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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 - up to now, I have only been trying to balance a system out where you only have a fish tank and duckweed beds, in order to determine how much duckweed can be produced by a specific amount of fish food input (and number of fish).  I have not been able to find a lot of data on the topic as far as duckweed feeding in aquaponics or aquaculture goes, but your thinking of combining pellets and duckweed is a good one - I think right now the obstacles faced by anyone feeding intensive fish culture only duckweed is a bit too much.  The data needed on nutrient quality and FCR, which ultimately will give you the growth rate and supplementation needs you are looking for, is sparce and with so many duckweed species out there, it is not easy to transpose results from one species over to another.  I do think this line of operation is well worth looking at, especially if you are not trying to rpoduce a fish at record time. 

 

On the upside, here are some thoughts:

  • I have heard many people argue that duskweed has a better FCR than any pellet out there, and with tilapia and perch being mostly vegetarian, you could potentially get very good growth with duckweed.  Tilapia can process a meal in 30 minutes, thus having some duckweed floating in the pond while you still feed pellets every now and then could potentially also result in better FCR
  • In my research, I have needed only 18 small / medium koi to produce 5 kg of wet mass duckweed per week out of a 7m2 growing area.  More time is needed but I do not think the logistics of producing enough duckweed is too great.  Fish tank with a number of fish, a straight flow through duckweed beds and back to a bio-filter or the fish tank.  The problem with having a bio-filter in there is that duckweed do not tough nitrates while there is still a sniff of ammonia around, leading it to build up and algae to bloom.  This has been my biggest problem to date.
  • If your duckweed bed capacity to remove nutrients is slightly larger than the amount of ammonia produced, the plant goes into scavenging mode, sucking a bunch of trace elements out of the water.  The potential for a nutritionally balanced fish food supplement is therefore very good if you treat (miss-treat) the duckweed correctly.   

Ok David, you might appreciate this carp Recipe then.

Get a cedar board a couple inches longer than the fish.  Clean fish, preheat oven.  Place fish on cedar board.  Bake in oven.  Discard fish, eat the board.

Enjoy

That was my last business commercial fishing for Carp and rough fish. We seined the undesirable species out, sometimes a couple million lbs. per year. Might be some opportunity for fish food/meal. Though even the rough fish aren't producing like they were in the past. Now here on the ocean the Mahi Mahi the locals depend on for their livelihood never came this last year. Actually gave me a bit more incentive to work on this aqua project. In the past we could fun fish and at least catch plenty to eat but seems to be getting worse every year. On the bright side I hear they had a very good salmon season in Canada. Unfortunately not the trend though.

David Waite said:
I know the only thing Dept of fish and game hate worse than tilapia are asian carp species. They are banned in most southern and western states. Not sure about up north but Im sure its the same. Very invasive due to can survive the cold. I have tried carp and honestly it tasted like s\\\. Not quite sure how the asian culture eats that boney thing but they do by the millions of tons.
TC very funny lmao.

TCLynx said:

Ok David, you might appreciate this carp Recipe then.

Get a cedar board a couple inches longer than the fish.  Clean fish, preheat oven.  Place fish on cedar board.  Bake in oven.  Discard fish, eat the board.

Enjoy

I think there is room for veggie fish in AP, specifically if people are not all that fussed about eating them.  In my country, there will be few takers for AP as a fish protein source when compared to the rest of the world, thus for me, I look at a fish first and foremost as a poop machine and then as food when I try to imagine possible AP systems for the local market.  There are goldfish, koi, indigenous small pretty things that cannot be eaten, and then stuff like carp and perch.  If we can get good trace elements and nutrients into a system with simple vegetarian fish diets, and people are not necessarily going to eat the fish, then we are closing the loop just a little bit more.

 

If you need to eat the fish, then there is supplemented tilapia and perch as good possibilities so far.

Possibly if water ran from fish tank to growbeds 1st then to duckweed. Wouldn't it then save the best nutrients for plants. Making the duckweed scavenge a bit to strain nutrients?  In your project Kobus you used 18 Koi (maybe 5KG?) for 7m2 = 5KG duckweed. Seems that would be over 5 times what would be necessary to feed those 18 fish?                                For 500KG fish would require a very large pond area for Duckweed.  I will leave my estimates of area out of the post. As they would seem to scale this out of feasibility. Just trying to get some idea of a formula for area required. Volume of feed needed.

Keep in mind that wet weight of duckweed is mostly water.  5 kg of wet duckweed is probably only similar to a few big hand fulls of pellets.

 

Yes it really does take a rather large surface area to really grow enough duckweed to replace commercial feed entirely.

 

As to sending the water to grow beds first, well it would provide for better veggie growth but you definitely don't get as much duckweed growth when you are feeding it nitrates instead of ammonia and there likely won't be much ammonia left after sending the water through media beds unless you are horribly overloading the media beds.

 

I had far better luck sending dirty water to duckweed but then you have the challenge of the muck and solids tending to get nasty in the bottom of the duckweed tanks since duckweed tends to like fairly still water.  Perhaps sending stuff through a swirl filter or settling tank first would help but then you are removing all those good nutrients from the system rather than breaking them down.  These are some of the challenges I've encountered when trying to figure out a good way to use/grow duckweed.  I'm sure it can be done some how but I didn't figure it out quickly.

Hello Steve

 

The idea of the experiment is not to feed the fish, but to work towards a ratio of how much duckweed filter is required to deal with a specific volume of fish waste.  The experiment does not feed the duckweed back to the fish at all - they are fed koi food.  The project aims to develop technology that can place aquaculture and soil conditioning fertilizer production in a rural South African setting.

 

That is why the data I'm developing will not exactly answer your question.  I use the feeding of 18 fish to generate the raw materials for fertilizer production.  From there, the fertilizer quality will be assesed, and if it looks good, the project will work at calculating how much fish feeding will result in how much fertilizer, and what the infrastructure will look like.

O yes - the duckweed will not touch nitrates thus you need clean, nutrient rish (ammonia) water for duckweed to be happy

 

Hi!! Kobus, Did that seem fairly accurate. 7m2 for 18 fish? Seems quite large for a couple fish. So after the duckweed there would be no nutrients left for other plants? Would need to be a separate water line. Somehow I feel if TC didn't figure this out I sure won't. Any estimates of area to feed 500KG fish half and half.

Kobus Jooste said:

Hello Steve

 

The idea of the experiment is not to feed the fish, but to work towards a ratio of how much duckweed filter is required to deal with a specific volume of fish waste.  The experiment does not feed the duckweed back to the fish at all - they are fed koi food.  The project aims to develop technology that can place aquaculture and soil conditioning fertilizer production in a rural South African setting.

 

That is why the data I'm developing will not exactly answer your question.  I use the feeding of 18 fish to generate the raw materials for fertilizer production.  From there, the fertilizer quality will be assesed, and if it looks good, the project will work at calculating how much fish feeding will result in how much fertilizer, and what the infrastructure will look like.

18 fish can generate enough ammonia to sustain 7 m2 of duckweed.  Ammonia stays at trace levels almost all the time, and I discourage nitrates because they cause algal growth.  This system is definitely not going to support much other than the duckweed, but I am cautious not to overload at this point because there is no back-up if the ammonia goes too high.  I've never tried to work out how many fish I could feed with this system as it is not the goal of the project.  If you look at the pics of my research system, you will see what I have in place in terms of the structure of the unit. 
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?

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