Smallmouth Bass and the 2007 Western Cape Province State of Biodiversity Review

Earlier this year, Cape Nature (http://www.capenature.co.za/) released the 2007 Western Cape Province (WCP) State of Biodiversity Review (http://www.capenature.co.za/downloads/biod%20review.pdf). The report focuses on progress in freshwater fish conservation management since the last State of Biodiversity report in 2002. The report notes that considerable progress has been made in improving freshwater conservation knowledge and identifies progress made in revising the conservation status and conservation genetics of indigenous fishes, the biology and ecology of Clanwillian sawfin Barbus serra, the identification of priority rivers for alien fish eradication and quantifying the impacts of smallmouth bass Micropterus dolomieu and rainbow trout Oncorhynchus mykiss in the province.

Regarding the management of alien fishes the Review notes that the severe impact of smallmouth bass Micropterus dolomieu on indigenous fishes in the WCP was quantified. Ongoing research has revealed that the impact of predatory alien fishes affects the entire ecosystem, with significant changes in aquatic fauna and flora once bass have invaded a river and eliminated the indigenous fishes. Other important objectives met were the identification of priority rivers for alien fish control in the CFR, and a project under the auspices of the Cape Action for People and the Environment (CAPE) that will enable invasive alien fishes to be eradicated from priority fynbos rivers. The Review found that the effect of invasive alien fishes has been particularly severe with more than 15 invasive alien fish species recorded, including several highly detrimental species (e.g. carp Cyprinus carpio, smallmouth bass, sharptooth catfish Clarias gariepinus and rainbow trout). Many rivers are invaded, including mainstreams and tributaries. Surveys have repeatedly revealed that in river areas invaded by smallmouth bass, indigenous fishes (especially small species and juveniles of larger species) are absent or very uncommon. Predation on fish is not the only impact alien fish have on indigenous biota; they impact on other biotic components such as aquatic macro invertebrates and can alter habitat structure, which in turn affect ecosystem functioning.

Review Findings: Smallmouth Bass

Among the main research findings cited in the Review, include:

Darragh Woodford’s MSc thesis on the impact of smallmouth bass on indigenous fishes in the Rondegat River, Cederberg. This work, part of the Table Mountain Fund project on alien fishes in the CFR, quantified the severe impact of smallmouth bass in this river. This is the first time this had been done in the CFR. The key findings, reported on in Woodford et al. (2005), were as follows:

“Fish populations in the Rondegat River, a mountain stream in the Olifants-Doring system in the Cape Floristic Region, South Africa were surveyed to assess the impact of predatory alien invasive smallmouth bass Micropterus dolomieu on the indigenous fishes. This was the first such attempt to quantify the predatory impacts of M. dolomieu within this region. The Rondegat River is home to five species of indigenous fish and is partially invaded by M. dolomieu, which has penetrated the lower river up to a waterfall barrier. Seasonal surveys were conducted at five sites above, and five below the waterfall. Physical habitat was measured at each site. Four of the five indigenous fish species were absent at bass invaded sites. Labeobarbus capensis, while still present below the waterfall, appeared to have suffered a near-total loss of post-spawning recruits. Analysis of the physical habitat quality failed to explain the loss of indigenous species below the waterfall, although sedimentation may have increased the vulnerability of the catfish Austroglanis gilli to M. dolomieu predation by obliterating benthic cover. Consequently, predation by M.dolomieu was presumed to be the critical mechanism explaining the loss of indigenous fishes in the lower Rondegat River.”

Steven Lowe’s post doctoral work on the ecological impact of smallmouth bass in the Witte River at Bainskloof. This work is different from previous studies on M. dolomieu in the WCP that have focussed on the impact of bass on indigenous fishes (see Christie 2002, Shelton 2003, Woodford 2005). Lowe’s work looks at the impact of bass beyond this, by focussing on changes in the aquatic macro-invertebrate community and algal biomass. Preliminary unpublished results by Lowe show that:

• invasive M. dolomieu deplete indigenous fish and reduce the fish biomass;
• the invertebrate community composition is significantly altered at bass-invaded sites in a seasonally-dependent manner;
• differences between sites are not due to the environmental factors measured;
• the diversity of invertebrate taxa at invaded sites is significantly reduced although the abundances of some taxa (e.g. Baetidae and Simuliidae) are significantly increased;
• the drifting behaviour of Baetidae (the most numerous mobile invertebrates in the water column) is increased in invaded reaches (areas of reduced risk from predation);
• the size-class composition of some invertebrate populations is shifted towards larger individuals in bass-invaded reaches (probably due to decreased predation and/or increased speed of development); and
• algal density is significantly reduced in bass-invaded reaches from spring to late summer, but not in winter.

Invasive Species Programmes

Table Mountain Fund Project: The Cape Action for People and the Environment (C.A.P.E.) has provided the springboard for the development of projects to rehabilitate rivers infested with invasive alien fishes. In 2002, the Table Mountain Fund approved a project to quantify the impact of smallmouth bass on indigenous biota of a Cederberg stream and identify the priority rivers in the CFR for alien fish control programmes. The project was completed in 2005, and through a series of expert workshops, identified the following: criteria for evaluating the most suitable rivers for alien fish control; a final list of the most promising rivers for alien fish eradication; and the most suitable method for eradicating alien fishes. More at: http://www.capeaction.org.za/

Cape Project: This project is part of a Global Environment Facility (GEF) funded Invasive Alien Species Programme, a key implementing focal area of C.A.P.E. The aim of the Invasive Alien Fish component is to determine whether invasive alien fishes can be successfully eradicated from rivers that are priorities for conservation. Four rivers were selected for this project, namely the Krom, Rondegat and Twee rivers in the Greater Cederberg Biodiversity Corridor, and the Krom River in the Baviaanskloof Conservation Area

Read the Full Review online at: http://www.capenature.co.za/downloads/biod%20review.pdf