I have twice grown duckweed in outdoor aquaponics tanks. In the first, the fish ate exclusively duckweed, but the duckweed grew so fast I had to harvest half of it every 3-4 days. The second time, the duckweed seemed mostly in balance with the fish who constantly ate it. I have used three species: spirodela, lemna minor, and trisulca. The first time, by the end of the summer there was no lemna minor left (the fish ate all of it). The second time, I was growing the lemna minor in a separated part of the tank so the fish wouldn't decimate it, until some animal blundered through my system and mixed it with the fish. The fish ate all the lemna minor before I could get it away from them.
A good mat of duckweed also makes a micro ecosystem for a number of arthropods and smaller fauna, which the fish like to eat. The second year I found a dragonfly nymph living there, happily eating any mosquito larve that appear. The fish liked the freshwater shrimp.
Somebody pointed out on another post a day or two ago that the introduction of wild insects, falling into the system, is a vector for disease and parasites which you do not want. I would prefer the more "self-sustaining" system, using local insects as an additional nutrient source, but I think he's right about the parasites and such.
Regarding insects, I had some success in the summer with fly traps: something like this with the bottom cut off and foam around the outside so it could float in the tank. I used bottles with handles so I could put the bait inside the handle (I heated and pinched it shut at the bottom of the handle). It worked very well; the flies would get trapped and lay eggs on the bait (meat cuttings), the maggots would fall into the tank too. The fish were checking it about every minute, eating the flies as soon as they hit the water, so I had trouble telling if it was working until I spotted the maggots.
I did worry about harmful bacteria (e-coli, salmonella) from the bait entering the water, but I sampled it and did some agar cultures and didn't find anything harmful. I still didn't want to take chances, so I switched over to peanut butter as bait, which didn't work quite as well. Now that it's cooler out it has stopped working, so I took them out.
That's a neat fly trap/food source. Did you have to re-stock the rotten meat occasionally, or just once per summer?
If I had a large enough system, I'd like to try something like having a bat house (high) over open water so the bat guano falls into the water. This would effectively be using bats to harvest the local ecosystem for insect nutrients which they then deposit into the aquaponics system. The system would have to be large enough to handle the extra poop, but I think the plants would benefit.
I've been pondering putting a bug zapper over the tank to help feed the fish. I'm not sure if that's a good idea nor if the fish would even eat the half-carbonized morsels. I fed the big goldfish some finely-chopped bits of (cooked) chicken once. They seemed to like it. [Read More]
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