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Sean Marr photographing sawfin (Barbus serra) spawning.

Many invasive species go unnoticed. In fact, most people in South Africa will view an Argentinean ant and a European snail just like any other native ant and snail species. The same is true for aquatic systems: even the most obvious unusual patterns go unnoticed. Despite the high levels of endemism in South African freshwater networks (which exceeds that of plants) and in the Cape Floristic Region (CFR) in particular, these ecosystems are also hotspots for freshwater fish invasions. In the CFR, introduced fish species - specifically black bass (Micropterus spp.) from North America, trout (Salmo trutta and Oncorhynchus mykiss) from Europe and North America and sharptooth catfish (Clarias gariepinis) from South Africa are recognised as the most serious threats to the endemic fishes of the region by the current IUCN Red Data Listing. At a global scale, most Mediterranean-climate regions are also recognised as being hotspots of freshwater fish invasions.

In a paper recently published in the journal Diversity and Distributions, C·I·B PhD candidate Sean Marr and colleagues highlight the extent of freshwater fish introductions in five Mediterranean-climate regions (California (U.S.A.), central Chile, south-western Australia, the south-western Cape (South Africa) and the Iberian Peninsula (Spain and Portugal)). Conducting a regional level analysis of freshwater fish introductions, species presence-absence for native and non-native fishes were collated across the five regions. Taxonomic patterns in freshwater fish introductions were evaluated by comparing the number of species introduced at the taxonomic order level to the numbers expected from a random sample of the global species pool. Finally, the authors assessed factors that may have influenced multiple introductions of freshwater fish species in Mediterranean-climate regions.

The endemic, Cape galaxias (Galaxias zebratus), can be found in streams of the Cape Floristic Region from tributaries of the Gamtoos and Krom River systems in the east to the Cederberg Mountains (Olifants River system) in the west. The introduction of alien invasive species and agricultural activities such as unsustainable water extraction and insecticides are recognised as some of the most serious threats to the endemic fishes of the region.

High levels of endemism, from 70 to 90%, were found for the south-western Cape, south-western Australia and Chile. Despite the high rates of endemism, all regions have currently more non-native species than endemic ones. Ninety percent of non-native species in these regions belong to five taxonomic orders, with Salmoniformes (trout and salmons) being significantly over-represented in all regions. In the south-western Cape, the Perciformes (basses and cichlids) were also over-represented. Interestingly, these two orders are mostly comprised of recreational angling and aquaculture species. Previous introduction success and latitude of the source region (as a proxy for net primary productivity) were the best predictors of whether a species will be widely introduced in Mediterranean-climate regions.

This study indicates that fish introductions in Mediterranean-climate regions are largely driven by taxonomically-biased human interests in recreational fisheries, aquaculture and pet species. It also underscores the importance of managing current non-native fish populations and preventing further introductions. While it is acknowledged that there are valuable recreational fisheries based on non-native species, the majority of involved parties (aquaculture practitioners, recreational anglers, hobby aquarists) are unaware of the long-term impact of non-native fish introductions. Improved conservation planning and education will be vital to the long-term conservation of native freshwater fish communities and the appropriate strategy will be context dependent.

Marr, S.M., M.P. Marchetti, J.D. Olden, D.L. Morgan, E. García-Berthou, I. Arismendi, J.A. Day, C.L. Griffiths and P.H. Skelton. (2010). Freshwater fish introductions in mediterranean-climate regions: are there commonalities in the conservation problem? Diversity and Distributions 16: 606-619.

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