Backyard Aquaponics
A place to discuss aquaponics

I think that as with many things AP, you will probably find that at the end of the day it doesn;t make a great deal of difference..

HI all, I'm avoiding starting a new thread, but I'm just trying to work out my ideal stocking densities.

I'm planning on expanding another FT (but then the system should be complete). I'm not worried about water volume, more how many fish I should have, to have a safe hassle free, but productive system. I won't be eating many of them, so I'm happy for them to grow big and strong, and maybe sell them to the aquarium trade (I'd imagine a large Murray cod going for a bit on gumtree.)

In total, I've got 1700L of 20mm quarts gravel. 3x1mx1mx0.3m beds, 1x2.4mx1.2mx.3m bed.

And 12.8M of DWC, but I know that doesn't add much to the stock densities, but surely it'd add something.

I've got a RFF followed by a settling tank, which is all pumped into a MT, and the water is added back into the system, so nothing is wasted.

So what's my idea stocking densities?

Thanks

An old and overlooked question to which you probably have already got an answer to. Plus some of this I know you know but...


Simple answer: Stock enough fish in your FT so that when they reach eating size you will have about 20-25kg/m3 of fish in your tank.

eg if you are going to eat your fish at 500g and you have a 2000L tank then you should stock your tank with 80 to 100 fish. (20kg/m3 / 0.5kg/fish x 2000L /1000=80)

Stocking density is limited by the lowest capacity component of your system. So if you have a big FT but small filtration then your stocking density is limited by the size of your filtration and not by the size of your FT.

Assuming you have enough filtration the stocking density of your FT is likely to be limited by either the ammonia concentration or the dissolved oxygen concentration.

The capacity of a FT in terms of ammonia or TAN is based on the turn over rate (flow rate relative to FT size) and the efficiency of your biofilter. You can avoid the need to do any calculations on the capacity of your ammonia concentration by simply ensuring your flow rate is turning over the tank about once per hour. This rate is going to be higher than you need but it is a good rule of thumb that will be good enough or better than is required for almost all BYAP systems.

Theoretical maximum stocking density of a FT without pure oxygen addition depends on the fish species being cultured but is generally 40kg/m3 (Trout, Barra, SP, Cod). Some species that have a lower tolerance to low levels of DO (Dissolved Oxygen) can be kept at higher stocking densities but they will often do better if DO is kept high. Please do not misquote this figure. It is a theoretical maximum only. In practice in a BYAP system you need to run a stocking density much lower than this or you will end up with dead fish. It is this theoretical figure that is used to set the recommended stocking density of 20-25kg/m3 (roughly half). It is arbitrary and you can push it higher or have it lower without too much concern but as the stocking density increases the chances of killing fish rises exponentially. No one can tell you where to draw the line but the higher you go the riskier things get. You can mitigate this risk through design, backups and redundancy but in a BYAP system I'd recommend keep the density below 25kg/m3.

Hi Stu.

I'd forgotten I'd asked this one. And it's the information I was looking for. But it's still allowing more fish than I was expecting. Which is good, I'm still under what I should have in there.

Just curious, does the stocking rates change if the fish get bigger?

I think I'm going to get in trouble with the wife if I try and eat 'Speckled Jim', or 'Spikey', so I think they are going to get rather large being Murray Cod, and she doesn't eat fish. My current plan is to grow out some fish to try and sell on gumtree, try and make back a small amount that I've spent on the system.

So if a Murray Cod gets to a 1kg, or 2kg. Will they still need higher levels of filtration? Or will their feeding requirements go down do to them being older and not growing as fast?

My goldfish are getting very fat, so I think they are about to drop eggs soon (or when it warms up maybe), I hear goldies are easy to breed.

It depends on how you measure the stocking rate but for our discussion YES.

Stocking rates in AQ and hence in AP are based on the MASS of the fish in the tank but the capacity of the system is generally based on the feeding rate. If you are doing the design work you need to account for both factors.

For example you could have 25kg of fish in an IBC and have only a small amount of filtration if you only feed them a small maintenance diet. If you want to push their growth you obviously want more capacity (filtration).

In RAS it is always said that you should not be impressed by high stocking densities but by high feeding rates.

With your cod as they get bigger their mass is obviously increasing and hence the number of kg of fish per m3 of water in your FT will increase so hence the stocking rate is increasing. This is why it is important to base you initial stocking density (in term of number of fish) based on the size the fish will get to before they start being removed (stocking density based on mass).

For example many people like to eat a smaller fish than 500g. 350g is a snack for me but for some is a good plate size. If you were to stock your tank based on harvesting some at 350g then you can get away with stocking a lot more.

eg 1000L IBC could be stocked with 57 trout. Then as soon as some get to ~350g you pull out 17 leaving 40 that you grow on to 500g. When they get to 500g you could pull out another 10 and grow them on to 650g. Then pull out another 10 and grow the remaining 20 to 1kg. Based on this schedule the stocking density (based on mass) never gets above 20kg/m3. Even if it does you have a reasonable buffer before the density gets over 25kg/m3.

For most people this is not possible with trout or barra because they can't keep them all year round but for cod, perch or those in climates where they can keep trout or barra all year it is a workable strategy. Initially the feeding rate will go up as the fish grow but once the fish start getting harvested the feeding rate should be roughly constant. Yes bigger fish eat less per % of body weight but since they are bigger they eat more than smaller fish do.

With your cod they won't need more filtration because they are bigger they will need more filtration if you are feeding them more. If they get to 1kg with you feeding them say 2% body weight per day which at 1kg is 20g/fish/day but don't increase their feeding rate beyond that then they will still grow for a while until they get to a certain size where 20g/fish/day represents a maintenance diet (~2.7-4kg ) but their filtration requirement won't go up if you don't increase their feeding rate.

Thanks Stu.

I'm not planning on pushing it, but just for future reference, apart from dead fish, what are the warning signs that the system is over stocked? High nitrite levels? High ammonia levels?

I'm thinking I'm well and truly under my stocking levels. I still haven't seen any nitrites or nitrates, which I think might explain my poor growth for tomatoes in the DWC.

I'm doing a tank upgrade at the moment, and once it's all completed, not including DWC beds volume, there should be 6,000L+ of water. Does that mean I could potentially safely stock 240 * 0.5kg fish. Sounds like a lot of fish.

The most likely sign is oxygen stress. Lack of appetite, gasping at the surface, listless, etc.

As long as you have reasonable flow it shouldm't be TAN overload unless your biofiltration is not up the feeding rate. While this is possible in an AP system it is not likely.


Lots of people have good growth with 0 nitrates.



Is that your FT or the whole system? For stocking density in FT you only consider thw volume of thge FT.

I was including the swirl filter, so probably 5000L all up for the FT.

The sump will probably be another 2000L, DWC will add up to 3000L, and 1000L for the swirl filter. So 6000L in everything else.

So 200 x 0.5kg fish? Still seems like a lot of fish.

That would only be 20kg/m3.

It is quite a few but you do have a 5m3 tank. If you were running a commercial operation you would have double that without O2 injection and At least 4 times that with O2 injection.

One of the reasons I wonder about people stressing the need for solids removal components.

I'm going to need more fish then

What do the commercial people do to stop the oxygen starvation and other issues? Just massive amounts of air being pumped into the tank? And higher rates of water flow?

Pure oxygen injection or pure oxygen injection and higher rates of flow.

Actually some sytems just increase flow.

There are some sytems in europe that are flow through systems set down stream of hydro plants. They habe raceways stocked to 120kg/m3 and flowrates of over 500 times per hour. Also the water is cold so also has a higher [DO].

I'm guessing they've done the sums and decided it's worth it. But I would have thought increasing the flow rates to that point would cost a fortune in electricity/pumps.

I don't suppose any of those systems have information freely online?

they are flow through systems down stream of hydro plants.

Ahh, I thought you meant hydroponic plants, like long beds or something. Not hydroelectric.