Aquaponic Fish are Not Garbage Disposals

By: Tawnya Sawyer

Dorm food

I’ve been doing a lot of radio interviews lately to promote my new book, Aquaponic Gardening. While they’ve mostly been local public radio stations, and aquaponics has been the star of the show, last week’s interview was different. It was my first shot at National Public Radio. My excitement at this quickly turned to nervousness as I realized that I was the one being brought in to declare that the Emperor has no clothes.

The reporter wanted to interview me as a continuation to a story she was already working on. This story featured an Ecological Engineering graduate student at Syracuse University who is experimenting with taking the food waste from a campus residence hall cafeteria, drying it, grinding it, pelletizing it and feeding it to the tilapia in an aquaponics system. Sounds great from an environmental perspective, right? But my first thoughts went to the health of the fish. Heck, much of what comes out of a college dorm isn’t even fit for growing healthy humans, let alone fish. Was doing this right as simple as correcting the human food mix for the known ratios of protein, vegetable matter, and carbohydrates that fish need (from a college cafeteria no less)?

While I had my suspicions that I was about to rain on this poor grad student’s wonderful project, I decided to check with some of my friends and colleagues who are aquaculture experts to see what they thought. Their responses? To summarize, the idea of using the food waste from a college dorm is great for the environment, but is probably harmful to the aquaponic fish as well as economically impractical. Those email conversations are excerpted below for those of you who are interested in how I drew these conclusions. But for the rest of you, here is the quick version.

Fish Health
When we raise animals in captivity it is both our economic and moral imperative to keep them healthy by feeding them a diet that closely replicates what they have evolved to eat in the wild. Fish live off algae, bacteria and protein (bugs, worms and other fish), not hamburger, cucumbers and pizza crust. What are the results of this?

  • The food will be unpalatable to the fish, just as if we were given a plate of algae, bacteria and worms. If you are hungry enough you will probably eat it, but you will certainly lose weight before you get to that point!
  • Some of what the fish do eat will not be digested – again, just like we can’t digest some things. This undigested food will pass through their waste stream whole and affect the water quality in the fish tank.
  • They will not get enough of some critical elements creating nutrient deficiencies. Dr James Rakocy, one of the experts I consulted, gave an example of this with vitamin C and broken back syndrome. (see his e-mail below for some specifics)
  • They will get too much of other things. Dr. Wilson Lennard talked about known diseases, such as ‘fatty liver syndrome’, caused by fish eating mammalian protein. Not to mention the empty carbohydrates from white flour, white rice, and processed foods in general.


Even if this approach were to somehow grow reasonably healthy fish to maturity, it is hard to imagine a profitable business model because of the following concerns:

  • Collecting and drying while still unspoiled will be costly.
  • Problems will arise in the milling process due to the inconsistency in the inputs.
  • Regulations will arise. It is currently illegal to feed food scraps to swine, so you can bet the USDA will quickly get involved.
  • Marketing stigma. Who wants to buy fish that have been fed garbage? We should be trying to develop better, not worse feed for our farm raised fish.


Of course, directing waste streams into productive uses is outstanding. If this food were sent to an on campus vermicomposting or Black Soldier Fly (BSF) waste recycling system, I would be unqualified in my support of this student’s efforts. Worms and BSF larvae have evolved to break down garbage, and both are delicious, and appropriate, food for fish! By adding either or both of these steps into his process, the student would shift from using the fish as garbage disposals to creating a potentially economic AND healthy food for the fish (BSF larvae and worms) as well as simultaneously creating an excellent, organic vermicompost fertilizer for the school’s landscaping. We simply can’t lose sight of the welfare of the animals in our care in our zeal for more sustainable environmental solutions.

To read more, please see the e-mails I received from those I consulted:

Myles Harston of Aqua Ranch

First of all tilapia are survivors. These fish will eat what they have to survive. What is determining the nutritional levels of the fish food scrape feed? Is there any sort of monitoring of the nutrients other than separating the different kinds of foods?E.g. Carbos, proteins. Assuming the human food is completely ideal for humans, the nutrient profile for fish is quite different. In the wild, tilapia will eat things like this; plant roots, algae, flok, moss, duck weed. So our challenge in captivity is to get as close as we can to something they would naturally eat. That is not completely possible right now. Now getting back to the human diet. We humans are often eating food that is overly refined like flour, sugars and artificial sweeteners. Many of the things that refined out of human food (like rice and wheat) are recaptured and used in vitamin pills. Crazy isn’t it? In our society, people are living longer but the quality of life in older ages has declined. That is diet related. Fresh produce and fruits are now (and for the past 5 or 6 years) irradiated. Irradiation kills the enzymes that break food down and it gives it a longer shelf life. The problem with that is that these are the same enzymes that we needed to digest the foods. How is this gentleman replacing the enzymes? Yes, I believe the fish will eat it. But with Tilapia being survivors, that is the reason that the fish coming from China are so dangerous with materials like melamine, heavy metals and manufacturing materials actually being found right in the flesh of the fish. What is being done about GMO and Roundup exposure? Most Americans are completely naive about this but by feeding it to the tilapia will double down on this.

Dr. James Rakocy, retired professor, University of the Virgin Islands

Tilapia have specific nutrient requirements, and food waste, which will be highly variable in nutritional quality, will most likely result in a low rate of growth. Fish need the right protein to energy ratio, the right protein composition, and the right types of fats and oils. They also have vitamin and mineral requirements that must be met. For example, fish must have vitamin C for healthy growth. Without enough vitamin C their spines break. This is called broken back syndrome. A random sample of food waste could have no vitamin C at all. Food waste might be more appropriate for fish that are scavengers like catfish. Tilapia are omnivores and prefer to eat algae, bacteria and insects. They will not eat a dead fish. There might also be a palatability problem. I can imagine that most ingredients in food waste would not be appealing to tilapia just as eating bacteria is not appealing to humans. The article is wrong in that fish feed is comprised mainly of fish. Tilapia feeds contains only 5% fish meal and researchers are close to eliminating fish meal entirely. The major ingredients in fish feed are corn and soybeans. The variability of food waste will also result in the variability of nutrients available from fish waste for plant growth in an aquaponic system, thereby increasing the risk of plant nutrient deficiencies. There have been thousands of peer reviewed journal articles on fish nutrition. Feed manufacturers take these results to formulate the best possible diet for maximum fish growth. Grabbing a bunch of leftover food and hoping to achieve haute cuisine for fish is comparable to finding a needle in a haystack.

Dr. Wilson Lennard, Aquaponic Solutions, Inc.

It is hard to comment not knowing what the source of the ingredients is. If there is any mammal protein, fat or carb in there, then no, it isn’t a good long term feed. It may grow fish well, but the proteins, fats and carbs in mammal meat are very different from those in fish or vegetables. There are well known diseases in fish that are fed a high mammal based diet. The most common is called “fatty liver”. This is where the fats from the mammal based feed deposit around the fish’s liver and eventually leads to death.


Tony Vaught of Pro-Aqua

Most of the time when you use the premise to use what you have rather than what the fish needs the feed is usually either deficient in some nutrient or element or it is un-digestible leading to poor water quality. In the case of un-digested feed in the water it will cause problems with suspended solids, possibly resulting in a clogged system. This will lead to additional unanswerable fat in the body cavity or it may lead to calcium deficiencies. Just because the fish eats it does not mean it is a healthy diet. Look to us humans for an example. We have a choice and even then eat unhealthy.

It may work to canalize the feed then add what is needed to make it complete, but it is not possible to take out fats or indigestible components in the ingredients. The other problem is waste food from institutions have other unknown contaminates that may get in the feed such as insecticides and cleaning agents. Each batch of waste food has different components making it necessary to test each batch then adjust it to fit the fish. Many milling problems and pellet quality issues will arise as well.

Great idea philosophically but not practical.

Kellen Weissenbach of White Brook Tilapia Farm

My main concerns would be about potential contamination issues (non-food items, chemicals, etc.) and a constantly changing “recipe” which could cause issues for the fish potentially and also delivers different levels of nutrients from batch to batch.