I keep being asked, “How many fish can you grow in an aquaponic system?” The answer depends on how much space you have to grow fish and vegetables.
The number of fish you can grow in an aquaponic system is limited by the number of vegetables you can grow to balance out the system biologically. There have to be sufficient plants growing in the aquaponic system to soak up all the nitrates excreted by the fish.
See Murray Hallam’s short video below which explains why you cannot just use your swimming pool and throw in a load of fish and pipe the water around to the hydroponic part of your aquaponic system.
The University of the Virgin Islands system, for instance, is on an area of 0.05 ha. It has four 10 foot in diameter tanks rearing tilapia fish in series each one with a total volume of 7.8 cubic meters. The total volume of fish in water at any time is therefore 31.2 cubic meters . The fish are in the fish rearing tanks of the aquaponic system for 24 weeks. The tanks are stocked with fish at a small fish per gallon, each tank being stocked in series, so they can be serially harvested, and the fish are left to grow out. Remember that the fish put on weight during that time and are harvested when they reach around one and a half kilos each. This is very intensive fish rearing and the tanks have to be constantly artificially aerated or the fish would die.
These tanks use water which is recirculated constantly through the gravity-fed solids removal and degassing section and then the liquid waste flows down to six hydroponic raceways which cover most of the rest of the site. Each of these hydroponic raceways contains 11.3 cubic meters of water. The total volume of water in the system is 110 cubic meters. The total amount of water in the hydroponic section of the system is 67.8 cubic meters. However, the water in the solids removal and degassing section amounts to a further 11 cubic meters which is not accounted for in summing together the water volume containing fish and the hydroponic volume containing plants. All this water is constantly being recirculated round the system day and night and provided with adequate amounts of air. Solids are regularly removed three times daily.
These raceways are four feet wide and two deep. The water circulates through them and these raceways are also constantly aerated. The plants grow suspended in net pots in floating rafts that float on top of the water.
The surface area under plants in raceways is many times bigger than the surface area of the fish tanks. However the volume of water in the hydroponic section is not substantially more than double the volume in the fish tanks, which is adequate to remove dissolved nitrates from the water AS LONG AS ALL PLANT SITES IN THE FLOATING RAFTS ARE FILLED AT ALL TIMES WITH GROWING PLANTS. The total plant growing area is 214 square meters, but plant site spacing is considerably closer than in soil, since plant roots do not need to grow sideways to find nutrition. Adequate nutrition is directly supplied by the flowing fish waste water. So many more plants can be grown per meter than in soil and planting density is high.
This high planting density is also a factor when calculating how many fish can be supported comfortably by the aquaponic system. Many more fish can be supported by aquaponic system fish tanks than can be supported by an equivalent volume of fish rearing pond water. When stocking Nile variety tilapia in the University of the Virgin Islands system, the stocking rate for Nile tilapia is 77 fish per cubic meter, harvested every 6 weeks on a 24 week cycle between the four tanks. The stocking rate for hybrid red tilapia in this system is higher, 154 fish per cubic meter, harvested every 6 weeks in the same way on a 24 week cycle. That means the fish go into each tank as fingerlings at the above rates and stay there to grow and put on weight until they are harvested 24 weeks later.
How many fish can you stock in each tank? For Nile tilapia that is 600 fish per tank, and for red tilapia, which are smaller, 1201 fish per tank. AS LONG AS YOU HAVE THE CORRESPONDING AREA OF HYDROPONIC RACEWAYS STOCKED TO THE MAXIMUM AT ALL TIMES WITH GROWING PLANTS. The aquaponic system fish waste water is cleaned by the plants. How many fish you can grow is basically determined by how many plants you can pack into the raceways and have growing robustly. The water has to be flowing round the entire aquaponic system at all times and the air blowers have to be pumping air into the system via air lines and air stones in the tanks.
Fish growth is also affected by water temperature. Tilapia fish are tropical fish, and will die at temperatures below around 20 degrees centigrade. They also start to suffocate at temperatures over 30 degrees centigrade, which is why their area is in a shed, see the above picture. The shed provides shade and prevents too much sunlight reaching the tanks and causing algae to grow. The Virgin Islands are in the Caribbean, and are tropical, so there is no need to heat the water, but in temperate or torrid desert conditions controlled climate greenhouses and additional fish rearing tank temperature control are necessary.
Also, tilapia are robust fish and really tolerate far more intensive stocking conditions than many other species of fish, for instance, trout. For many other species of fish stocking rates should be at around 30 fish per cubic meter or even less. Rearing periods for fish such as trout and perch are not 24 weeks, but more like 2 years from fry to plate size in each tank. There are species specific conditions that have to be observed, for instance, trout are cold climate fish and prefer temperatures below 20 degrees centigrade, but above 10 degrees. For my article about rearing brown trout in aquaponic systems, see http://aquaponicsglobal.com/?p=162
Trout require much more fussy water conditions than tilapia, too, but trout water still produces bumper crops of vegetables.
You just cannot grow nearly as many of them in an aquaponic system as you can tilapia. And they grow more slowly.
An aquaponic system is a calculated balancing act between the number of fish and the number of plants it contains.