Effectiveness of a mechanical removal of invasive spotted bass to conserve endemic fish in a Western Cape headwater stream

Mix, Miranda

Effectiveness of a mechanical removal of invasive spotted bass to conserve endemic fish in a Western Cape headwater stream

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

FitzPatrick Institute of African Ornithology

Faculty

Faculty of Science

Abstract

Freshwater fish are among the most threatened taxa globally due to many human-induced threats, such as pollution, habitat degradation, flow alteration, climate change, overexploitation, and the introduction of non-native species. The Cape Fold Ecoregion, located within the Western Cape and Eastern Cape provinces of South Africa, is a biodiversity hotspot for freshwater fish. The Olifants-Doring River System is home to more endemic fish species than any other system in the Cape Fold Ecoregion, but many of its native fish species are threatened by the presence of invasive black bass (Micropterus spp.). The Breekkrans River, a tributary to the Doring River, is of heightened conservation value due to the presence of five endemic species, one of which is Critically Endangered and has a population estimate of only a few hundred individuals (Doring fiery redfin, Pseudobarbus sp. nov. ‘doring'). Native fish populations have been reduced dramatically throughout the Breekkrans River due to the presence of spotted bass (M. punctulatus). From January 2023 to December 2024, an eradication effort was conducted to remove spotted bass from 8 km of the Breekkrans River using mechanical methods to reclaim critical habitat and extend the ranges of endemic fish species present in the river. I analyzed data from the removal effort and from three monitoring surveys that were conducted throughout the upper Breekkrans River to assess how the spotted bass population responded to the removal effort, which gear types were most effective at removing different bass age classes, and how the native fish populations responded to bass removal. While spotted bass abundance has been reduced to near zero within the upper Breekkrans, they have not yet been completely eradicated. Specific gear types strongly selected for incomplete size ranges. Hand nets and seine nets selected for young-of-year (YOY) bass, whereas fyke nets, gill nets, and spearguns selected for juvenile and adult bass. Even though spotted bass persist in the system, native fish recovery is already underway, and there are signs of recruitment of native fish within the bass removal zone. Despite the partial success of the removal effort, it appears that in certain situations, mechanical removals can be an effective way to remove invasive species and restore native fish communities while providing employment opportunities to people who live in areas with little economic opportunity.

Keywords

Pseudobarbus sp. nov. 'doring'

invasive

Reference:

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Masters

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