When rangers found a cluster of crocodile carcasses in the Olifants Gorge last year, scientists had more questions than answers. They could not find any significant fish kills which let the most probable suspects off the hook. Until now.
Summer has returned to the Kruger National Park and with it the cessation of crocodile deaths in the Olifants River. From May to September this year, 28 crocodile were found dead in the Olifants Gorge. This is a sharp drop from the previous years' 170 that fell over exactly the same time frame. This year, rangers were prepared and early detection, removal and burning of carcasses may have seriously brought down the mortality peak.
The disease that kills them is called pansteatitis, an inflammation and peroxidation of body fats, usually brought on by the consumption 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 of this year.
More interestingly, the sharp-tooth catfish (Clarias gariepinus) in the area, which usually would comprise about 60 % of crocodile diets in these systems, are showing signs of oxidative stress and 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.
In order to survive, our bodies require oxygen. However, this very chemical that gives us life also has the potential to kill our cells, by oxidizing our cellular membranes, the reason we age. Our bodies have evolved antioxidant systems to deal with the onslaught by the free oxygen radicals.
However if we were to be exposed to a large number of pro-oxidants like pesticides, herbicides, heavy metals and other toxic chemicals, our bodies' antioxidant systems may be overburdened, which is what is happening to both the fish and the crocodiles in the Olifants Gorge.
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 favoured 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 bio-monitoring 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.
Photos: Dr Danny Govender
By Dr Danny Govender