I recently read a reference about our systems on an aquaponics gardening forum and the author of the post inaccurately implied that we were misrepresenting how much fish we can raise in our multi-tank systems. This post demonstrated a total lack of understanding of staggered stocking and multi-tank systems. Given that the person writing the post runs the forum, and can delete posts that are in conflict with hers, I don’t have a means of replying directly to that group. But, it made me realize that the concept of staggered stocking is a great topic for this Q and A column.
The reason all (except for our smallest hobby system) of our Clear Flow Aquaponic Systems® use multiple fish tanks is so that a grower can use staggered stocking to increase fish production and, therefore, increase the nutrient load to grown more plants. In staggered stocking, you have multiple fish tanks and stock them sequentially. The fish are stocked at a given size, typically 50 grams and then grow to maturity in that same tank, along with those same fish that are growing at the same rate.
Let me give you an example. We will assume you are raising tilapia in our Family Plus system, which has 4 100-gallon fish tanks, along with the proper filter tanks for converting fish waste to nutrients and raft tanks for plant growth. First, let’s look at the growth rate of tilapia. With proper feed rates, quality feed and good water quality, it takes 24-26 weeks to raise a tilapia from 50 grams to about 600 grams (1.5 lbs.), which is the size most growers harvest at.
When starting your system, you initially stock one tank with about 38 – 50 gram fingerlings. Then, 6 weeks later, you stock the second tank with the same number of 50 gram fingerlings. And, every 6 weeks, you repeat this in the remaining tanks. After 24-26 weeks from the date you stocked your first tank, and every 6 weeks after that, you will have a tank full of fish ready to harvest. When you harvest a tank, then you restock it with 38 – 50 gram fingerlings.
This means that in each fish tank in the system, you will raise 2 full crops of fish, one every 24-26 weeks. So, if you harvest 38 1.5 lb fish (57.5 lbs) from each tank twice a year, you will annually harvest 460 lbs. from the Family Plus system. The real beauty of this staggered stocking is that you never have more than one tank ready for harvest and, with sequentially smaller fish in each tank, you are never over the system capacity and have a steady supply of nutrients for the plants.
The density of the fish in the tank ready for harvest is 1/2 lb of fish per gallon of water. But, if you look at the total water volume of our systems, you are actually raising fish at a density of one twelfth lb of fish per gallon of water. This is a much safer ratio than any other method of stocking that I know of.
And, there are other benefits which include better nutrition and less stress for the fish. With staggered stocking you can get the right feed to the fish at each stage of growth. With a mixed size population in one fish tank, it is impossible to make sure the fish are eating the right sized pellet and getting the right nutrition for the stage of growth they are at.
When you have a tank of fish ready for harvest, you can net those fish out of that tank. If you have a mixed size population in one tank, you stress all of the fish when you are trying to net just the big ones.
So, all in all, when you compare using 4 – 100 gallon fish tanks to using 1 – 400 gallon fish tank, you can easily see the value of the four tank system. When you apply this to our larger systems that use 6 fish tanks, your output is every 4 weeks. And, if you install two of these modules, your harvest of fish is every 2 weeks. With three modules, the harvest is every 1 week, and so on. From both a production and marketing point of view, this method is far superior to a single fish tank aquaponic system.
All of our Clear Flow Aquaponic Systems® use the same multi-tank design and have the same volume ratios, water flow dynamics and components. We do that so someone who starts with one of our hobby system is using and learning a design that is scalable to commercial production.