Suspended Fixed Raft?

[Harold Sukhbir]

I have seen people suspend coated plywood over(could even be done with styrofoam i guess) over the top of DWC beds. I know this would factor a reduction in nitrification and would have to be considered to balance fish numbers and probably aeration and filtration to some extent. Some say there are advantages with this type of arrangement over floating raft.Can we discuss the advantages or disadvantages of this?

[Chris Smith]

This could be a good alternative for small systems but not for commercial farms. I do not like the idea of suspended wood over the troughs for commercial systems because it would reduce the mobility of the farm. When I remove and harvest some rafts I can push the rest of the rafts down the line with one finger to open up the opposite end of the trough. I then place rafts of small plants in the opening. This would not be possible with suspended wood.

I do not think suspending rafts would decrease nitrification because the bacteria live in the water column. That is why the water is a tea color.

 

[Ryan Chatterson]

I dont see a big benefit to it either. You get more oxygen on the upper part of the roots but you also have to deal with the initial shock to the plant of the starter media drying out from not being in the water. Plus like Chris said you lose the mobility which is a huge aspect of a commercial operation. Ive done some side by side testing with this, some in 5 gal buckets with a few inches of water in the bottom vs plants in a traditional raft and the raft plants grew faster. (Harvested lettuces 4 days sooner over a 30day growout)

To touch on what Chris says- The bacteria live everywhere, in the water column, on all surfaces, in the air, on your fingers right now, etc. The reason we account for the biofiltration that the rafts provide is because the you need X amount surface area for biofiltration per pound of fish. In a low density system you might not need the additional biofiltration that the rafts provide but in a heavily stocked tank, you definately will (Unless you provide a sperate biofilter, gravel bed or something else to make up the needed surface area). A few months ago we took a test system with a semi heavy load of fish (1/4lb per gal), no plants(no one re-planted after harvest)and did a heavy raft cleaning. The fish had been fine for months but the rafts were the main biofilter. Cleaned the troughs, rafts and everything till they looked new, allowed the water to start flowing again and went home. The next day the nitrites were through the roof and we had about 20 of 400 tilapia bite the dust.

Calculate the surface area of biofiltration needed for your specific load of fish and you can tell exactly how much surface area you should be provideing.

[Harold Sukhbir]

So i get from both of you Chris and Ryan that there is the ease(labor) of production,  no shortfall in nitrification, and more efficient grow-out times when using floating raft.What about cost considerations?, as this may well be a factor for those considering commercial AP. I'm not sure,but fixed raft might save a little on aeration, foam replacement etc.,I hope to hear from our fixed raft people on this. As Ryan mentioned calculating the fish to raft ratio, maybe a good place for you all to put a rule of thumb on this conversion for floating raft?