A love affair or a tragedy in waiting? (recovered blog)

[Kobus Jooste]

AP system, how do I build thee, let me count the ways…………………I have been tangled up in ratios and scaling for the last couple of days.  Visualizing a cubic foot is just not possible for a decimal person.  It is also not the way in which my head worked when I started designing.  I am a scientist, thus was naturally drawn to the publications around the UVI and Alberta Raft aquaponic systems.  Not because I thought that they were better, but because they supplied data.  Real sink your teeth into data while the background movement systems were still quite young and offered no long term data sets to ponder.  These systems were all about fish per tank space, feeding rate per raft area and planting densities in those rafts. 

 

Going from that to pounds of fish per cubic foot of grow bed felt like waking up in a place where nobody speaks your language.  It made me think hard about why I started down the line. I was also looking at some posts of people frantically trying to grip this slippery eel and thought about a post that my at least soothe my aching head.  It may also prevent classic poetry from being abused or turning from poetry to tragedy.  “Bubble bubble toil and trouble……?”  My thoughts started drifting from how to design an AP system to why.  Why?  Why indeed.  If you can answer “why” then “how” starts making a lot more sense.  Here is my two part why rationale.  First, you want to keep fish happy.  Thus you keep their water clean.  You feed them, which makes the water dirty.  Dirty water can get clean again, but then you will need filtration and keep everything aerobic.  On the other side you have plants.  The plants are happy, because they have all the water they need, but you also have to keep the water aerated to keep them from “drowning” and then you need to make sure that they get enough nutrients.

 

That, as far as I am concerned, is why we build the system.  It would also help a lot with design issues.  Let us stand still at one overlapping “why” for now.  Feed.  Fish need it, plants need it, and you have some of both, thus it can become a combined quest.  I would like to make that quest the focus of this blog, as it will also lead to some of the “how” issues floating around.  Fish and plants split from each other in terms of relatedness Kingdoms ago, but fortunately for us, the split did not come before most of the foundations of metabolism and cell structure were laid.  Thus the macro and micro nutrients needed for the two groups are very similar.  Here and there are concentration variations, and for some reason plants developed a habit around Molybdenum and Boron that animals frown upon.  Other than that you can pretty much find a middle road with the two focus crops in aquaponics. The following tables and graphs serves to illustrate this point.  Please note that the ratios given here do not apply across the board, as they can only be ascribed to the feed used in the experiments, the fish used in the experiments and the age of fish studied, but within a few percent this way or that, they serve to illustrate a point.  That point is your pick of food is the first important choice you make in the running of the system.  This statement in a way goes against my desire to have a low cost food production line attached to AP, but bear with me, there is hope!  A well balanced food equates not only to a well balanced nutrient mix for the plants, but may also help a bit with water quality management. 

 

As can be seen from Figure 1, we are well on our way with feeding our plants once we have thrown the fish food in the water.  Ignoring the Boron and Molybdenum that we know would not be in the fish food, and the Carbon, Oxygen and Hydrogen that is all around, the rest is pretty much there.  A little booster of trace elements I use contain nothing that is not in good food mixes.  The only thing I have against some food manufacturers is that they push the protein levels a bit high for an omnivore such as tilapia.  Now, obviously we cannot just toss the fish food at the plants and walk away.  The food sustains the fish, and from there, we manage what wastes are derived from the fish culture side for good plant growth.  The following data sets will give you a good idea what happens to the food once it leaves your hand, but there are only two options really – through the fish or into the sludge (with or without the fish helping).  Before Figure 2 and 3 are taken too seriously by the experts, it is based on two publications, from which only the tilapia stats was culled.  Different fish have different ratios of uptake and excretion.

 

 

 

 

Figure 2 had me scratching my head a bit, as the amount of Nitrogen they state ending up in the water column is much lower than reported in most other databases I have, but as it is a nice way to present all of the fish nutrients, I’m still using it.  The fish feed for tilapia used only had 24% protein, and the fish were juveniles, which could account for a different metabolism than the adult fish output typically reported.  What I want this picture to show people is not perhaps that there is a problem with the data, but for them to look at the sludge with a different angle.  It is not something, in my mind, that is to be clarified and tossed out.  It is plant food.  It is worth holding on to but it has to be dealt with otherwise it is going to rob your system of oxygen and potentially also give spikes in nasty chemicals as the decomposition pathway takes is course. 

 

Figure 3 is based on adult tilapia data from a different publication (I will attach both for people to have) giving people an idea of what happens to the water column after a fish feed in terms of macro nutrient release.  This is why we obsess about Ammonia.  As can be seen from Figure 3, up to 73% of the protein content (not total mass of food, just the protein mass) is Ammonia in your water a few hours after you tossed it into the water as a food pellet.  This is why I would like to have a low protein pellet (28%) rather than the one I have now.  My fish do not have to grow like Frankenfish, and I may have less Ammonia to deal with.

 

So there.  With the addition of some trace elements and the control of pH, we have happy fish and happy plants when the temperature is just right, or thereabouts.  To get back to the start of the blog, we now have an answer, although basic, for one of the key “why” questions.  Why build the system from a nutrient perspective is answered as follows:

 

If we manage the solid and dissolved wastes that comes from aquaculture correctly, we can grow happy, productive plants in the same water stream.  The how now becomes a matter of personal choice, available space, materials etc and something that I will steer clear of in this edition other than to say this: Media beds is the place to put your solids, and the place to nurture the bacteria that turns all that Ammonia into Nitrates.  Some plants like a raft, some plants like a gravel bed, but they ALL like the nutrients that we can provide.  Thus the first design step ought to be understanding how much fish you want to keep, how much filter space and plant space you need to deal with the fish, and then you find an optimal way of putting it all together.  In reverse the statement can look like this: If you know how many plants you want, you can reverse engineer their nutrient requirements, which would give you the fish stocking rates you will need. 

 

I will get my 2 cents worth in on that one, but at a later stage.  For now, I hope this latest rambling from the ecologist was worth the time reading it.  Happy farming!