From May to September 2009, 28 crocodile were found dead in the Olifants Gorge.
This is far less than the 170 carcasses that were found over the same time last year.
This year, rangers were prepared and early detection, removal and burning of carcasses may have seriously brought down the mortality, says Dr Danny Govender, ecology scientist at the Kruger National Park in an article in the Kruger Park Times.
“The disease that kills them is called pansteatitis, an inflammation and peroxidation of body fats, usually brought on by the consumpt
ion of rancid fish fats. In this case death could not be attributed to a large scale fish mortality incident; however there was a small fish kill recorded in the Olifants gorge in June 2009.”It appears the sharp-tooth catfish (Clarias gariepinus) in the area, which usually would comprise about 60 % of crocodile diets in these systems, are showing pathology very similar to the crocodiles. It is hypothesized that these affected fish would be quite lethargic and may fall victim to a largely opportunistic predator, such as a crocodile. Consumption of this oxidized fat from live fish may be enough to trigger the disease.
Several simultaneous changes have occurred in the Olifants Gorge in the period preceding the crocodile deaths, including increases in river flow rates, rising of sluice gates at the downstream Massingir dam, a progressive eutrophication with subsequent blue-green algal blooms and a build-up of fine clay sediments in the areas favored by the crocodiles. Clay particles are also known to bind a large number of organic pollutants and heavy metals. Could a fish like the catfish that lives in the interface of water and sediment, close to the sediment bottom, be exposed to a chemical that is causing this change, this is what scientists are determining.
Catfish are usually not considered in aquatic biomonitoring due to their extreme tolerance to pollution and reduced flows; however in an interesting twist of fate they appear to be the most affected.
It is believed that it is not one chemical alone (as none have been found in high enough concentrations), but a concoction of a multitude of compounds that have acted synergistically to produce the current situation.
The good news for Kruger’s crocodiles is that though there has been a localised decline of crocodiles in the Olifants Gorge, the general picture has been that of growth, with all Kruger’s rivers showing an increase in population numbers in the latest census।
