Kruger National Park (KNP) rangers and conservation management officials started draining operations at Shilolweni Dam near Tshokwane picnic site on Tuesday April 15, 2008 to prevent algal poisoning of the wildlife in the area.
KNP rangers from Tshokwane first became aware of the situation at the dam when vultures were seen circulating on April 4, 2008. After investigation, five zebra carcasses were found and Tshokwane section ranger, Steven Whitfield, suspected that blue green algae (cyanobacteria), known as Microcystis, was responsible for the deaths.
State veterinarian Dr Roy Bengis, from the department of agriculture's veterinary services, confirmed that the animals died from algal poisoning after full post mortems were done on two of the carcasses. Water samples from Shilolweni Dam confirmed that the water contained a high concentration of blue green algae.
One of the oldest known organisms on earth, blue-green algae were the first green ‘plants' on earth and are found in most water bodies in low numbers. However, they can multiply rapidly under the right conditions to produce algal blooms, great masses of algae that discolour the water. The algae produce strong poisons, which can damage the nervous system and the liver of animals. In the case of an algal bloom, these toxins concentrate in the water at much higher levels than normally occur.
The wind plays a major role in concentrating the algae, which blows across the surface of the water, piling up into dense clusters whose location depends on the prevailing wind direction. Although the algae were directly to blame for killing the animals, a variety of factors had combined to create the algal bloom conditions.
Most winter months the hippo population gradually fills up to about 70 individuals in the Shilolweni Dam as the surrounding waters and Orpen Dam dry up. This large hippo population defecates and urinates in the water, effectively fertilising the water for the benefit of the algae. Despite both dams filling up quickly after the first summer rains this year, the hippos never dispersed to Orpen Dam, as they did in the past.
Though not investigated in depth, it could be possible that the high density of Pistia stratiotes, the alien invasive water lettuce which the park is managing using a bio-control weevil, covering the surface water of Orpen Dam by mid-summer, may have deterred the hippo from moving there. "We understand that the large concentration of this algal poison was built up due to the high concentration of hippo currently found in the dam and we have found that the most effective way to discourage the hippos from visiting the dam is to lower the level of the water," explained Dr Freek Venter, the park's head of conservation management.
Rangers were also concerned about the high populations of both black and white rhinoceros which often visit the dam, and the decision was taken to burn the surrounding grass in an attempt to discourage grazing animals from visiting the dam until the draining operation
The pump apparatus has been visible from both the H1-2 tar road between Skukuza and Tshokwane and the tourist parking area near the dam itself. Shilolweni Dam is very popular as a game viewing site and is visited by thousands of tourists every year. "A number of options were considered before it was decided to drain the dam using floating pumps set up in the middle of the dam and, as this is the second case in the last 12 months where algal poisoning has caused multiple animal deaths, we are currently looking at permanent solutions to this problem," concluded Dr Venter.
The Nhlanganzwane Dam near Crocodile Bridge rest camp was breached in July 2007 after it was discovered that the same algal poisoning had killed more than 54 animals.