Gallons per fish: does a sweet spot exist?

[Ken Westervelt]

I heard a rule of thumb recently that you don't want to go much more dense than 4 or 5 gallons per full-grown fish.  I assume this has something to do with the amount of nitrifying bacteria available to support the system.

Anybody else have a different rule of thumb they go by? If less, do you need more equipment than a larger DO source?

[ericjf7]

The original question being, "does a sweet spot exist?". My general response aimed at the sweet spot being: the soup pot (somewhat smaller fish included), deep frier (a la street vendors in Asia increasingly selling small D.F. fish as a supreme delicacy), the back yard barbecue, or any other method of preparing delicious food, not necessarily only big fish . The larger number of fry for stocking only referred to setups with enough size to accommodate them, even in the interim, the higher the stocking numbers, the more important circulation and aeration becomes. The 'existing hole in the ground tank' I'm considering will hold upwards of 4000 gallons (plus the beds) and may have inspired me to use larger stocking numbers, the emphasis being on harvesting and removal as a valid option to keep the overall system in balance as the size of the fishes increases. For those not too squeamish, there could be other uses for (extra) fish, considering that a considerable portion of the fish harvest depleting our oceans goes towards manufactured feeds for commercial fish farms, farm animals, and domestic pet foods, although these would not be my first choices..

[TCLynx]

ericjf7 said:

For those not too squeamish, there could be other uses for (extra) fish, considering that a considerable portion of the fish harvest depleting our oceans goes towards manufactured feeds for commercial fish farms, farm animals, and domestic pet foods, although these would not be my first choices..

Yep, there are uses for extra fish, for those who are willing (gotta stay really on top of water quality when running this way but it is a valid option.)

I've found that my chickens really love the pumpkin seed sized tilapia fry. They really appreciate moving food that doesn't get away. I'm sure cats would enjoy such treats too but you might not appreciate the fish breath such a pet might grace your lap with in the evenings but it's All good

[dano]

I know this is very early on for me to say much on the matter. I was worried I wouldn't have enough fish emulsion to feed the plants even in the early stage of growth, but it seems the fish out pace the plants pretty fast. My fry have doubled in size in two weeks, and the plants are doing great [check my pics] but I wish I had more roots cleaning the water right now. PH is staying at 7.5

[John Lang]

New here, hello all
If the MAX rule of thumb is 1 lb of grow out fish weight to 5 gal water-grow bed media, this assumes that you harvest at 1 pound. I am trying to calculate if I had 100 cu ft of grow bed (similar to Rachel's example) I would be able to use max 100 - 1lb fish. My dilemma is, if my primary goal is vegetation and not fish harvest, my fish will continue to grow. In my case I have started with a small system and have 4 Rocky Mountain White Tilapia. I haven't had them for a year yet and have no idea how big they will get. I plan on breeding them or getting more of the same but only harvest on rare occasion for a meal, here and there. So most of them will grow to full size. What is the full size of a Rocky Mountain White? Is it 2, 3, 5 lbs? This I believe will be a major determining factor in the sizing of my system, maybe not. Any thoughts on this?
John

[TCLynx]

If you don't want to be eating or harvesting fish regularly, keep the numbers of fish much lower than the max or even recommended levels. You should still have plenty of nutrients for the plants.

While the recommended fish level for a system with 100 cubic feet of grow bed media may be 100 fish (grown out to 1 lb each.) Such a system could easily be supported by far fewer fish once it it stabilized. How much fewer fish, well that might depend on how greedy the particular plants being grown are and how high the protein is in the feed being used for the fish.

How big do tilapia grow, well the biggest Blue Tilapia I've heard of are around 9 or 10 lb. I've probably never seen any bigger than 3 lb myself but I suppose if you are keeping the tilapia for years then perhaps you could approach the 9-10 lb range though the Whiles are a cross between two types of tilapia and I've read that the blues tend to be some of the larger species of tilapia.

Now just to figure things right here, you are saying the system has 100 cubic feet of media (that would be over 700 gallons of media.) How much fish tank? And is there a sump tank? If the fish tank is 300-400 gallons and the sump tank takes care of the water level fluctuations then 30-50 small fish might be a good starting number and as they get bigger, select for the traits you like best and eat the rest until the system has the appropriate number of breeders. If that system also has the fry/fingerling tanks, then the really high protein feeds for them will help make up the difference in nutrient levels to get good plant growth.

John Lang said:

New here, hello all
If the MAX rule of thumb is 1 lb of grow out fish weight to 5 gal water-grow bed media, this assumes that you harvest at 1 pound. I am trying to calculate if I had 100 cu ft of grow bed (similar to Rachel's example) I would be able to use max 100 - 1lb fish. My dilemma is, if my primary goal is vegetation and not fish harvest, my fish will continue to grow. In my case I have started with a small system and have 4 Rocky Mountain White Tilapia. I haven't had them for a year yet and have no idea how big they will get. I plan on breeding them or getting more of the same but only harvest on rare occasion for a meal, here and there. So most of them will grow to full size. What is the full size of a Rocky Mountain White? Is it 2, 3, 5 lbs? This I believe will be a major determining factor in the sizing of my system, maybe not. Any thoughts on this?
John

[John Lang]

Thanks for the info TCLynx
I am thinking of just expanding on the barrel idea for grow beds, so 100 sq ft surface using 1/2 barrels (not 100 cu ft) I believe calculates to around 60 cu ft of media or around 450 gal of media.This would bring the fish numbers down to around 90 - 1lb fish, so I can calculate from there using your above guidelines. The fry/fingerling integration also sounds like the way to go, to insure I have future fish for our plates.

Not sure of the difference between the feed requirement for adults and fry. Would you not have similar output lb for lb, adult vs fry?
Thanks for your input, it is greatly appreciated.
John

[TCLynx]

I think your numbers of 1lb fish per amount of grow bed might be a little high. For a recommended stocking density of a backyard system wanting to produce edible fish, 1 fish per cubic foot of media (assuming growing out to 1 lb) is a good recommended stocking density. 1 lb of fish per 5 gallons of media is MAX and probably pushing things for a backyard system.

If you are more interested in growing the plants and really not wanting to eat fish very often, you can/should definitely stock less fish.

As far as the feed for fry/fingerlings. Small tilapia have an extreme protein hunger and should be fed a much higher % of protein than the adult fish. Very high protein feed generally relates to higher amounts of nitrates being provided for the plants.

I definitely don't recommend trying to put the full MAX or even recommended weight of fish into a system to start with. As in don't put in 50 lb of fingerlings because some one told you that the system could support 50 one pound grown out fish. If the recommended stocking level of a system (like with 60 cubic feet of grow bed) is 60 fish (to be harvested at 1 lb) then start with 60 small fish and as they grow, the bio-filter can grow with them. If one were to put 60 lb of fingerlings into such a system, it would be immediately overwhelmed and already at max, what happens as the fish grow? It doesn't work well to operate that way.

As I have noted before, if you are not desiring to eat the fish often, you don't need to stock so many. Since media based systems keep the solids in the system, the balance between fish and plants does not need to be nearly so carefully balanced. If there isn't any nitrate showing and the plants show signs of nitrogen deficiency, one can always use a tiny bit of pee ponics or fish emulsion or urea fertilizer to boost the nitrogen levels if there are not enough fish or the weather too cool for them to eat enough to satisfy the plants.

I've left a system fishless for periods of time and never experienced nitrogen deficiency in my plants. I know of others who have had to add supplements to keep the plants happy, probably all depends on how heavily planted the system is and with what kind of plants as well as the system pH (mine has normally been high which slows the plants down.)

John Lang said:

Thanks for the info TCLynx
I am thinking of just expanding on the barrel idea for grow beds, so 100 sq ft surface using 1/2 barrels (not 100 cu ft) I believe calculates to around 60 cu ft of media or around 450 gal of media.This would bring the fish numbers down to around 90 - 1lb fish, so I can calculate from there using your above guidelines. The fry/fingerling integration also sounds like the way to go, to insure I have future fish for our plates.

Not sure of the difference between the feed requirement for adults and fry. Would you not have similar output lb for lb, adult vs fry?
Thanks for your input, it is greatly appreciated.
John

[John Lang]

Thank you again
This all helps to clarify my dilemma, which thanks to you is not a dilemma any more.
Appreciate the help.
John

[TCLynx]

Always happy to help. Too many fish is more often a problem in backyard system than not enough. There are many people that run huge amounts of Veggies just with some goldfish so don't feel that you need to have a large number of tilapia in order to do aquaponics. Finding the right balance for your situation is what you should be going for.