Abstract
Non-native rainbow trout Oncorhynchus mykiss have been widely introduced in the Cape Floristic Region (CFR) at the south-western tip of Africa and may pose a serious threat to endemic freshwater fishes in the region. Quantitative information about trout impacts in the CFR is scarce but is urgently needed to guide legislation and management efforts. We used a combination of comparative and experimental approaches to evaluate impacts of introduced trout on native fish populations in headwater streams draining the upper Breede River catchment in the CFR. Fish populations were surveyed, and 19 environmental variables measured, in 24 minimally-disturbed streams, half of which had been invaded by trout. The mean densities of native Breede River redfin Pseudobarbus burchelli, Cape kurper Sandelia capensis and Cape galaxias Galaxias zebratus, were 89–97 % lower in invaded streams than in streams without trout. Furthermore, while native fish were present at all 12 sites without trout, they were not recorded at seven of the 12 invaded sites. None of the measured environmental variables differed significantly between sites with and without trout, and distance-based linear models identified trout density as the best predictor of redfin and kurper density, while galaxias density was best predicted by other environmental variables (riparian vegetation, canopy cover, substrate length, site slope). Native fish ≤40 mm in length were largely absent from invaded streams, but generally abundant in streams without trout, and a field experiment confirmed that trout selectively consume small redfin. Taken together, these findings constitute evidence that trout have depleted the abundance of CFR-endemic fishes through size-selective predation. It is recommended that managers aim to prevent new trout introductions and consider eradicating trout populations where they pose a threat to highly threatened native species.
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Acknowledgments
Financial support was from the DST-NRF Centre of Excellence for Invasion Biology (CIB), the German Academic Exchange Service (DAAD), the University of Cape Town (UCT) and the Cape Tercentenary Foundation. JMS also acknowledges a UCT Postgraduate Publication Incentive Bursary. This research was conducted under permit 0035-AAA 007-00057 issued from CapeNature, and animal ethics clearance for the study was obtained from the University of Cape Town. We thank K Tilanus and MS Bird for field assistance, and ND Impson, SM Marr and SR Lowe for constructive discussions. H Waldron of the UCT Marine Research Institute at UCT conducted the nutrient analysis. We thank CL Moloney and D Pillay for analytical advice and I Hampton, OLF Weyl and two anonymous reviewers who provided valuable comments on the manuscript.
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Shelton, J.M., Samways, M.J. & Day, J.A. Predatory impact of non-native rainbow trout on endemic fish populations in headwater streams in the Cape Floristic Region of South Africa. Biol Invasions 17, 365–379 (2015). https://doi.org/10.1007/s10530-014-0735-9
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DOI: https://doi.org/10.1007/s10530-014-0735-9