Ecosystem change and the Olifants River crocodile mass mortality events

Nile crocodile (Crocodilus niloticus) mass mortality events in the Olifants River between the
Letaba River confluence in South Africa and Lake Massingir in Mozambique have been attributed to
pansteatitis: a disease that affects fat depots of the animals. The disease is also found in sharptooth catfish
(Clarias gariepinus) in the same area, and the cause of the disease is attributed to pollution. Although the
Olifants River Valley is polluted, the impact of interventions such as dam construction on biodiversity
receives little attention. We show that the onset of the pansteatitis epidemic in crocodiles and sharptooth
catfish at the Olifants/Letaba confluence coincided with back-flooding of Lake Massingir that changed the
Olifants River from a rock and sand substrate river to a clay substrate lake. Isotopic analysis shows that
sharptooth catfish shifted from a predominantly vegetarian to a piscivorous diet that is highly correlated
with pansteatitis prevalence, and crocodiles and tiger fish (Hydrocynus vittatus) show coincident trophic
level increases. The evidence suggests that the ecosystem change altered the structure of the lotic foodweb
and that an exotic or extralimital fish has invaded the confluence and is the vector of the pansteatitis
epidemic. The invasive fish species is yet to be identified. The pansteatitis epidemic is an unintended
ecological consequence of damming this river.