Aquaculture and Aquaponics Nutrition: How Fish Shape the Growth of Greens
Trenton L. Corby, MS in Aquaculture Aquatic Animal Nutritionist and Water Quality Specialist
INTRODUCTION TO MODERN AQUAPONICS
Aquaculture is universally defined as the culture of aquatic organisms i.e., fish, shellfish (both crustaceans and bivalves), reptiles (crocodilians and testudines, largely), amphibians, algae, and/or corals. Hydroponics, conversely, is the cultivation of plants utilizing nutrient rich water as opposed to more traditional farming practices, such as soil.
When performed in unison, aquaponics is achieved. Aquaponics is then, by definition, the culture of both aquatic animals and plants together within adjoining systems, where one organism directly impacts the other. In theory, this is done largely through nutrients, effluence, and water chemistry, and while modern aquaponics is still in relative infancy, different forms of it can be traced back centuries.
Ancient Egypt, China, the Roman Empire, and Ancient Mexico all had varying degrees of plant cultivation that largely resemble modern aquaponics, primarily through the usage of fish waste as a nutrient source for crops. Even today in its simplest form, aquaponics is being done within rice paddies around the world, from the integration of carp in Southeast Asian rice paddies, to crayfish (or “crawfish” as they are colloquially known) as seen in the Deep South region of the United States.
As the animals naturally produce waste within the rice paddies, the nutrients occurring within said waste act as fertilizers for the rice plants.
The rice, in return, helps to “clean” the water that pools within the rice paddy, creating more favorable water conditions for the crayfish.
Modern aquaponics is rarely so simple but follows the same basic principle. Aquatic organisms produce the necessary nutrients that the plants need to grow. Seldom discussed, however, are the implications in question as to how commercial diets, ones offered to cultured fish and shellfish species, impact the nutrient load of the aquaponic system, as well as the nutrient acquisition of the plants.
PROTEIN IN AQUAFEED AND THE IMPACT OF NITROGEN ON AQUAPONICS PRODUCE
The differences in nutritional requirements vary considerably for both plants and animals. For animals, the primary macronutrients are defined as complex biomolecules: proteins, lipids, and carbohydrates. Likewise, the macronutrients for plants are defined much more simply, as the elements nitrogen, phosphorus, and potassium (or N-P-K as seen on fertilizers).
Animal nutritionists largely follow the requirements of these nutrients when formulating a diet for an animal, just as chemists and agronomists do when developing fertilizer for plants. In interconnected systems, however, the nutritional requirements of one organism may drastically dictate the availability of nutrients to the other.
Proteins are composed of amino acids, which in turn are composed of bonded atoms and elemental groups composed of hydrogen, carbon, oxygen, and nitrogen. The protein level found in aquaculture diets directly impacts the nitrogen levels required to grow desired produce, with fish only being able to metabolize 25-30% of protein consumed in these diets (Dr. Khanal, Bioenergy Research Group, Hawaii). The remaining nitrogen is excreted in the form of ammonia (NH3) during metabolism.
The excreted ammonia is broken down by naturally occurring bacteria within the culture system, known as ammonia oxidizing bacteria and nitrite oxidizing bacteria (historically known as simply Nitrosomonas and Nitrobacter bacteria).
It is through these bacteria-driven processes that ammonia is converted first into nitrite (NO2), then finally nitrate (NO3). This nitrate is what is utilized by plants as the available nitrogen source. Therefore, it would stand that diets formulated to contain a higher protein percentage (potentially for more carnivorous fish or for younger, larval fish) would also yield higher potential nitrates for plants.
Protein sourcing has also shown some significance. In a study by Shaw, Knopf, and Kloas (2022), four diets with varying protein sources were formulated for Nile tilapia (Oreochromis niloticus) within an aquaponics system. Two of the primary goals of the study included various protein sources as replacements to fishmeal in aquafeed, as well as to determine aquaponics specific feed based on the differing protein sources.
The results of the study showed that fish offered the various diets grew similarly regardless of the protein choice. However, diets formulated to contain poultry by-product meal as well as black soldier fly larvae meal resulted in the most nutrient-rich RAS water.
The system where the fish were offered the black soldier fly larvae meal generated one of the most favorable nutrient profiles for the plants, with significantly higher concentrations of potassium (23.88 mg/L), magnesium (20.19 mg/L), and phosphorus (3.08 mg/L) than the fishmeal and poultry blood meal treatments, while maintaining comparable calcium (134.28 mg/L) and nitrate (42.96 mg/L) concentrations.
This furthers the implication that selecting ingredients carefully for aquaponics-centered diets is crucial as certain ingredients yield different nutrient loads via fish waste in aqueous systems.
PHOSPHORUS AND ITS IMPACTS IN ITS GIVEN ENVIRONMENTS
Phosphorus levels in aquafeeds are also something important as they present a catch 22 of sorts. Phosphorus is a crucial nutrient for both plants and animals. In fish, phosphorus is critical for the production of ATP, DNA, and RNA. The element, as well as calcium, also acts to form bones and scales. Without adequate phosphorus supplementation, growth becomes stunted, FCRs suffer, and skeletons become deformed over time.
Similar effects are also observed in farmed crustaceans, as phosphorus deficiencies impact energy levels in Pacific white shrimp as well as impair molting and exoskeleton development. Therefore, maintaining adequate amounts of phosphorus needed in aquafeed diets is paramount.
However, if diets are oversaturated with phosphorus-rich ingredients, or if the phosphorus is in a form that does not allow for easy uptake, it leaches out into the natural environment. In fact, both phosphorus and nitrogen are considered two primary nutrients that have the potential to adversely affect surrounding waterbodies.
This is considered undesirable for several reasons, with examples such as algal blooms, proliferation of aquatic plants and bacteria, and degradation to water quality, with parameters such as turbidity and dissolved oxygen being most impacted.
So, when aquatic organisms are cultured through ponds or raceways (culture systems open to the exposed environment), phosphorus must be in high enough quantities to allow for fish and shellfish development, but not so high that it pollutes the environment and risks killing off the species in question.
This, however, changes within controlled agricultural systems, such as Recirculating Aquaculture Systems (RAS).
RAS is the primary way in which modern aquaponics takes form, via open loop or closed loop systems. Fish and shellfish are kept in tanks and offered manufactured feed. Their waste stays within the system but is moved out to the area of the system where the plants are kept (whether the waste returns to the fish section of the system is determined via the system being closed loop or open loop).
Appropriate CaP ratios are also crucial for plant growth and development as well. Previous studies have also shown that feed supplemented with trace amounts of phytase improved phosphorus absorption. It should be noted that aquaponic-based feeds should be more carefully tailored towards optimizing phosphorus-calcium balances, or the incorporation of ingredients that could increase phosphorus utilization (i.e., phytase).
MICRONUTRIENTS
Within an aquaponics system, iron (Fe) is more often than not listed as one of the primary limiting micronutrients. Other limiting micronutrients include boron (B), copper (Cu), and zinc (Zn). These named micronutrients, of which all are metals (with the exception of boron, which is classified as a metalloid), are found in trace amounts within aquaculture feeds. Accordingly, it would stand to reason that once they enter into an aquaponics system via waste excretions, they would be available in even smaller quantities.
This is due in part to both the fish and the plants needing very minute quantities of these elements. Over-supplementation of such substances could potentially lead to a toxicity point in either (or even both) individual species. Using iron as an example, Liu et al. (2025) examined iron supplementation in Mirror Carp (a variant of common carp, or Cyprinus carpio) within aquaponics systems and determined that offering carp between 200 mg/kg and 400 mg/kg of iron within a diet improved individual fish’s health, while over-supplementation damaged the liver and stunted growth.
Within open loop aquaponics systems, where fish effluence and system water is unidirectional, it is much easier to supplement these micronutrients sparingly within the areas of the system containing the plants, not the fish or shellfish.
Chelated iron is the primary way to deliver iron to aquaponic systems and is usually done this way via a reservoir or “head tank”. That way when plants show deficiencies in iron (usually in the form of chlorosis), this area of the system can be supplemented with the appropriate levels of iron without harming the fish, nor overfortifying feeds with excess amounts of these nutrients.
OTHER CONSIDERATIONS
Although it could largely be considered semantics, it is worth mentioning that there are many factors that could contribute to nutrient load within an aquaponics system, and that species selection is as important as selecting the appropriate diet. Selecting a fish species versus a crustacean species yields different nutritional requirements that would also change waste properties that would impact plant growth and development.
Age of species is also crucial, as younger individuals require different diets as opposed to fully mature individuals, or even individuals reaching the end of their natural lifespan, as cellular deterioration could impact nutrient absorption and digestion. Stocking density will also be proportional to feed load being delivered to the system, which then will impact waste production as well.
In fact, Feed Rate Ratio (not to be confused with FCR) is directly utilized to determine feed input within a system to growth potential and nutrient load within the plant section of the system. And just as aquatic animals have different diets and nutritional requirements, such as being carnivorous, omnivorous, or herbivorous, plants also have different nutritional requirements based not just on species, but whether they are fruiting crops as opposed to “leafy greens”.
CONCLUSION
In conclusion, aquaculture diets and aquaponics diets, while similarly formulated with fish or shellfish in mind (typically), differ in the inclusion of plants within the production system. Therefore, when formulating a diet for an aquaponics system, it is important to consider both the nutritional needs of the animal as well as the plant.
REFERENCES:
- Liu, Y., Dou, Z., Ji, C., Zhou, Q., Zhao, J., Wang, K., … & Liu, Q. (2025). Effects of dietary ferric EDTA levels on vegetables and mirror carp (Cyprinus carpio var. specularis) in aquaponics system. Animals, 15(6), 792.
- Shaw, C., Knopf, K., & Kloas, W. (2022). Fish feeds in aquaponics and beyond: a novel concept to evaluate protein sources in diets for circular multitrophic food production systems. Sustainability, 14(7), 4064.