By: Paul B. Brown*
Whenever you have dynamic interactions between 300 million people and the American economy acting in really complex ways, that introduces a degree of almost chaos theory to the system, in a literal sense.” Nate Silver (1978-), American writer.
Nate Silver’s description of the American population and their interaction with the economy come close to describing the complex interactions in intensive aquaculture systems. All food production systems tend to intensify over time to improve efficiency and gain control. Aquaculture systems have been in the process of intensification since the explosion in production that began in the 1990’s. The most intense systems imagined to date are the closed loop, recirculating systems (RAS). In our previous article we considered the nutritional needs of the plants that can be incorporated into the system (aquaponics); however, there is a third organism in an aquaponic system, or a second organisms in a RAS that must be fed.
Intensive systems rely on bacteria to oxidize the excreted ammonia-N from the fish/shrimp. Ammonia is toxic and if not oxidized can increase to concentrations that cause harm, even death, of the target organisms. Ammonia is oxidized in a 2-step process, initially to nitrite, which is also toxic, then finally to nitrate, which is almost nontoxic. Facilitating the oxidation of ammonia-N is almost identical to the N cycle found on land, the same one farmers use to fertilize field crops. Common bacteria carry out this 2-step process. However, do we provide the nutrient needs of these organisms? Can they survive on simply nitrogenous products and oxygen? Unfortunately, the typical perception is that bacteria need a source of N and oxygen, when in fact, they have relatively complex nutrient needs.
Table 1 depicts the nutrient needs of the two basic bacteria needed to complete the N cycle in RAS, Nitrosomonas and Nitrobacter, compared to the mineral nutrient requirements of fish. Many of the nutrients required for pure culture of bacteria are indeed required by fish and supplemented into the diet, but several are not. Qualitatively, molybdenum, cobalt, and chromium have not been clearly identified as required nutrients for fish, although they are considered essential nutrients for other animals. Based on this simple comparison, we might suspect that fish/shrimp diets are inadequate for sustainable growth of N oxidizing bacteria. As we were working on this topic several years ago, we incorporated a quick analysis of several RAS systems in the Aquaculture Research Laboratory.
Table 2 contains the results of mineral analyses from existing RAS system operated at various temperatures and containing three different species. Tilapia were operated at 28º C, perch at 22º C and trout at 14º C. Each fish species was fed a specific commercial diet that met their nutritional needs. All systems had been in operation for over 3 months, and some as long as 9 months. Other than the macrominerals (Ca, P, Mg, Na, K and S), all microminerals were below detectable limits (BDL) of the instrument, despite the fact these minerals were being added to the system daily (via the fish food). Our conclusion was that the bacteria we rely on to complete the N cycle were uptaking the minerals from solution at a faster rate than they were being added to the system. The question that needs addressing is at what point do we not provide adequate mineral nutritional needs for the bacteria? This will be a function of stocking density, feed input, mineral availability through the animal, and solubilization of minerals once excreted. Water quality will likely play a role in feeding the bacteria. Can we look at the data in Table 2 and feel comfortable the bacterial populations are being provided their nutrient needs? Is this a sustainable scenario? The answers are we do not know.
So, 300 million people interacting in a complex economy. Chaotic, yes, which might be why so few people understand the nuances of the economic system. Data in Table 2 includes 18 minerals, which interact with a target organism (fish/shrimp), then bacteria, then potentially plants in an aquaponic system, all influenced by water quality parameters (pH, alkalinity, hardness, dissolved oxygen, carbon dioxide). Our producers can see nutritional deficiencies in fish/shrimp, and those are rare these days, and they can see the more common mineral deficiencies in plant species in aquaponics. We cannot easily detect nutrient deficiencies in bacteria, until ammonia-N or nitrite-N increases to dangerous concentrations. These systems are complex, poorly understood and not adequately modeled to allow producers to have a high degree of certainty whether all organisms have sufficient nutrients. We have a great deal of research to do in this area, but the basic systems and operational parameters are working for those pioneers in this field. Further intensification will require a far more precise understanding of these and other interactions.
Dr. Paul Brown is Professor of Fisheries and Aquatic Sciences in the Department of Forestry and Natural Resources of Purdue University. Brown has served as Associate Editor for the Progressive Fish-Culturist and the Journal of the World Aquaculture Society, among many others.
pb@purdue.edu
