches of rivers are characterised by high richness that is mediated by trophic diversification that allows different fishes to occupy different trophic positions (Guo et al, ; Ou et al, ). Secondly, the general absence in the Great Fish River's upper mainstem section of the E. anoplus , which was historically widespread (Laurenson & Hocutt, ), suggest the likely impact of the IBWT in influencing both habitat availability for this species, together with the proliferation of multiple non‐native fishes, which include generalist predators such as C. gariepinus (Kadye & Booth, , ).…”