A common question that I encounter is “How many fish can I keep in my tank?”
Stocking densities are often the subject of pretty useless generalisations. I often hear people (who ought to know better) advise novices to put a given number of fish into a tank…..without qualifying the advice.
Because fish come in different sizes from tiny fingerlings to big ones, fish numbers are pretty meaningless when it comes to deciding on the capacity of an Aquaponics system. A much more accurate standard is kilograms of fish per 1000 litres of water.
The weight of fish that can be kept in an Aquaponics tank will depend on the following factors:
- the size of the tank – larger tanks offer a generally more stable fish rearing environment.
- the size of the fish – a hundred fingerlings in a 5,000 litre tank will hardly register while a hundred 500 gram fish in a 1,000 litre tank will be a real handful.
- the species of fish – jade perch and silver perch are relatively tolerant of poor water quality where barramundi will be less so. Less active fish like sleepy cod can be stocked in higher numbers than other species.
- the Dissolved Oxygen levels – trout require high oxygen levels; jade perch less so.
- the bio-filtration capacity of your system - needs to be adequate to deal with the peak ammonia levels that are likely in your system. The capacity should take account of periodic overfeeding and the effects of temperature extremes.
- the ability of your growing systems to remove nitrates from the water. While nitrates are not toxic to fish to the same extent ammonia or nitrite, they will become problematic once they reach a certain level.
- the level of risk that you wish to assume – you will be getting less sleep if you run 60kg per 1,000 litres than if your tanks contain 20kg per 1,000 litres.
Theoretically, the amount of fish that can be kept in any system is limited only by its ability to convert ammonia and nitrites to nitrates (bio-filtration) and to maintain appropriate levels of oxygen in the water.
For practical purposes, however, the level of risk is proportionate to the stocking rate. What this means is that, if you are just managing to stay ahead of the requirement for ammonia/nitrate removal and dissolved oxygen in the system, you will have very little margin for error or mishap.
A properly managed 2,000 litre tank could produce up to 100kg of fish per year. Higher stocking rates may be possible depending on your skills and your willingness to invest in good equipment.
In general, I recommend that you cut your teeth on twenty or or thirty kilograms of fish (per 1000 litres) until you have the appropriate level of knowledge and skill.
Assuming a harvest weight of about 750g (about 1.5lbs), this means that you could keep about 40 fish in a 1,000 litre tank. In anticipation that you might lose the odd fish, you might start off with about 50 fingerlings.
Note added – 2 May 2010: A comment from a reader has caused me to revisit the advice contained in the preceding couple of paragraphs. My reader’s observation that, responding to generalised advice with more generalised advice, is a point well made.
Also, premised on what I now believe about aquaponics systems that are not fitted with adequate mechanical and biological filtration, I suggest that, for many systems (and most species) even 20 – 30kg per 1,000 litres may be unsustainable.
Even stocking at this rate will require that you:
- exercise commonsense in your feeding
- have a regular water testing regime
- have warning or back-up (preferably both) systems in place
…if you are to avoid injuring or killing fish in the event of a power or equipment failure.
To summarise, generalised advice on stocking densities is worthless.
-o0o-
{ 9 comments… read them below or add one }
Ultimitaly it comes down the the amount of feed going into the system. Some species require1.5-2.0 lbs of feed per pound of fish grown and will have a much lower stocking density than a species that converts close to 1 to 1. Also, species that are slow to grow out, will eat less per day and can have higher densities, than species that grow quickly (tulapia and trout for example.) The bio load is all about the amount of input, in a perfect world, we would determine the amount of feed our biofilter could handle at peak and work backwards for the species we want to stock. UVI has some good data on how many kg’s of feed a 4×8 raft system will remove per day, with or without pre-filtration. Our challenge is we tend to stock fingerlings and grow them out in a single tank and so our bioload goes from small to maximum when the fish are full size. UVI uses 4 fish tanks pre system so they can maintain an even bio load, by stagering when they stock fingerlings. It’s not practical for us, but is the ideal setup. I have an extra growbed that is not currently in my system, but the system is plumbed and sized for it. As the fish grow, I’ll add the last grow bed and add enough plants to handle the growing load. I hope to catch this by watching the ammonia and nitrate levels.
Neal……the point of the post (which you seem to have grasped) is that stocking density is subject to a range of variables and is not something that easily lends itself to a doctrinaire guideline.
Agreed the one size fits all does not exist, when in doubt, stock lite.
Great post Gary!
When someone asks me a question about stocking densities, I often start out with the phrase, “It all depends”. I have carried over 2 pounds (.9kg) of Tilapia per gallon (3.8 liters) using a combination of excellent biofiltration, aeration, mechanical filtration and plant absorption along with small amounts of weekly water changes (10% or so). Many people prefer not to do regular water changes in their aquaponics systems, but if you are fish centric instead of plant centric (which I am), it helps tremendously with adding additional ongoing carrying capacity.
One last comment, many people ask, “how much can I GROW per gallon/liter”. This is not the same question as how much can I CARRY at any one time per gallon/liter. The “grow” subject is a matter of comparing a weight to time interval. I can grow a lot more pounds per gallon annually than I am carrying at any one point in time, as I am constantly harvesting (removing) some of the existing biomass (harvest size fish) that in turn is replaced with new biomass (new fingerlings). For a home grower, the best route is to harvest and replace to the extent that you maintain a steady biomass ratio. The system will run smoother and be far more predictable. To be fair though, it is easier said than done.
-Kellenw
Kellenw……I think that “it all depends” is the only way that anyone can answer that question…..and all of the other questions that relate to the aquaponic mythconceptions that I’ve identified.
Water changes don’t bother me unduly…..so long as we use the water we change out to irrigate plants.
Thanks for your contribution.
Though I agree with all (or most of) the above, I believe there is an even better way to express stock densities: not in kgs/m³ but in kgs/m³/hr as this includes the recirculation rate which is the defining factor for water quality. Look at nature: many fish live in schools in super high density. Some fish like Eurasian Perch have a long period in their life that they live in schools. Only when over 25 cm they become solitary. So density is not really an issue as long as fresh, healthy water is available. I have personally visited an aquaculture facility where catfish was stocked at densities over 400 kg/m³ (!!!), and the fish seemed very happy and healthy and doing well. Of course there was a huge biofilter to ensure water quality. These super high densities (mind you, I do not promote this) seem to help in avoiding cannibalism.
my two cents
frank
Frank……it seems that the stocking density for many aquaculture systems is limited only the capacity of the system for nitrification and oxygenation……so, in my opinion, your observation is sound.
Unlike humans (and other organisms like rats and pigs) which begin to demonstrate deviant behaviour when seriously overcrowded, some species of fish appear to react differently to high density living. I’ve been advised by very experienced aquaculturists to crowd certain species to subjugate their territorial tendencies…..thereby limiting cannibalism and deaths.
I think that some of the perception that attends overcrowding of fish (as being cruel) is probably another example of anthropomorphism……a tendency to attach human feelings and emotions to non-human organisms. I’ve heard this referred to as the ‘Bambi’ syndrome.
Hi Gary
I am not sure how suggesting that generalised advise on stocking densities is worthless and then offer generalised advise on stocking densities is helpful.
Further suggesting people start (cut their teeth) at 20 or even 30kg per 1000 liters without knowing what filtration they have is downright careless.
So, in part I agree that it is pointless to suggest stocking densities where no actual design has gone into the systems. This is a massive failure in duty of care by the folks selling systems without qualifying the design. It is not enough to calculate stocking densities based on the grow bed acting as a filter.
Further, stating that you “might lose the odd fish so start out with 50 fish”, is not a qualified statement of mortality rates. What does one do if they all live? It will mean your calculation goes from 30kg/m3 to 37.5kg/m3 and that is pushing into levels that require a great deal more skill. The starting out enthusiast or even professional should calculate their stocking densities without a mortality rate and work on not losing any. If none die (which is the ideal) then their filtration is designed for the entire load. If some die, it only makes a difference to the bottom line not the bio load. Trying to predict mortality rates that you can not possibly know and include that as part of your stocking density calculations will leave you and your fish in a bad position.
As much as I enjoy people trying to make a difference, I prefer that the information being provided is pertinent and accurate. If not it simply contributes to the confusion and ignorance surrounding aquaponics.
Cheers
Paul V
Paul V…….you can find my reply to your comments on the APHQ forum here.