The dominant fish species in Kruger is the common barbel (Clarias gariepinus). Its popularity with crocodiles has led to its description as the "impala of the river". Barbel prefer the slower-moving parts of the river unlike stronger swimmers such as the mudfish (Labeo), yellowfish (Barbus marequensis) and the well-known tigerfish (Hydrocynus vittatus).
One of the most extraordinary phenomena in Kruger is the life cycle of the freshwater eel (Anguilla). During their feeding stage – also known as the yellow eel stage – they live in Kruger's rivers, preying on fish and crabs. According to natural history scientist Dr Andrew Deacon, this stage can last between three and 20 years.
Dr Deacon explains that at some point the eel is ready to breed and makes its way downstream to the Indian Ocean. During this journey the eel undergoes a dramatic metamorphosis from a freshwater creature into a sea fish – the silver eel stage. It changes colour from yellow to silver or white, its eyes become larger and its snout more pointed.
On its way to the ocean the eel stops feeding altogether, its digestive system shrivels and its anus may even close. As it gets closer to the ocean its reproductive organs develop and breeding begins. Amazingly, the eels swim to the other side of Madagascar to lay their eggs and are never seen again.
Millions of their eggs are washed back down the African coastline where they hatch among plankton into transparent, leaf-shaped larvae. As they get swept past river mouths they undergo another transformation, triggered by the smell of fresh water in the sea. In what's known as the glass eel stage, they rapidly develop muscles and make their way to the source of the fresh water. As they swim upstream into the lowveld's rivers, they become miniature eels known as elvers.
They are able to clamber up waterfalls and rapids until they reach a point in the river conducive to developing into the yellow eel stage. There is a record of an eel that swam over 1?000km up the Zambezi and somehow managed to get up the 100-metre cliffs of the thundering Victoria Falls.
It was found in yellow eel stage in a deep pool above the falls. Other sea fish sometimes make their way upstream too – a saltwater bream was caught in the Crocodile River near Crocodile Bridge in 1970.
Amazingly, a Zambezi shark was caught at the confluence of the Limpopo and Luvuvhu Rivers in 1950. Although Zambezi sharks can adapt to freshwater conditions, this was an extraordinary occurance as the 2-metre fish had swum some 400km from its natural habitat in the Indian Ocean.