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.