The introduction of alien freshwater fish species often acts as pathways for the introduction of alien parasites into South African aquatic ecosystems. Unintentional introductions of alien parasites raise concern among ecologists as these introduced parasites can have harmful effects on the populations of species that fulfil important roles in aquatic ecosystems. The presence and spread of introduced parasites can furthermore affect species that are of economic importance for the aquaculture industry.

A recent paper published by C·I·B researcher Olaf Weyl and co-authors, Denham Parker and Horst Taraschewski, reports on an assessment of an alien parasite that, once established, could have profound effects on South African eel populations. For the study, the authors examined the distribution, prevalence, intensity and abundance of the alien gill worm, Pseudodactylogyrus anguillae, on eels from four river systems in the Eastern Cape. This parasite is native to East Asian river eel populations and once settled on the gills of its host, P. anguillae feeds on gill epithelia and blood which, at high infection levels, can result in hyperplasia of gill tissues, fusion of secondary lamellae, and in some cases leads to hyperanaemia and reduced fitness of the host.

Two hundred and twenty seven Longfin eel (Anguilla mossambica) and twenty-six giant mottled eel (Anguilla marmorata) sampled from the Great Fish, Nahoon, Sundays and Kowie River systems were examined. Results from the study indicated that P. anguillae is not widespread in the Eastern Cape because the parasite was only present on the gills of A. mossambica sampled from the Great Fish River system where it infected 73.2% of the sampled population at a mean intensity of 63.8±34.3 parasites per fish.

Physiological tolerances are a key factor when establishing suitable habitats of a species, and therefore are an important tool for predicting the spread of an alien species. Pseudodactylogyrus anguillae is primarily a freshwater parasite that is also often found on eels from brackish environments, but is absent on eels from marine environments. As the range of P. anguillae appears to be restricted, the most likely pathway for introduction is a result of the freshwater introduction of infected eels. Unfortunately there is a lack of official documentation of eel introductions but the possibility of undocumented eel importations for experimental culture purposes cannot be ruled out.

While the study indicates that P. anguillae appears to be restricted to the Great Fish River system and/or its tributaries, there is evidence that infected eels have already been transported through the country and the threat of anthropogenic-facilitated translocation remains high. It is therefore clear that precautionary approaches are needed. These should include steps to mitigate the translocation of eels from the Great Fish River system to other water bodies and steps to prevent introductions of nonindigenous eel species to South African aquatic systems.

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To read the paper: Parker, D., Weyl, O. and Taraschewski, H. 2011. Invasion of a South African Anguilla mossambica (Anguillidae) population by the alien gill worm Pseudodactylogyrus anguillae (Monogenea). African Zoology 46(2): 371-377.

For further details contact Dr Olaf Weyl, e-mail o.weyl@saiab.ac.za