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No to earth ponds and cages for tilapia in SA

BY Leslie Ter Morshuizen

The Government through the DAFF is promoting the expansion and development of aquaculture across South Africa.  This is delightfully good news for the Industry and people who are going to be trained, employed and become stake holders in this growing business.  However, I wish to raise a caution regarding the trend of wanting to erect pond and cage culture systems within the Country.
 
Both earth pond and cage farming rely on the use of (relatively!) inexpensive infrastructure whereby the ambient environment provides the correct growing conditions for the species and no form of heating or cooling is employed.  As such this works rally well to grow cold water species in a cool temperate climate or warm water species in the tropics, where the optimal growth temperatures are available for most months of the year.  However, in SA we have hot summers and cold winters across most of the country, making summers unsuitable for cold water species and winters too cold for warm water fish.  When I say too cold or too hot I am not referring to this threatening survival, as the fish may often be able to survive under the ambient conditions.  Rather I am concerned that the temperature it is above or below the optimal required for growth.  If you have a goldfish pond for aesthetic pleasure then this is fine, but in a commercial setting we need to be sure that we achieve near optimal growth from our fish throughout the year, otherwise our venture is unlikely to be economically viable.
 
People often say that they are based in a warm part of the country and will therefore be able to grow tilapia in earth ponds.  Please understand that this is untrue, SA has a cool to warm temperate climate, and you need tropical conditions to grow tilapia commercially in earth ponds and cages!
 
By way of example I have inserted a graph below which compares the optimal growth temperature for tilapia to the temperature at which growth is halved and the temperature below which there is no growth.  This has been overlaid with the average monthly ambient temperature, which is a good approximation of the temperature in earth ponds or reservoirs, for two `warm’ localities.  Mtubatuba is in the north of Zululand, one of the consistently hottest parts of the country and Polokwane is situated in Limpopo which similarly enjoys of the warmest weather in SA.
 

 
What you see from the chart is the following:
Mtubatuba – 0 months of optimal growth, 4 months of moderate growth, 8 months of minimal growth
Polokwane - 0 months of optimal growth, 3 months of moderate growth, 9 months of minimal growth
 
I respectfully request that we stop making plans to farm tropical species in earth ponds and cages in SA.  We have plenty of research data showing that growth is sub-optimal and it borders on misleading to continue giving people false hope by telling them they can farm tilapia or catfish under ambient conditions in SA.  The ONLY way either species can be economically viable is through the use of a recirculating system inside an insulated environment.
 
Carp would be a better species for farming in earth ponds due to their lower optimal growth temperature being around 24?C.  Admittedly the market and price for carp are restricted.
 
Trout is a different matter as the high lying areas of the country due provide micro climates that are cool enough throughout the summer for the farming of this species outdoors.  Be sure you select your spot carefully, otherwise you too will be erecting a recirculating system.
 
We welcome the support of DAFF in growing the aquaculture industry but I stress the caution that earth ponds and cage farming is not economically viable for tilapia and catfish in SA.  Yes they will survive in most places but they will not grow adequately for profit to be realised.The Government through the DAFF is promoting the expansion and development of aquaculture across South Africa.  This is delightfully good news for the Industry and people who are going to be trained, employed and become stake holders in this growing business.  However, I wish to raise a caution regarding the trend of wanting to erect pond and cage culture systems within the Country.
 
Both earth pond and cage farming rely on the use of (relatively!) inexpensive infrastructure whereby the ambient environment provides the correct growing conditions for the species and no form of heating or cooling is employed.  As such this works rally well to grow cold water species in a cool temperate climate or warm water species in the tropics, where the optimal growth temperatures are available for most months of the year.  However, in SA we have hot summers and cold winters across most of the country, making summers unsuitable for cold water species and winters too cold for warm water fish.  When I say too cold or too hot I am not referring to this threatening survival, as the fish may often be able to survive under the ambient conditions.  Rather I am concerned that the temperature it is above or below the optimal required for growth.  If you have a goldfish pond for aesthetic pleasure then this is fine, but in a commercial setting we need to be sure that we achieve near optimal growth from our fish throughout the year, otherwise our venture is unlikely to be economically viable.
 
People often say that they are based in a warm part of the country and will therefore be able to grow tilapia in earth ponds.  Please understand that this is untrue, SA has a cool to warm temperate climate, and you need tropical conditions to grow tilapia commercially in earth ponds and cages!
 
By way of example I have inserted a graph below which compares the optimal growth temperature for tilapia to the temperature at which growth is halved and the temperature below which there is no growth.  This has been overlaid with the average monthly ambient temperature, which is a good approximation of the temperature in earth ponds or reservoirs, for two `warm’ localities.  Mtubatuba is in the north of Zululand, one of the consistently hottest parts of the country and Polokwane is situated in Limpopo which similarly enjoys of the warmest weather in SA.


What you see from the chart is the following:
Mtubatuba – 0 months of optimal growth, 4 months of moderate growth, 8 months of minimal growth
Polokwane - 0 months of optimal growth, 3 months of moderate growth, 9 months of minimal growth
 
I respectfully request that we stop making plans to farm tropical species in earth ponds and cages in SA.  We have plenty of research data showing that growth is sub-optimal and it borders on misleading to continue giving people false hope by telling them they can farm tilapia or catfish under ambient conditions in SA.  The ONLY way either species can be economically viable is through the use of a recirculating system inside an insulated environment.
 
Carp would be a better species for farming in earth ponds due to their lower optimal growth temperature being around 24?C.  Admittedly the market and price for carp are restricted.
 
Trout is a different matter as the high lying areas of the country due provide micro climates that are cool enough throughout the summer for the farming of this species outdoors.  Be sure you select your spot carefully, otherwise you too will be erecting a recirculating system.
 
We welcome the support of DAFF in growing the aquaculture industry but I stress the caution that earth ponds and cage farming is not economically viable for tilapia and catfish in SA.  Yes they will survive in most places but they will not grow adequately for profit to be realised.

 April 06, 2017
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