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Sustainable water management
by Gert Hendrik Oosthuizen
20 August 2012
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I often read about people being admonished for removing carp from our dams and thought I'd share a few thoughts on the matter.

A female carp generally lays around 300 000 eggs per spawn and can lay over 1 million eggs per year. The eggs are succeptable to fungal and bacterial infections and the spawn are preyed upon by a variety of small predators and birds yet thousands survive in an open water environment.

In good quality water and with enough food, the surviving spawn can grow to around 3kg in the first year. In optimal conditions (warm climate, clean flowing water, a stable natural food supply), carp can reach up to 5kg in their first year.

Carp are omnivores. They can survive on a vegitable diet but prefer high nutritional food sources such as insects, snails, mussels, crabs, shrimp, bloodworm, eggs, frogs/tadpoles, etc.

A stable waterbody can only sustain x amount of fish. Once the fish population in a body of water exceeds x the growth rate not only slows down, but larger fish start losing condition. I have banked carp with hollowed out bodies that should have weighed 6-7 kg due to their length but only managed half that on the scale due to abject conditions caused by overstocking. For the most part, the bait we use while practicing our sport offer little or no nutritional value to the fish we target so we are not adding to the sustainability of the body of water.

Any aquaculturist will tell you that over population leads to increased nitrate levels in the water. In a stable system, the nitrates are consumed by nitrobacter bacteria however, a natural system can only support a certain amount of waste be it through rotting vegitation, unconsumed bait or undigested excrement. The nitrate build-up leads to algal blooms which lead to catastophic biological melt down of the system killing almost everything in the water. In a home aquarium, this manifests itself as a murky substance in the water. In larger bodies of water it is not so easy to observe until thousands of dead or dying fish start washing up on the bank. During 2012, we have had two such occurances in Cape Town, one at the water treatment plant at Zeekoevlei and the other at Zandvlei, one of the most important estuaries in the Cape.

What is needed, is a manner of curbing the population much in the same way that game farms and wild life preserves practice culling. The easiest way of managing a body of water is through the introduction of predatory species. This has to be done responsibly or the carp population could be wiped out completely. Recommended species would include bass and certain species of talapia (kurper). Barbel (sharptoothed catfish) are not advisable as they will eat anything that fits in their mouths. Additionally, smaller fish can be removed in order to lessen the impact on natural food sources. If you do not eat carp yourself, there is always the odd farm labourer or roadside beggar that will really appreciate the donation.

As a specimen angler, I generally fish smaller closed systems that are easier to manage than large water bodies such as Bloemhof of Hartebeespoort for instance. On smaller water bodies, I advocate the removal of fish under 7kg in weight. This might seem extreme but if one considders the amount of fish spawned per year vs the relatively small amount caught on a body of water, there will always be smaller fish to catch and continue the propagation of the species as bigger fish die off due to natural causes.

We have a vast amount of water bodies that are old enough to produce world record class specimen but due to over population, we seldom see fish over 30lbs comming out. Anglers fish their entire lives and never breach the 20lbs mark. You can count on one hand the amount of 50 pounders banked during the past 5 or 6 years.

Only through successful water management can we hope to achieve sustainability and growth.



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