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I have been looking at the various basic multi cropping extensive aquaculture / agriculture world wide, especially those related to duckweed culture for a research project that I am busy with.  It got me wondering why people all over are not giving edible carp species a go.  In places like Bangladesh, waste water pits are used to grow duckweed, which is then fed to fish  (in fertilized ponds, but that is not crucial) like carp, which is then harvested.  They also add a small amount of salt to the ponds - not more than what is often recommended for adding to koi systems.  In Australia, Jade Perch is also an excellent option.  Duckweed likes ammonia more than nitrates, thus if you set up a system that has a fish tank, some duckweed tanks, and then grow beds, (could the combined trace elements from minor sea salt addition and the proceeds from a worm bin or two) you could potentially be closing the loop a bit on your inputs - only the sea salt at this point. 

 

Are there any examples of systems such as these in operation already? 

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Steve - I was hoping to start consolidating info in the advanced aquaponics group, but am a bit swamped.  Tell me what you are looking for in terms of duckweed and as soon as I am back from picking up my daughter, I can have a look see.

steve said:
Good Morning Kobus and all interested, I wonder if it would be good to start a new thread on Duckweed. There is a lot of info scattered  here, is there a way to add it to a new thread? I've researched for hours on subject only to find a few sentences of relevant info. for me. Seems to be the case with most all issues I research on net. Countless hours putting together bits of the pieces. Interesting but unfortunately much wasted time for many people. I have heard Kobus tried to consolidate info on other issues. Just wondering if you had some success?
Duckweed  just seems almost too good to be true. Much potential in many areas. I'm thinking it deserves its own topic or thread. I was concerned about mosquito's in the slow ponds only to learn it is reported to deter their breeding. Was wondering about a ballpark idea of size requirement for its growth. One site recommended .5 M tank depth. Maybe if I started out with an area equal in surface area to fish pond. Then expand if it works. An interesting Duckweed link.         http://www.fao.org/ag/againfo/resources/documents/DW/Dw2.htm

Kobus Jooste said:
Steve - I was hoping to start consolidating info in the advanced aquaponics group, but am a bit swamped.  Tell me what you are looking for in terms of duckweed and as soon as I am back from picking up my daughter, I can have a look see.

steve said:
Good Morning Kobus and all interested, I wonder if it would be good to start a new thread on Duckweed. There is a lot of info scattered  here, is there a way to add it to a new thread? I've researched for hours on subject only to find a few sentences of relevant info. for me. Seems to be the case with most all issues I research on net. Countless hours putting together bits of the pieces. Interesting but unfortunately much wasted time for many people. I have heard Kobus tried to consolidate info on other issues. Just wondering if you had some success?

Steve, you can always start your own thread on Duckweed and ask for contributions to it.

 

I would always add some minnows or some type of mosquito eating fish into a duckweed pond or use some mosquito dunks to make sure to control mosquitoes around here just because they can be a major problem here.

 

Anyway, as to consolidating/splitting a thread or information, there is no "easy" way.  And no one on here paid to do it so your best bet if you want it done is probably to start copying and pasting it yourself into your new thread.

Steve - I have a mountain of data on duckweed for the work I'm currently busy with.  The problem is to work waste water treatment theory over into aquaponics or aquaculture.  In waste water, culture systems are typically deeper (500 cm +) and the water is left stationary in it for days to weeks.  The duckweed cuts out light and eventually can cause anoxia in the water below - killing even bacteria in the water.  This will obviously not work so well in aquaponics.  I use 30 cm deep beds for duckweed (there are recommendations for even shallower) and the water is constantly in motion.  The 7m2 of duckweed is capable of keeping 18 koi going with very good water quality.  I choose to work with Lemna gibba, which can absorb up to 1100 mg Ammonia per liter m2 per day, but then you need to be operating at waste water ammonia concentrations.  I cannot let it get above 2 mg/L in my system, thus I am still learning about what capacities I will have.

 

The challenge is to find a way to deal with aquaculture standards using waste water treatment as a reference.  In many developing countries, earthen ponds are fertilized or connected with human latrines and pretty much left to their own devices.

steve said:

Duckweed  just seems almost too good to be true. Much potential in many areas. I'm thinking it deserves its own topic or thread. I was concerned about mosquito's in the slow ponds only to learn it is reported to deter their breeding. Was wondering about a ballpark idea of size requirement for its growth. One site recommended .5 M tank depth. Maybe if I started out with an area equal in surface area to fish pond. Then expand if it works. An interesting Duckweed link.         http://www.fao.org/ag/againfo/resources/documents/DW/Dw2.htm

Kobus Jooste said:
Steve - I was hoping to start consolidating info in the advanced aquaponics group, but am a bit swamped.  Tell me what you are looking for in terms of duckweed and as soon as I am back from picking up my daughter, I can have a look see.

steve said:
Good Morning Kobus and all interested, I wonder if it would be good to start a new thread on Duckweed. There is a lot of info scattered  here, is there a way to add it to a new thread? I've researched for hours on subject only to find a few sentences of relevant info. for me. Seems to be the case with most all issues I research on net. Countless hours putting together bits of the pieces. Interesting but unfortunately much wasted time for many people. I have heard Kobus tried to consolidate info on other issues. Just wondering if you had some success?
Some very interesting work there on the sewage water treatment aspects. We have really gotten ourselves in a mess. I have chickens their manure would be a great addition to the duckweed tank. Save the fish nutrients for plants. I last read how this Duckweed really needs to be dried to be effective. But a Tilapia diet can be fed half and half and growth rates are quite similar to straight pellet diet.  This could be such a huge step in sustainability if it would work. Probably does very well for some?

duckweed just needs to be dried to be accurately compared to commercial feeds which are also dry.  Like when people give the numbers like oh duckweed is x% protein and so on, that is % of dry weight.  So the tricky part is how much wet duckweed do you need to get that same amount of protein in order to accurately compare.  There is a big part of the challenge.

 

I will caution about using chicken manure to fertilize duckweed and then using duckweed to feed the fish.  Unless that chicken manure is properly composted, it isn't very much better than simply putting the chickens over the fish tank.  And I don't think warm blooded manure should be used in aquaponics without first sterilizing or composting it.  At least not if you eat any salads or raw carrots.

The duckweed does not have to be dired to be a fish food, but as TCLynx pointed out, it can be almost 95% water and the nutrient component only refers to its dry mass.  One of the biggest stumbling blocks for the use of duckweed where wet use is not possible is effective large scale drying.  You can yield 50% of your water surface every 2 - 5 days, and to get this dried is where a lot of issues develop.  There are many nutrients in duckweed that breaks down over time or when a lot of heat is applied.  This is why I am trying to find a soil-based fertilizer application for duckweed, as it cuts out some of the risks of using manured water supply and negates the issues of damage to stuff that could be beneficial as an animal feed but not so important to plants.  You can even "tailor" your duckweed by adding nutrients to their culture water.  They are excellent nutrient scavengers and will pick up almost everything in the water, including contaminants such as heavy metals (which can be a big risk if you are working with contaminated water and do not know it.).  I think duckweed has a lot going for it, but most humans are in a mindset of synthesizing, engineering or mining the things they want- there is a need for the culture of useful elements such as algae and duckweed to become mainstream before all the tech issues will be resolved.  It works very well in low tech apps though. 

 

Still, I'm in the mindset of developing a fermentation and bacterial decomposition process for duckweed rather than drying, to get the nutrients that the duckweed absorbed made available to plants again.  In this way I'm not all that badly challenged by trying to figure out what to do with all the duckweed

Thanks for the caution on the manure use TC. I know this is a very tricky. I will drop this from the equation now. I've looked at different ways to utilize the animals waste. I have a biodigester to treat the manure, a considerable improvement. Have many land plants, trees to fertilize. Thanks for your help and patience! I try to keep my ideas ahead of myself enough so that when I get there I know where I'm going.
Always good to throw around ideas.  That way you know which ones splatter when they hit the fan

This may be slightly off topic since the conversation seems to center around duckweed, but does anyone feed their fish purselane?   I only learned that it was a vegtable last year and now this weed is one of my favorite vegtables.  Because of this, I now leave it alone when weeding and I'm growing more than I can eat.  It's high in Omega-3, which makes be think it would be a good supplemental for tilapia or any other fish that would eat it.

My chickens love it, would my fish?  I just started my first system, and my tilapia are still too small to eat it, but I definitely plan on trying it once they get big enough to eat it unless someone can point out a reason I shouldn't.

it's worth a try.  I don't think my tilapia really paid it much attention though.  Might need to chop it up and mix it with other stuff to create a food you can trick them into trying out.  Often times I noticed the tilapia would ignore new food quite stubbornly until they finally try it.
Thanks for the tip TC.  I'll try it when they get big enough and post how it goes.

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