Saturday, April 30, 2011

Aquaponic Gardening – The Duckweed Dilemma!

Aquaponic gardening is a terrific way to grow your own organic veggies and chemical free fish.  Let's take a moment to  ponder the subject of what to feed your "wet" pets so that you can economically harvest the best. The subject of duckweed usually will come up in any conversation when folks passionate about aquaponics get together ... it seems that his tiny aquatic plant can make a big impact!


Duckweed is a species of small floating aquatic plants found worldwide.  It is often seen growing in thick, blanket like mats on still or slow moving, nutrient rich fresh or brackish waters.  Anyone who has ever had duckweed “sneak” into an aquarium or pond knows that this tiny plant can double their mass in under 2 days given the right conditions … this is faster  than almost any other higher plant.  It is for this reason duckweed is considered an invasive plant in many parts of the country and is on the “hit” list in some states. 

Now the real dilemma regarding duckweed is whether it is something you should be feeding it to your fish in your aquaponics system.  The nutritional value of duckweed varies, but most species have protein contents in the range of 15-45% which is good and duckweed is a convenient feed for fish.
  • It can be readily grown locally 
  • It can be fed fresh and since it floats, it will be consumed by your fish and not decay at the bottom of your system.
  • It is used very efficiently by fish such as tilapia and carp, but other species might well cope with duckweed as a component of the diet since it is particularly low in fiber and high in protein when grown under ideal conditions.
  • It is relatively inexpensive to produce or may be regarded to have no cost where the opportunity costs of family labor are not taken into consideration.

From the studies I have read, it seems that tilapia do the best with a combination of duckweed and pellets versus just duckweed alone.  See below:

*Gaigher, et al. (1984) compared the growth of hybrid tilapia fish on commercial pellets vs. duckweed. The fish were cultured at high densities in an experimental recirculating unit for 89 days with duckweed (Lemna gibba) or a combination of duckweed and commercial pellets. They conclude that a combination of pellets and Lemna gave the best performance:

When fed on duckweed alone, intake rate was low, feed conversion ratio good (1:1) and relative growth rate poor (0.67% of bodyweight daily). Sixty-five percent of the duckweed consumed was assimilated and 26% converted to fish. When the fish were fed on pellets in addition to duckweed the rate of duckweed consumption decreased and growth rate of the fish doubled with feed conversion ratios between 1.2 and 1.8. Seventy percent of the mixed diet was assimilated but only 21% converted. Fish grown on the mixed diet performed similarly to fish grown on pellets but had a better feed conversion ratio.

I am sure that everyone has had their own adventures with duckweed.  I would love to get everyones input .. so please leave your comments below ...

BTW ... whatever your decision on duckweed dilemma is, I would highly recommend growing in a separate system.  Duckweed enjoys calm or almost still waters, whereas most fish require vigorous aeration of the water and, of course, you want to be able to control its explosive growth and not find yourself up to your ears in it!  




References:
*Gaigher, I. G.; Porath, D.; Granoth, G. (1984) Evaluation of duckweed (Lemna gibba) as feed for tilapia (Oreochromis niloticus X O. aureus) in a recirculating unit. Aquaculture 41: 235-244.

6 comments:

  1. Tilapia can survive on duckweed but not grow at rates fast enough for commercial viability. But most aquaponic systems should not be focused on fish production anyway. BTW the FCR numbers above are wack. FCR of 1:1 means 1 lb feed in makes 1 lb fish growth. That is a better conversion ratio than 1:2, which is a very high FCR anyway. Most operations only achieve around 1:6-1:8 depending on the feed.

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  2. Thanks for your comment Dave ... I agree about the focus for aquaponics ... at least in the US.

    I took the FCR numbers directly from the study, of course studies can be flawed also ...

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  3. Some more clarification might be helpful for some people. When the protein % on duckweed is generally given that is usually for dry duckweed so if you are feeding fresh duckweed, you are not feeding that much protein. A dozen tilapia in a shallow (large surface area) 300 gallon tank will consume all the duckweed from the surface of that tank in under a week if not fed commercial feed as well. If you want to grow enough duckweed to really feed a lot of tilapia you will definitely need a separate body of water to grow it and you will need to fertilize it to get the high protein and mineral content. It would take a fair bit of extra space for the operation to provide enough duckweed to substantially support a large amount of fish. Luckily this space need not be full sun as duckweed does well in shade and partial shade.

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  4. Thanks TC ... You really know your stuff! Your set up is amazing ... to check out TCLynx site go to:
    http://www.aquaponiclynx.com/

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  5. Great site! Three cheers for knowing how important wastewater remediation with duckweed is as the missing link in sustainable crop production.
    Tamra www.duckweedgardening.com

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  6. Very nice and informative blog posting. Among all the fish, tilapia fish is Awesome! Tilapia fish farming is very profitable and easy. I like tilapia fish dish very much for it's unique taste and nutritional value.

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