Comment

Comment by Chris Carr on May 11, 2012 at 10:38am

Plus they cant live on 100% duckweed, so would have to half that value of course. The point is the number is still pretty big, and you can only find what it is with experimentation. Time of year and how fast it doubles can also change the area required in a big way.

Comment by Chris Carr on May 11, 2012 at 10:36am

What is the protein content of duckweed again? Assuming perfect conversion and assuming the constants are accurate, the nitrogen seems to be all accounted here, but showing only 10% protein content of the fish. So it is likely you are correct that your number is a little big, because even going backwards and coming to a smaller SM value, it is still too big, but not totally off.


 

I think the degree of error here is the density of what a square meter of duckweed is, and the degree of error is very big, and will vary depending on localized conditions. In conclusion, likely not something you can solve with pure math, you need to grow it, dry it, and measure it all out :)

 


consumption is 1500kg feed/year = 4.11kg/day
1m2 duckweed = 45g
daily requirements 4.11kg/day / 0.045kg/m2 = 91.33 m2/day
duckweed per harvest every 4 days = 365.32 m2
Required space for doubling effect 730.64 m2

ammonia production per day (1500kg x 30% x 95g ammonia/kg = 42.75kg ammonia) / 365day = 117g/day
daily duckweed ammonia requirements = 91.33g/day
remaining ammonia 25.67g/day
ammonia consumed 78%


remaining ammonia per year back to fish protein 9.37kg ammonia /95g ammonia/kg protein = 98.63kg protein
protein content of 1000kg of tilapia / 98.63kg protein = 10.1%

 

 

Comment by marty lininger on May 11, 2012 at 8:46am

I want to estimate the size of duckweed bed that a specified fish population can support.  Any feedback I can get on my assumptions validity would be greatly appreciated!  Perhaps there is a logic error in my steps to calculate the size, please let me know!

My final calculation below seems to be way too much.  I do not know where I am erring.

Assumptions:

fish incorporate 30% of protein in feed, the other 70% is converted to ammonia  (1)

can get 95 gram of ammonia from 1kg of protein fed    (1)

1 gram of ammonia will feed 1 SM of duckweed  (2)          

duckweed can double in 4 days or less

constant population mass 1000kg of tilapia,       FCR of 1.5

consumes 1500kg of 30% protein feed in 1 year, consumes 450kg protein

 450kg protein x 95g ammonia/KG protein  = 42,750 g ammonia

which can support 42,750 SM growth of duckweed

harvesting half the crop every 4 days = 365/4 = 90 harvests

42,750 SM / 90 = 475 SM growing area per 1000kg tilapia fish

 

Other:

1SM of optimal density duckweed weighs  0.1 – 1.2 kg (0.65 avg) wet and is 93% water (3)

1 SM of optimal density avg 0.65 kg wet duckweed weighs 45 grams dry

1 SM naturally occurring duckweed in Orlando, FL dried weighs 100g  (4)

 

ref;

(1)    http://www.fao.org/docrep/X5744E/x5744e0e.html

(2)    Kobus Jooste post

(3)    Various sources

(4)     Own observation -but was it optimal density?

Comment by Carey Ma on May 6, 2012 at 9:41pm

Wow! So much interest recently. I apologize I haven't and can't respond at this time, however, I promise I'll do my best next weekend. I just came back from down south for a consultation and am leaving in a few hours for another consult so catch ya guys next week.

Cheers

PS There are basically two schools of AP: Clear-water vs. Greenwater culture. Both have their own requirement and methods of feeding. In clear-water culture, it is best if each function is separated; grow algae in the algae tubes and grow duckweed in the duckweed trays (they have different needs).

Just remember, unless you overstock your tanks, you will have to share "energy/ nutrients" with your veggies so maybe choose which is more important, raising feed or raising crops for market?

IMO the purpose of AP is not so much to raise an organic crop for market but to raise fish and have a natural, ecological way to filter "pollutants" with edible crops as a nice byproduct. Soil based crop raising is still by far the easier and better way to grow veggies vs. AP crop production.

What I would like to see is a volunteer group that is willing to compile a list of feed inputs and their approximate nutrition values. I'll start a page in this group for y'all to help yourselves so please stay tuned next weekend.

Comment by NTS on May 4, 2012 at 9:21pm

Why not grow both duckweed and azolla. In the wild they grow together. This would give your fish a more diverse diet. Azolla makes a great green fertilizer because it is nitrogen fixing. I would stay away from growing Salvinia.

Comment by TCLynx on May 4, 2012 at 7:32pm

LOL, yes separating worms from castings is labor intensive.

BSF larva though do self harvest and if you are in a warm enough climate they are great, cold climates require a big greenhouse to keep them going year round though but I believe Growing Power has done it.

Duckweed can supplement for up to 50% of commercial feed for tilapia without a loss of weight gain, that is the only easy guideline I've heard so far.

Worms and BSF larva are also great supplements but they are too high in fat to be a complete feed.

Algae and green water culture can be a great way to feed tilapia but that is not really appropriate to aquaponics but is more for extensive pond culture or solar algae pond production.

Now Carey has some great ideas for feed so watch his posts closely and pick up what you can.

As to feed mills or extrusion.  A pellet mill really just compresses bio-mass or whatever you put in it and I don't expect that it is easy to make floating feed that way.  Extruded feed is probably a bit more like making a dough and extruding it in an appropriate environment that it will contain enough air or light material to make it float.  That is probably a gross oversimplification of the concepts so hopefully some one with more technical experience in feed making can chime in and share more on the subject.  I have used a meat grinder with it's blade removed to extrude something like fat spaghetti that could be cut into pellet size pieces and dried or baked but that was an incredibly labor intensive way to make fish feed and I wouldn't recommend it for more than just testing a recipe.  (and using the kitchen to make fish feed is at your own risk because the cook may ban you from the house if you get caught. )

Comment by Paul Trudeau on May 4, 2012 at 5:46pm

Marty, whether or not one uses BSF should not hinge on removing them from their grow medium.  They do self-harvest (crawl out into a collection bin).  I don't have lots of worm experience but my impression is that removing them from their grow medium is not as simple as for BSF.  Glenn Martinez also points out that pound for pound his worms fetch a higher price for him than his fish.

Comment by Chris Carr on May 4, 2012 at 5:19pm

I would love to eventually see a community sourced guideline document that can outline supplementing feed come about from this group. I think developing a cut and dry DIY feed recipe that can be used as a complete source is not practical to apply for universal purposes (mostly due to cost, labor, and complexity). Using control groups it would be interesting to see how far you can supplement commercial feed with various combinations of ingredients without impacting an overall aquaponics system.

Comment by Dave & Yvonne Story on May 4, 2012 at 4:46pm

Chris & Marty &

thanks for the great info.

I am hoping to get off the grid and limit my driving to town, Making my own fish food is of interest for me. I like the azolla, but I also want Omegas. I will do further research. Thanks

Comment by Chris Carr on May 4, 2012 at 3:55pm

FYI there was a derailed thread with some links posted here.

This has a couple interesting links embedded.

http://documents.ponics.org/sections/aquaculture/aquaculture-world-...

This one is an interesting case study regarding small scale feed formulation in developing countries.

http://www.worldfishcenter.org/resource_centre/WF_2462.pdf

 

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