Black bass (Micropterus spp.) in the Olifants- Doorn River system: distribution, distribution barries, predatory impact and management

Abstract

In the Cape Floristic Region the Olifants- Doorn River (ODR) system is a known biodiversity hotspot in terms of endemic freshwater fish. Eight of the 10 described native freshwater fish species are endemic to this river system. One of the main threats to these fish is predation by introduced predatory fishes. Three species of alien invasive black bass (Micropterus salmoides, Micropterus dolomieu and Micropterus punctulatus) were introduced into the ODR system during the 1930s but prior to this study, their distribution and impacts had never been quantified on a system-wide basis. This study aimed to clarify the current distribution, distribution barriers, predatory impact and best management options for black bass in the ODR system. This was achieved by conducting a system-wide survey of 578 km of stream covering 41 tributaries in the ODR system. Black bass presence was tracked upstream within each tributary to its uppermost distribution point where physical barriers preventing further spread were identified and described. Fish species composition, abundance and size were recorded directly above and below these barriers to quantify black bass impact on the native fish. This research demonstrated that since introduction, natural and human assisted dispersal has facilitated not only the establishment of black bass in the Olifants and Doring main streams but also facilitated the invasions into 22 tributaries. Based on survey results it was estimated that 81.5 % of the ODR system that was previously occupied by native cyprinids is now invaded by black bass. Assessments of native fish abundance and size distribution above and below black bass invasion barriers demonstrated that in invaded tributaries only adults of larger cyprinids (Labeo seeberi, Labeobarbus capensis and Barbus serra) were able to co-occur with black bass species. Smaller fish such as juvenile L. seeberi, L. capensis and B. serra and native minnows (Barbus calidus, Pseudobarbus phlegethon and Barbus anoplus) were absent from the black bass invaded reaches. The findings of this catchment scale study are consistent with findings from other studies in the region. As a result, most native fishes are now restricted to streams above the natural barriers that limit the upstream invasions of black bass. Black bass eradication from invaded reaches is therefore necessary for habitat restoration. Effective eradication will however depend on the presence of barriers to prevent re-invasion from downstream sources. To better understand what constitutes the nature of such barriers, this study characterised the natural barriers that inhibited black bass invasions in 17 tributaries. Natural barriers comprised of 15 waterfalls, two cascades and one chute ranging in height from 0.49 m to 3.5 m with an average vertical drop of 1.21 ± 0.67 m. These findings suggest that black bass have poor jumping abilities and the recommended height of artificial barriers as part of a black bass management program should be between 80 and 100 cm depending on the size of the tributary. As a result, the presence of natural barriers or the construction of artificial barriers to prevent black bass invasions is considered a vital component of native fish conservation projects. Finally, the study assessed the feasibility of black bass eradication from the 22 invaded tributaries in the ODR system based on eight criterion covering aspects of biological, physical, anthropogenic and logistical importance. This assessment showed that effective eradication was most likely only feasible in seven tributaries. Prioritisation of these seven tributaries for black bass eradication based on the threatened status of the resident native species, the land-use in the respective catchments and the tributary length available for rehabilitation indicate that the Breekkrans, Biedouw and Thee Rivers should receive the highest priority.