24 Jun 2014

A New Beginning for South African Native Fish

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A school of Fiery Redfin in the Rondegat River; it had been extirpated from the lower reaches of the river by Smallmouth Bass. Credit: SAIAB/O. Weyl

A school of Fiery Redfin in the Rondegat River; it had been extirpated from the lower reaches of the river by Smallmouth Bass. Credit: SAIAB/O. Weyl

In an article in this month’s Fisheries, an international collaboration between AFS and South African institutions resulted in South Africa’s first non-native fish eradication using rotenone in February 2012 in the Rondegat River, a small headwater stream in South Africa’s Cape Floristic Region (CFR) that had been invaded by Smallmouth Bass. The CFR is a global biodiversity hotspot with an exceptional degree of biodiversity and endemism. Better known for its rich plant communities, the region is also home to 17 fish species which occur nowhere else on earth. Most are restricted to a single river, or a tributary within individual rivers, which makes them particularly vulnerable to human impacts such as non-native fish introductions, habitat destruction, and pollution. This has caused many of the CFR native fishes to now only occur in headwater streams where these impacts are absent. Consequently, more than half of the endemic fishes are considered to be endangered. Hence, CFR rivers are key areas for conservation of biodiversity and in headwater refuges the main threat to the native fishes are non-native fish introductions. In the CFR, predation impacts from sportfishes such as Smallmouth Bass and Rainbow Trout are particularly severe because the native fishes did not coevolve with native predatory fishes. As a result, most native fishes now only occur in river reaches where non-native fishes have been unable to invade. To preserve the unique endemic fish fauna, removal of non-native fish from conservation areas is therefore a priority in this region.

In the Rondegat River, native redfin minnows and Clanwilliam Rock Catfish had been extirpated by Smallmouth Bass predation. It was hoped that removing the bass would result in the recovery of native fish populations, which were still abundant in the stream above a small waterfall that marked the upper distribution limit of the bass. Treatment and subsequent monitoring demonstrated that the project had been successful and that there was an almost instantaneous increase of fish diversity following bass eradication. The successful treatment culminated from a decade-long process of impact assessments and determining operational best practice, which was facilitated through collaboration among a South African nature conservation authority (CapeNature), the South African Institute for Aquatic Biodiversity, and the American Fisheries Society Fish Management Chemicals Subcommittee. The successful removal of alien fish and almost instantaneous increase in biodiversity was a significant triumph for native fish conservation in the region and should encourage further endemic fish restoration efforts within South Africa.

Threatened Endemic Fishes in South Africa’s Cape Floristic Region: A New Beginning for the Rondegat River, by Olaf L. F. Weyl, Brian Finlayson, N. Dean Impson, Darragh J. Woodford, and Jarle Steinkjer. Fisheries 39(6):270-279.