Ecosystem change and the Olifants River crocodile mass mortality events

Abstract

Abstract. 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.

Perfluorinated alkyl acids in the plasma of South African crocodiles (Crocodylus niloticus)

“…The mortality events of the crocodiles, as well as other aquatic species, were attributed to environmental pansteatitis, a disease hallmarked by inflammation and subsequent hardening of adipose tissue (Osthoff et al, 2010; Huchzermeyer et al, 2013; Lane et al, 2013). To date, no definitive origin of for pansteatitis has yet been determined, though several hypotheses have been noted, which include possible contributions from anthropogenic sources (Larsen et al, 1983; Swanepoel et al, 2000; Ashton, 2010; Masango et al, 2010; Huchzermeyer et al, 2011; Oberholster et al, 2012; Woodborne et al, 2012; Dabrowski et al, 2013; Bouwman et al, 2014; Dabrowski et al, 2014). Of particular interest are those contaminants from anthropogenic sources that have been previously shown to affect lipid pathways and adipose physiology.…”

Pansteatitis in polluted Olifants River impoundments: nutritional perspectives on fish in a eutrophic lake, Lake Loskop, South Africa

“…The stable isotope analysis followed the protocol used by Woodborne et al . () and Dabrowski et al . ().…”

“…() and Woodborne et al . () were able to show that a concurrent trophic shift in sharptooth catfish and Nile crocodiles in the Olifants River Gorge was linked to an excessive assimilation of polyunsaturated fats in animals with pansteatitis when compared to healthy individuals from the same locality. This pointed to a change in feeding behaviour possibly linked to sporadic hydrodynamic change within the gorge and to presence of an oil‐rich prey species not usually preyed upon by catfish and crocodiles.…”

“…() and Woodborne et al . () who argue for a strong role of piscivory in the Olifants River Gorge. Here we explain the apparently anomalous occurrence of pansteatitis in Lake Loskop when compared to occurrence of the disease in the Olifants River Gorge and to provide a perspective on why the disease is not yet known from Lake Flag Boshielo.…”

Chlorinated, brominated, and fluorinated organic pollutants in Nile crocodile eggs from the Kruger National Park, South Africa

“…Parasitic and bacterial diseases have been excluded. The exact cause(s) of the steatitis is not yet known, but speculation includes microcystins from cyanobacteria in the water and food of the crocodiles (Myburgh and Botha, 2009), trapping of pollutants settling out of the water due to the effect of the river slowing down in the upper reaches of the Massingr Dam across the border in Mozambique (Osthoff et al, 2010), crocodiles feeding on dead and rancid fish caused by anthropogenic ecosystem impacts (Ashton, 2010), crocodiles feeding on dead and rancid fish from fishnets (Huchzermeyer et al, 2011), broad-scale cascades of environmental deterioration and pollution (Ferreira and Pienaar, 2011), crocodiles feeding on local catfish (Clarias garipienus) with steatitis with no known cause but pollution is suspected (Huchzermeyer et al, 2011), changes in the food web due to changes in the ecosystem, combined with a yet to be found extralimital fish species as vector of the cause (Woodborne et al, 2012), high concentrations of aluminium in the body fat of Nile tilapia (Oreochromus mossambicus) that interferes with cellular metabolism (including lipid-peroxidation) and affects predators such as crocodiles (Oberholster et al, 2012), a seasonal dietary change due to upstream migration of alien silver carp (Hypophthalmichthys molitrix) with a fatty acid composition different from local fish (Huchzermeyer, 2012).…”