Longfin eel, Anguilla mossambica (Photo credit: SAIAB/Bruce Ellender).
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.
Rhodes University student Bruce Ellender sampling eels with a fyke net.
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.
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 email@example.com