Rapid recovery of macroinvertebrates in a South African stream treated with rotenone
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South Africa’s Cape Fold Ecoregion supports a unique freshwater fish assemblage with many endemics. To mitigate impacts of alien invasive fishes on this unique assemblage, nature conservation authority CapeNature used rotenone to remove smallmouth bass (Micropterus dolomieu) from the Rondegat River. We investigated whether the rotenone treatments had an adverse impact on the aquatic macroinvertebrate community over the long-term, the first study of its kind in Africa. We monitored macroinvertebrates within treated and untreated (control) sites on multiple sampling events for 2 years before and 2 years after two rotenone treatments. We analysed the difference in invertebrate abundance between treatment and control sites before and after treatment, using generalised linear mixed models with sampling event as a random factor to partition out natural fluctuations in abundances over time. Populations fluctuated widely in control and treatment sites over the study period, and we found no effect that could be clearly attributed to rotenone. We conclude that macroinvertebrates recovered rapidly after treatment, probably through drift from untreated areas upstream, with no long-term adverse effects. We recommend that the presence of uninvaded upstream refuges that may provide demographic rescue be used as a key discriminating factor for future conservation purposed rotenone deployments.
KeywordsRiver management Ecological resilience Alien fish removal Non-target effects Biological monitoring
We thank Jenny Day, Dean Impson and Patrick Lane of CapeNature for support during the planning and implementation of the monitoring programme; Jannie and Sarie Nieuwoudt, land owners on the Rondegat River, for access and accommodation; and Francois Lamont and Edlyn Wolhuter, SAIAB, for logistical support. We acknowledge use of infrastructure and equipment provided by the SAIAB Research Platform and funding channelled through the NRF-SAIAB Institutional Support system. Two anonymous reviewers are also thanked and acknowledged for providing helpful and constructive comments which improved our manuscript. This work was financially and logistically supported by the Water Research Commission (Projects K8/922; K5/2261; K5/2538), the National Research Foundation (NRF) of South Africa, DST-NRF – South African Research Chairs Initiative of the Department of Science and Technology, Grant No. 110507, Professional Development Programme Grant No. 104911, and the DST-NRF Centre of Excellence in Invasion Biology. Any opinions, findings, conclusions or recommendations expressed are those of the authors and do not necessarily reflect the views of the NRF. Permit No. 0035-AAA004-00702 to conduct this research was issued by CapeNature.
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