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Climate change is projected to alter river discharge in every populated basin in the world. In some parts of the world, dam removal now outpaces their construction and the diminishing cost efficiency of dams in drying regions is likely to further increase the rate of removals. However, the potential influence of climate change on the impact of dam removals has received almost no consideration. Most dams have major biological and ecological impacts and their removal would greatly benefit riverine ecosystems. However, using model regions in the Southern Hemisphere, we highlight that artificial lentic habitats created by dams can act as refuges for increasingly imperiled freshwater fishes, and dams may also prevent the upstream spread of invasive alien species in rivers. We argue that, in these and other regions where the major impact of climate change will be to reduce streamflow and aquatic refuge availability, a shifting balance between the negative and positive environmental impacts of dams requires policy makers to include climate change predictions in prioritisation processes for dam removal.

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1. Invasions by non-native species can compromise the conservation value of otherwise pristine headwater streams. While both developed and developing countries recognize this threat, few of the latter have suitable budgets to implement control programmes. 2. This study assessed the effectiveness of a mechanical project to remove non-native rainbow trout Oncorhynchus mykiss from a 6 km section of the upper Krom River, a small headwater stream in the Cederberg Mountains in South Africa's Cape Floristic Region (CFR). 3. From October 2013 to February 2014, 354 O. mykiss were removed by angling (58%), fyke netting (28%) and gill netting (14%). This resulted in a marked reduction, but not eradication, of the O. mykiss population (fish relative abundance decreased from 0.53 ± 0.09 fish per net per night in October 2013 to 0.21 ± 0.09 fish per net per night in February 2014). Following the cessation of manual removals, the relative abundance of O. mykiss had increased to 0.56 ± 0.18 fish per net per night by March 2016, suggesting that without sustained removal effort, the population will rapidly return to its pre-removal abundance level. 4. Further work is needed to refine the methodology and test the effectiveness of mechanical removal of non-native freshwater fish in a variety of ecological settings in the CFR. This approach holds potential for meeting the dual goals of reducing the ecological impacts of non-native fishes and generating employment opportunities in line with the policy objectives of developing nations.

1. Freshwater fish are in decline worldwide as a result of introduced non-native species, impoundment, water quality changes, over-abstraction, and climate change.2. The Clanwilliam sandfish Labeo seeberi is an endangered migratory cyprinid endemic to a single river system in South Africa's Cape Fold Ecoregion. It has declined across its range and persists as fragmented populations in the Doring River system.3. One of the last recruiting populations occurs in the Oorlogskloof River, where three non-native fish species are present: smallmouth bass Micropterus dolomieu, bluegill sunfish Lepomis macrochirus (both alien and invasive), and banded tilapia Tilapia sparrmanii (extralimital). Impacts of these non-natives, together with climate change, may pose a serious threat to this important sandfish population and to other cohabiting native fish species.4. A 6 year data set, collected over 9 years and spanning 25 km of the Oorlogskloof River, was analysed to characterize spatio-temporal variation in sandfish abundance and size structure and to evaluate the relative impacts of biotic and abiotic factors on population trends. 5. Sandfish experienced a 92.6% decline in relative abundance from 2013 to 2018, driven by a 99.6% decline in young-of-the-year individuals. A combination of extreme rainfall events and drought appear to have played a key role in the decline and subsequently prevented recovery.6. Small sandfish (≤200 mm) were almost entirely absent from the centrarchidinvaded section of the Oorlogskloof but were relatively abundant where these centrarchids were absent, suggesting a strong adverse impact of these non-native species. Banded tilapia co-occurred with sandfish without any apparent adverse impacts on the relative abundance or size structure of sandfish. 7. Improved water resource management and preventing the further spread of nonnatives must be conservation priorities. Localized eradication of non-native

Morphological identification and molecular data (mtDNA COI) were used to resolve the taxonomic identity of a non-native freshwater shrimp in the Cape Floristic Region (CFR) of South Africa and to evaluate levels of genetic diversity and differentiation in the species' core natural distribution. The species was morphologically and genetically identified as Caridina africana Kingsley, 1882, whose main natural distribution is in the KwaZulu-Natal (KZN) Province, more than 1200 km from the point of new discovery. Subsequently, sequence data from natural populations occurring in seven rivers throughout KZN showed the presence of nuclear copies of the mtDNA COI gene (NUMTs) in 46 out of 140 individuals. Upon removal of sequences containing NUMTs, levels of genetic diversity were low in the alien population (possibly as a consequence of a bottleneck event), while varying levels of genetic diversity and differentiation were found in natural populations, indicating habitat heterogeneity, fragmentation and restricted gene flow between rivers. Following the present study, the alien shrimp has survived the Western Cape's winter and dispersed into a nearby tributary of the Eerste River System, hence posing an additional potential threat to endangered endemics. Understanding the biology of this alien species will aid detection and eradication procedures.

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