How many fish? Stocking Density.
Just how many fish you choose to put into a new system is governed by so many factors. Fish species, volume of grow beds attached, through put of water, how well you manage the system, how big you allow them to grow before harvest.......on and on we can go.
Personally, I purchase 1 fish per 10 litres of water in my fish tank. Sometimes a few more if the mood takes me....right now I have 300 fingerlings in a 925 litre tank. They will be split up soon into two tanks, then later into 3 tanks, and so on.
Having learned my craft by hard experience I am very happy to stand by this formula without fear. They will all grow well and healthy and will all reach my dinner table in due course.
Whether one expresses stocking density in KG or number of fish is a moot point....It all depends.
Some will express the stocking density in number of fish per cubic metre, or in Kg per cubic metre.
If you wanted to apply some formula to the exercise then the stocking density would be expressed in Kg per M3. Most householders are content to know the answer to the “how many fish” question. To express an upper stocking density for home systems as a Kg measurement, then 30 to 40 Kg per 1000 litre fish tank volume is a good manageable number for a well found home based system. These numbers are at the very top end for home systems and should not be exceeded.
Another important variable is how much food will be put into the system. Most beginners tend to over feed their fish. Overfeeding puts extra load on the system and is just a waste of food. We will deal with feeding the fish in another article.
Let’s examine how it works.
100 new Jade Perch or Silver Perch or Tilapia fingerlings would be lucky to weigh one Kg, so at the beginning we could say we have stocked our tank to 1Kg per 1000 litre.
Obviously fish grow (well we hope they will) and at some time that 100 fish may well collectively weigh 30 or 40 Kg. When that day arrives start the BBQ immediately!!!!
That is the whole idea, isn't it? Eat them as soon as they are big enough !
I find that a 300g Jade Perch is a beautiful serving for one person. Even for me a fish lover, a 500g fish is a bit on the big side. My darling likes them to be around 200g so I catch a smaller one out for her.
If you had 100 fish weighing 300g each you will then have a total of 30Kg of live fish in the tank.
If you start your Aquaponics system with a population of one fish per ten liters of water then you need to understand that you must commence harvesting the bigger fish as soon as they are ready.
The fish will not always grow at the same rate (most species that is) and therefore the bigger ones will be harvested out as soon as they are ready. Not only will we keep the overall weight of fish in the tank in hand, but the actual number of fish will diminish. (because we are eating them in case anyone is wondering how we will do that.)
Variable fish growth rates are actually a really good feature for the home grower. The home grower can gradually harvest their tank of fish over several months rather than having to have a total harvest in one day. As they harvest two or three of the bigger fish a week, the smaller ones grow and the numbers in the tank diminish. The total weight of fish in the tank will remain within limits.
It is a reasonable expectation that no one will grow that 100 fish out to become 5Kg each monsters, or even one kilo fish, and have them all in the same tank all at the same size.
Don’t try to grow them out to one or two Kg fish because you have in your mind the same regulated minimum size limits as for wild caught fish. These are your own fish, you can harvest them whenever you want!
I have not hauled out an entire 100 fish at the one year mark and carried out a weigh in to see if I have gone past the total magic number of 30 or 40 Kg. It is just impractical to do that. They don't all mature at the same time...they just don't.
Even fast growing species such as Barramundi and Trout have growth rate differences, but not to the same extent as say, Jade and Silver Perch. If you are intended to grow out one of these species you will need to adjust your initial stocking numbers down accordingly.
If you are the kind of person who knows in your innermost self that you will not harvest the fish because you will start to see them as pets, then do not start with one fish per one thousand liters. Start with only twenty or thirty fish per one thousand litres .
Then there is the opposite of that, I call it the “bloke” effect. Some “blokes”, when starting out get excited about the idea of being able to go and harvest a fish whenever the mood takes, so they decide to double the number of fish to two or even three hundred fish per thousand liters. Don’t do it !
Those same “blokes” often have a “I must have the biggest fish” phobia. That is a bad combination which will lead to difficulty.
Some of the very experienced operators on my forum delight in posting photos of their monster Trout ..... damn those Trout. Trout grow so well in home Aquaponics systems.
"A" represents an empty tank holding 1000 litres or 250 gallons.
"B" We have 100 fingerlings in the tank. Total mass at the start , no more that one Kg.
"C" Some of the fish have "bolted" ahead of the rest and are approaching harvest size. Commecce harvesting as soon as they are ready.
"D" As you harvest the remaining fish will catch up and will be progressively harvested.
You must be prepared to manage the system well , and yes... harvest as soon as they are ready, you will have a wonderful result.
Have fun as you utalise Aquaponics to move towards self sustainability.