How were the fish in the Olifants affected when the river dried up?

By Dr Andrew Deacon In Kruger National Park

Between July and September 2005 the Olifants River stopped flowing for 78 days in the Kruger National Park (KNP). To assess the possible effects that this non-flowing period had on the ecology of the river, Dr Andrew Deacon was asked to do a fish survey and compare it with his previous monitoring efforts in the Olifants River. During the last week in July sites were sampled at the Mamba gauging weir on the western boundary where the Olifants River enters the Park. In the eastern part of the Park sites were sampled downstream of Balule towards the Mozambique border.

Andrew was assisted by a team from Scientific Services (KNP), as well as Dr Pieter Kotze and his team who are currently busy with studies on sediment releases from the Phalaborwa Barrage that caused enormous problems to the fish populations in the past.

The surveys were conducted by sampling fish with different kinds of nets and electro shocking. Using these techniques ensure that very few fish die during the sampling effort. Shallow water riffles and rapids were sampled with the electro-shocker (less chances of crocodiles getting to the researchers in the shallow water), deeper channels sampled with cast nets, and deep pools with seine nets dropped from a boat.

An effort was made to sample all the important habitat types: stony riffles for fish that need flowing water, fast flowing rapids for the fish needing the oxygen rich water of the turbulent water, marginal vegetation with overhang that acts as cover for smaller species, deeper flowing channels for the larger fish such as tigerfish, and pools with little flow for the fish that feed on the muddy bottoms.

The problem with non-flowing situations is that all the flowing habitats, such as riffles and rapids cease to exist as the water level dwindles; leaving only pools that become stagnant with time if no inflow is received. All the riffle- and rapid dwellers are forced into the pool situation where predators such as tiger fish, catfish, crocodiles and piscivorous (fish-eating) birds make use of the easy prey out of their habitat.
 

MAMBA GAUGING WEIR

Starting at the Mamba gauging weir the researchers were surprised that the diversity of fish did not differ much from the pre-2005 monitoring results. The sensitive habitats were healthy and they even encountered some of the more sensitive fish species such as the Lowveld catlet (Chiloglanis pretoriae). A number of minnow species (Barbus) were also present, all signs indicating that the earlier situation at the site was not as detrimental as expected.

On the western side of the Park, some 50km downstream of Mamba, more sites were sampled at Balule and further downstream. Starting with the riffles, a healthy population of the flow-loving catlets (Chiloglanis paratus) and mudfish (Labeo species) were encountered, indicating the re-population of these controls (connections between pools) after the no-flow situation of 2005.

However, when the marginal vegetation was sampled, both the researchers noticed a near-complete absence of fish in this normally busy habitat. Increased sampling efforts in the overhanging vegetation habitats did not reveal more than a few hardy catfish species (sharp-tooth catfish, silver catfish and squeaker). Only a handful of adult minnow individuals of about three species were found in all their efforts.

The situation was baffling. How did it happen that the Mamba area was relatively diverse in species, but the downstream area so impoverished? And even more surprising is the quick recovery of the sensitive flowing-water habitats, while the slower marginal habitats were showing a complete lack of species diversity.

Jacques Venter, KNP river bio-technician at Phalaborwa gave an answer to the first problem. The river at Mamba never stopped flowing completely for more than a few days. As water was released from the Phalaborwa Barrage in an effort to alleviate the situation in the river, some areas were kept flowing for most of the time, even if it was just a trickle. This kept the fish alive and at least no populations were lost.

When the river came down in flood during the rainy season, these fish spawned and apparently reproduced at a healthy rate. Certainly some fish moved down from the Blyde River catchment. The Blyde River did not suffer the same fate as the Olifants River since it never stopped flowing during 2005.

SO THIS EXPLAINS IT?

So this explains the better condition of fish populations at Mamba Weir. The lower part of the Olifants River lost all the surface flow downstream of Mamba Weir. The Olifants River has a very sandy bottom, and the sediment deposition increased due to huge sediment loads coming from the catchment. The sediment is dumped into the river because of massive erosion problems which originate in overgrazed areas, especially in the Sekukhuneland area.

When the river flow is very low, all the water just disappears into the sandy river bed. It might replenish pools, bringing some fresh water to the system. However, with most of the pools in the KNP containing herds of hippos that add to the deterioration of the already polluted water with their dung, the trickle of water does not alleviate the water quality problems of the river.

With no flowing water in the stonesin-current habitats (riffles and rapids), and lowered water levels retracting completely from the overhanging vegetation habitats, all the fish that sheltered in these habitats had to resort to the pools with their larger predators. Flocks of fish-eating birds were reported feasting on fish in isolated pools in the river.

Now this explains the disappearance of some of the fish, but why have the riffleand rapid dwellers recovered so well while the marginal vegetation assemblages are lacking? The answer most probably lies in the presence of downstream waterfalls and the Olifants River Gorge.

THE OLIFANTS RIVER GORGE

The Olifants River Gorge is a spectacular ravine carved into the rhyolite rock of the Lebombo Mountains just before the river flows into Mozambique. Due to the fact that the ravine consists of a narrow slit through the bedrock, the energy of flowing water keeps this gorge clean of sediment. Because of this the pools are very deep and the stony habitats clear of sediment.

The water does not heat up as much as in shallow pools and thus contains more oxygen. It is in these deep pools with cooler water that still retained some oxygen where many of the smaller fish species survived the noflow situation. The cracks in the bedrock and the sediment-free cobble on the bottom provided adequate shelter.

When the river started to flow strongly during the first good rains, these fish began their migration upstream under the protection of turbid and turbulent water. However, reaching two cascades or small waterfalls (three metres or more) in the river (one below the Trails Camp and one below Olifants Camp), it was only the catlets (Chiloglanis paratus) and mudfish (Labeo species) that were able to scale these obstacles. Both these fish groups have sucker mouths that enable them to slowly and deliberately work their way up wetted bedrock until they are over the migration barrier. All the other smaller fish species were stopped by these natural barriers.

Although the fish assemblage in the river might recover to near the 2005 levels due to fish moving down from the Blyde River, or some remnant populations breed and recover, too many setbacks might eventually compromise the resilience of the system. For instance, some sub-tropical species might not occur in the Blyde River and thus have no refuge. Or, since the sluice gates of the Massigir Dam are in place, the Olifants River Gorge will be flooded and the lack of the scouring effect due to flowing water will result in sediment deposition.

The sediment will then cover the exact habitat that was the reason for the survival of these fish. It therefore is very important that the river remains flowing without any stoppages. It takes weeks to refill subsurface reservoirs once a river stops flowing even for a short while. Constructive talks between the KNP and the Department of Water Affairs that took place this year might improve management of the system and prevent the river from losing its very important integrity.