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.