The Sundays Valley water-transfer canal, which distributes water and fishes from the Orange-Fish-Sundays
Inter-Basin Water Transfer Scheme to hundreds of irrigation ponds used for citrus farming.
Freshwater fish invasions are a major source of environmental homogenisation around the world; unique species
assemblages are increasingly being overwhelmed by large numbers of introduced species. While active human-mediated introductions
contribute to this process, there are also many passive pathways of fish introductions. Inter-basin Water Transfer (IBWT) schemes
is such a pathway, which transfers water from one river basin to another via a network of canal systems. These canal systems behave
as corridors, allowing fish species to move from one catchment to another. South Africa, being a water-stressed country, is one of
the world’s biggest users of IBWT to transfer water resources to areas with high water demand for agriculture and industry. The
socio-economic importance of these schemes makes managing them for the ecological integrity of the receiving catchments especially
Research into the effects of water transfer schemes as an invasion pathway for alien fishes was recently completed
in the Eastern Cape Province, South Africa. In a paper published in Ecological Applications, C·I·B researchers
Darragh Woodford, Olaf Weyl, Cang Hui and David Richardson assessed the effect of continuous propagule pressure delivered by a canal
network, on fish invasions in farm dams. By surveying fish communities in a series of farm dams with known ages and filling histories,
together with the number of fish entering them through the canal network, estimates of propagule pressure (the number of fish
transported per unit volume of water in the canals) and establishment rate (the time taken for populations to become established within
the ponds) could be produced.
The River Goby (Glossogobius callidus) was a rapid coloniser of most surveyed ponds, and received
far more propagules from the irrigation network than it needed to successfully establish populations.
The experiment showed that propagule pressure is a significant driver of establishment. Fish species are more likely
to establish populations in new habitats with increased propagule pressure, and the speed of establishment increases asymptotically
with increased propagules. A more troubling outcome of the experiment was that all species collected in the canal network were found
in one or more surveyed ponds, showing there are no barriers to arrival within the canal invasion pathway. This highlights the dangers
posed by IBWT infrastructure for increasing freshwater fish community homogenisation. The irrigation network provided the researchers
with an unprecedented opportunity to quantitatively assess the number of propagules involved in the establishment of introduced fish
populations. This natural experiment empirically demonstrated the long-held theory that the magnitude of propagule pressure can
determine the success or failure of an invasion, provided the invader is capable of reproducing in the receiving environment.
Read the paper:
Woodford, D.J., Hui, C., Richardson, D.M. and Weyl, O.L.F. (2013). Propagule pressure drives establishment of introduced freshwater fish: quantitative evidence from an irrigation network. Ecological Applications, 23(8), pp. 1926–1937.
For more information, contact Darragh Woodford at D.Woodford@saiab.ac.za
The African Sharptooth Catfish (Clarias gariepinus), here held by C·I·B Associate
Darragh Woodford, was uncommon in the canal network and failed to establish in most ponds. It’s presence in many of the ponds
despite its low propagule pressure nonetheless demonstrates how easily fish can use irrigation canals to enter new ecosystems.