Hooo-Hooo Hooo-Hooo Vol. 13 Issue 01 - Page 4

WildLife Group of the SAVA No good deed goes unpunished Dr Emily Mitchell In the 1960’s and 70’s, drought and water uptake by industry, agriculture and forestry upstream from rivers upstream from the Kruger National Park (KNP) led to boreholes and dams being built to stabilize the surface water supply. From 1973-1994, excess hippos were lethally removed to keep the population of hippos at the estimated carrying capacity of the rivers. Small man made dams only contained a few animals, mostly displaced bulls. However, from 1994, hippo culling was curtailed and rainfall was abundant. The numbers of hippo in the rivers increased significantly, and many hippo colonised small dams as a result of territorial aggression. In 2005, following a dry summer and abnormally warm winter, the water levels in the Nhlanganzwane dam were low and the dam hosted a large hippo population. Fifty-four animals (including 9 zebra, 23 blue wildebeest, 7 white rhino, 2 lions and 2 cheetahs) were found dead around the dam between March and June. Many were too autolysed to determine the cause of death but macroscopic findings (swollen mottled friable livers, icterus, widespread subserosal and tissue haemorrhages and pulmonary oedema) as well as histological findings (liver necrosis) in the fresher cases were consistent with cyanobacterial poisoning. A severe blue-green algal bloom was present on the dam, with a raft of green scum on the down-wind side. The algae were identified as Microcystis spp. and the presence of toxin confirmed by a mouse toxicity assay. Subsequently, a fire in the area destroyed the grazing so most of the hippo moved away and the alga bloom subsided. In 2007, again in early winter, a similar mortality event associated with an algal bloom occurred around the same dam affecting at least 9 white rhino, 10 zebra and 10 wildebeest. The dam wall was breached and no further mortalities occurred. In April 2008, a mortality event occurred around the Shiloweni dam, 4 Extensive hepatic haemorrhage (white rhino). Image provided by Roy Bengis Note the raft of blue-green algae on the water’s edge, hippos and low water level in an affected dam. Image provided by Roy Bengis which also supported a large hippo population and had a significant algal bloom. The Shiloweni dam was drained and the hippo moved away. By July 2010, after the dam had re-filled and again supported many hippos, 17 zebra, 7 wildebeest and 3 white rhino were found dead. No further mortality occurred after the dam wall was breached.