Recirculating Aquaculture System Fundamentals (Part 5 of 6)

Recirculating Aquaculture System Fundamentals (Part 5 of 6)

Heating a commercial farm is still the biggest fundamental left unattended in the Highveld of RSA.


Heating a commercial farm is still the biggest fundamental left unattended in the Highveld of RSA. A commercial farm that produces for example 8 tons a month will run a cycle of introducing fry and harvesting 400g to 500g fish monthly, with an uninterrupted grow out period between 6 and 8 months. Without heating, fish will not eat or grow. Remember the fish must grow to a set size with in a defined period. If not, the farm will not be financially viable. Heating should take place for 6 months of the year. Temperature fluctuations need to be kept within 2˚C. Rainy weeks in summer and the entire winter period need heating to maintain cycles of health and growth.

Two methods can be applied:

1. Build within an insulated building with large capital expenditure and lower operating costs 2. Greenhouses designed to contain the heat with tanks that are insulated which is low capital expenditure with higher operating costs.

The system designer again will design a farm with two things in mind, capital investment to build the farm and the end return on the investment.

Personally I feel a well-designed greenhouse designed to contain heat is the way to go. All water surfaces can be insulated from conducted heat loss, bottom and sides of tanks. Today’s technology allows this to be done close to 90% efficiency at relatively low cost. Surface evaporation is the biggest loss of heat; it is also the biggest absorber of radiated heat from the sun. Trying to heat a million litres of water is almost an impossible task without spending a fortune. The idea is to build heat during the day using the radiated heat from the sun. And by heating at night reduce the loss of heat to an acceptable 1˚C to 2˚C. Bigger losses than this will cause stress to the fish and bio filtration. Portions of water can be covered to reduce evaporation if the system will allow. Low DO is more dangerous than the loss of temp. Many forms of heating can be employed but few are economically feasible.

The second fundamental the designer would need to address is how the heating is to be distributed between all tanks evenly and efficiently within the budget. Some systems between 40 000l and 1 000 000l are run with a central sump making heating a lot easier. Other systems are made up of smaller individual tanks normally 10 000l numbering between 4 and 100 tanks, not so easy from a heating perspective.

For heating to take place there are a few steps that are required; first, if the body of water that needs to be heated is 27˚C and ambient temperature is below 27˚C the water temp will drop. The heat source needs to exceed the 27˚C by at least double 54˚C minimum. 60˚C - 80˚C heated water is ideal. Secondly the point of heat exchange needs to be large and efficient enough to discharge the heat into the water. Culture water is filled with life, mostly bacteria and an assortment of organic and inorganic materials. Running this water through the heat source would kill and sterilize the water, consequently contributing to waste and potential contamination of the system. The bio film created would quickly block and or effect efficiency of the heat source.

The easiest form of heating is electricity using either heat pumps or heating elements. However both are expensive and unfortunately the supply of electricity has been unreliable. This is only an option for small systems.

Solar is a great way of complementing the sun during the day but cannot be used alone. When the sun is removed during cloudy days or night time the temperature will drop. Temperature drops larger than 2˚C will stress the fish and the growth and health will be compromised. To be effective, solar needs to be used in large volumes at great expense.

A kettle, boiler, geyser, donkey or furnace will heats a body of water on a continual bases using different sources of energy; electricity, diesel, heavy fuels, LP Gas, coal, wood or solar. This water is then exposed to the culture tank allowing the heat to be absorbed by the body of water by a closed system either high or low pressure. The heated water and the fish water do not mix.

Wood fire heat exchangers are effective with smaller volumes up to 200 000l of water if designed and sized correctly. Gas geysers complement a wood burning heat exchanger to increase the heat to the desired temperature. Wood can be stock piled during the year reducing the winter monthly expenses.

Gas geysers are very efficient at heating water but the cost can be huge. Unfortunately gas cannot be stockpiled easily and would be costly. Diesel and heavy fuels are in the same category.

Coal boilers or kettles are efficient but come at a large capital expense but the monthly operating cost is relatively low. Coal can be stockpiled easily.

Fish stress

In the natural environment Tilapia are exposed to large bodies of water where they can choose to be in fast running water or still areas. Food for the fish is normally abundant during warmer summer months with good growth and sparse in winter where the fish will be dormant.

Predators will remove any sick or old fish, and stocking densities are extremely low. Breeding fish will have their territories, and potentially life is bliss.

In a commercial farm none of the Tilapia’s natural environment is evident. The water current unceasing, food supply is consistent in size, flavour and quantity. The weak and unhealthy live in general population till death. Depending on strain and genetics of Tilapia the fish can be so aggressive they cannot make eye contact with each other. Some fish are hormonally treated, others with steroids and antibiotics.

Stress is a real and present danger to all livestock of any kind within a commercial environment. A stressed fish will not grow at an optimum rate. Stress causes severe impact on health and immune function of fish. Stressed fish develop high ecto-parasite burdens, organ damage, stomach ulcers, lowered reproductive performance, and lowered resistance to bacterial and viral diseases.

Stress is induced by the living conditions of the system. If the system is not correct and condition not perfect the fish will be stressed. As mentioned before, Tilapia are the diehards of the cichlid family, and as their reward we believe they can live in any conditions. They can, but they will only grow to be a financially rewarding commodity if the conditions are perfect not just adequate.

The fish needs to rest at some point of the day. The flow of water in the tank cannot resemble a waterfall because we want no settled solids or dead spots. Or a still tank with no exercise.

Have a qualified fish veterinarian check and maintain the health of your system on a regular basis, take the vets advice to heart and fix or upgrade where necessary. Address mortalities early and keep the fish healthy. Sick fish are stressed to death. A suboptimal system is a breeding ground for parasites, pathogens and disease.

Feed needs to be of the best quality. Growth and health go hand in hand. Ultra cheap feed is cheap for a reason. Seek advice from the TAASA farming community and don’t just follow the norm or availability of feed trends. Most tilapia farmers who have been around for a while will offer you critical perspectives on feed – they know what an important component this is in the tilapia growth cycle.

You are not going to change the identity of tilapia by stocking them at sardine like levels. Rather ensure every parameter of water quality is perfect (or very close to it). Less than perfect conditions with stressfully high stocking densities can be fatal to your fish and business.

In the last part of this series I will offer quick perspectives on farming Tasks, Biosecurity and the Environment

Written by:  - 21 Oct, 2020  
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