Closing the Loop on Aquaponics

By Kyle Brookens

As discussed in the Aquaponics blog post, the system is far from completely closed, but it is possible to make the system a little bit tighter by just focusing on fish food. Tilapia mostly feed on algae, plankton, some insects, and duckweed. They also require vitamins and trace minerals in order to grow healthy in a short period of time. Believe it or not, tilapia can even eat vegetable scraps if they are blended up to a decent size. Live insects provide a great source of protein and fat, which fish do need some of both. Common insects that they eat are earthworms, crickets, snails, slugs, flies, moths, beetles, and caterpillars. Red wigglers are great because they provide an excellent source of vitamins, not to mention that tilapia love to eat them. If they have to, they will even eat scraps of meat, but I would not recommend feeding tilapia meat products (Woods). Tilapia can be fed myriad foods, and some of them are a great supplement for your fish because they are so easy to raise and produce high yields. Both red wigglers and duckweed serve as excellent forms of supplemental fish feed.

Duckweed is a great supplemental food for tilapia because this plant is naturally part of their diet, and it is extremely easy to cultivate. All you need is a stock tank full of PH neutral, de-chlorinated water, a small culture of duckweed, sunlight, space, and a source of fertilizer. Yields of duckweed can start at “a few hundred kilograms per hectare/year,” but quickly escalate “to 10 tonnes/ha/year” (Leng et. al.). Its growth “resembles the exponential growth of unicellular algae,” where its mass may double in less than a day (Leng et. al.). Duckweed usually grows like a blanket on the surface of ponds that have large amounts of decaying organic matter in them. The decaying organic matter serves as a source of fertilizer. Regardless, Duckweed can be found in a wide variety of climactic zones and can go dormant if the temperatures get too low. Duckweed prefers a PH between 6.5 – 7.5. The plant prefers temperatures between 50 – 80, just as most plants do. The leaf itself has very little fiber, up to 43% protein, 5% fats, and “highly digestible dry matter” (Leng et. al.). As with any plant, duckweed needs a source of fertilizer. Manure from livestock, food processing wastes, sewage sludge, compost, and vermicompost are all great sources of fertilizer, it is just important to make sure that the solids do not float. It is also important to not over fertilize; too much fertilization will burn and kill the plant. Duckweed may also need a mineral supplement such as sea salt. If duckweed does not sound amazing enough already, it also contains many essential amino acids that resemble animal protein (Leng et. al). For this reason, duckweed may be used as a feed for many different kinds of livestock, such as pig, duck, chicken, and cows. While growing duckweed, it is important to maintain a density of about 1kg/square meter. This is important to inhibit algae from taking over. Duckweed may be frozen or dried for later use. Although this plant is amazing, it cannot be relied upon to supply the entire diet of the tilapia in an aquaponics system, and for this reason, vermicompost is a great addition to help close the loop.

As described in the Vermicompost blog post, worms are extremely useful consumers of food waste that produce rich, healthy worm castings. In closing the loop of the aquaponics system, the earthworm castings could be used as an excellent supply of fertilizer for the duckweed operation. All the organic food, stems, cardboard, and paper waste that is generated by the garden will help feed the worms, which will help feed the plants and duckweed, which will help feed the fish. Furthermore, as the worms reproduce, the population will need to be controlled, and what better way than to add a few worms to the diet of the fish. Red wigglers may also be added to the grow beds of the system to help serve as yet another bio-filter (in conjunction with the plants and bacteria). Red wigglers have a high content of protein and vitamins, just as the duckweed, but the worms provide different kinds of nutrients (Woods). Red wigglers should only be used as a supplement to duckweed, for tilapia only require about 35% protein in their diet. Plus, the younger the fish, the more protein they need, and the older the fish, the less protein they need. With this in mind, it might be a good idea to chop up the worms a little bit before they are fed to the fish when they are young. It is also a good idea to feed tilapia red wrigglers in moderation.

All things considered, both duckweed and red wigglers can help close the aquaponic system, but it is also important to know that both duckweed and red wigglers cannot be relied upon to completely feed your fish. In order to make sure that both your fish and plants are healthy, you should only feed them duckweed and worms as a supplement. Both of these organisms combined do not supply all of the required minerals and nutrients that the fish and plants need, and that is why they should just be treated as supplements in your aquaponics system. Unless you are an expert in fish and plant nutrition, it is wise to feed your fish mostly a diet composed of scientifically balanced food pellets. With this said, feel free to experiment with your fish if you are not planing on selling your produce for profit. There is a lot to learn in the aquaponics cycle, and it would be ideal to completely close the loop (including a pump run by renewable sources of energy). It can be done, but it will require a lot of research, resources, time, money, and experimentation to get such a system dialed and running smoothly.

Basic duckweed bins. Photo by Fakhoorian, Tamra.

 

Works Cited

Leng, R A, J H Stambolie, and R. Bell. “Duckweed – a potential high-protein feed resource for domestic animals and fish.” Livestock research for Rual Development. Oct. 1995. <http://www.lrrd.cipav.org.co/lrrd7/1/3.htm&gt;. Retrieved 12 Dec. 2012.

 

Woods, Jonathan. “The Urban Aquaculture Manual.” Web of Creation Chapter 4. 2 Mar. 2009. <http://www.webofcreation.org/BuildingGrounds/aqua/Chap4.html&gt;. Retrieved 12 Dec. 2012.

 

Fakhoorian, Tamra. Duckweed Gardening. “Duckweed Growing Tips and Tricks.” 7 Mar. 2012. <http://duckweedgardening.com/2012/03/07/duckweed-growing-tips-and-tricks/&gt;. Retrieved 12 Dec. 2012.