Designing feeds for aquaponics
Fish feeds for aquaponics can be home-made or bought from specialized feed companies that formulate specific diets depending on the species and age of the fish. Normally commercial producers use specialized feeds since they are guaranteed to meet all the nutritional needs of the fish, and tend to be more cost effective compared to making and formulating one’s own feed. However, formulated feeds are not always perfect and may have varying effects on the quality of the water where fish live and excrete waste. Only recently have scientists and engineers begun to look at specific diets for fish in recirculation systems and in aquaponic units. Theoretically it seems possible to provide fish with pelleted feed, which will help them to grow quickly, while providing enough nutrients for the plants that will later ‘feed’ on this water. In practice, however, things are more difficult, and depend on many complex parameters such as the temperature and pH of the recycled water, as well as microbiota in fish intestines and in biofilters. An aquaponics practitioner should know the basics of feed composition in order to have some way to judge which feed would be best to start off with. Although it may not be necessary to design feeds from scratch, students should be able to choose the best feed for this system after reading the following sections.
Fish growth and nitrogen retention
The nitrogen that will eventually be eliminated as ammonia by the fish comes from the protein in the feed. Although there is some nitrogen in other components of feed, almost all of the nitrogen absorbed by fish and eliminated as waste is from amino acids since, as their name suggests, they all contain nitrogen in the chemical makeup.
If we know the percentage of nitrogen in the feed, we can then calculate the approximate amount that will be excreted as ammonia into the water by a process similar to that of urination. That ammonia will later be turned into nitrate which will be provided to the plants. It should be noted here, however, that fish do not really urinate but, as opposed to most mammals, they eliminate nitrogenous waste through their branchia (similar to our lungs). In the following sections we will follow the source and fate of nitrogen in an aquaponic system, based on Seawright et al. (1998), who were one of the first groups to publish studies on nutrient cycling in aquaponic systems, several decades ago. In their paper they provide an equation for calculating the nitrogen balance in the system, which we will use as a guide. After calculating the nitrogen present in the feed, we calculate how much is retained in the fish, lost as uneaten feed, and lost in faeces, to end up with the concentration of ammonia in the surrounding water.
Nitrogen source
Feed is the main nitrogen source in our aquaponic system. In order to calculate the total amount of nitrogen placed into the tank via the feed we first need to know the exact amount of feed used, in grams or kilograms. Next we need to know the percentage of protein in the feed. This is normally shown on the feed label or available from the feed producer. As mentioned in previous sections, fish feeds have high proportions of protein, normally between 25% and 50%. Once we know the protein percentage we can calculate the percentage of nitrogen by dividing it by 6.25. We use that number since nutritionists assume that 1/6.25 or approximately 16% of all protein is nitrogen. Thus, for a feed for tilapia with 35% protein we know it has 35% * 16% = 5.6% nitrogen. If we added, for example, 120 grams of feed to the tank in one day, we are adding 120 * 5.6% = 6.72 g of nitrogen.
Nitrogen absorption by the fish
The fish will absorb nitrogen into its protein deposits, which is mostly its muscle. However, most of the fish body weight is water, so that weight has to be discounted since the nitrogen is only present in what can be called the ‘dry weight of the muscle’. In general, and based on results in our lab and findings from the literature (e.g., Seawright et al. 1998), the dry weight of tilapia is about 27% of its body weight or, put another way, 73% of tilapia muscle is water.
Next we need to know the feed conversion rate (FCR). The FCR is the ratio between the feed provided divided by the weight gained. The inverse of the FCR is called the feed efficiency, or the weight gain divided by the feed ingested. The FCR is typically around 1-2 in fish. The feed efficiency, on the other hand, can be viewed as 1 divided by the FCR. That is, for a conversion index of 1.5, the feed efficiency is 1/1.5 = 66.73%. To put it another way, about two thirds of the feed eaten by the fish will be absorbed by the muscle of fish and counted as growth.
Of course it would be better to have a high feed efficiency (close to 100%); the higher it is, the more economically advantageous it is. However, fish have a maximum limit for how much muscle they can accrue over time. As muscle grows, the amount of protein will grow (as well as the amount of total nitrogen in the muscle), but the proportion of protein in the muscle will stay more or less stable. The total percentage of nitrogen with respect to body weight is around 8.8% in tilapia. This may vary among species, but is a good approximate number.
So, depending on the feed provided, we can estimate how much nitrogen will be retained in the fish. If we provide 120 g of feed using the values suggested above, then the nitrogen retained in fish will be found by multiplying the feed by the dry weight, by the feed efficiency and by the percentage of nitrogen in fish muscle, i.e., 120g * 27% * 66.73% * 8.8% = 1.90 grams of nitrogen from the feed will stay in the fish.
Nitrogen lost in solids
Apart from being lost as urine, nitrogen waste can be lost via faeces. We can measure the protein or nitrogen content of faeces since it accumulates in the solids filter of our system, or we can siphon it up daily and store it. The solid waste could also contain feed that was not ingested but, as mentioned above, it is difficult to measure exactly how much feed was not consumed by the fish, so we lump together faeces and feed not consumed as solid waste. Before analysis, the solid waste is dried in order to calculate the dry weight, and then the nitrogen content is measured. In a RAS system the total amount of solids is around 10%, i.e., 10% of the feed provided to the fish ends up as solid waste (including fish faeces and pellets that are not ingested). When analysed we found that the nitrogen content of the faeces was 4.8%.
As we explained earlier, protein is 16% nitrogen, or that is what nutritionists estimate. Thus, if we only have a measure of nitrogen, to obtain the amount of protein it came from originally we need to ‘back-calculate’ by dividing the amount of nitrogen by 16%, which is the same as multiplying it by 6.25% (1/16 = 0.0625 or 6.25%). So in the case where the nitrogen content of the faeces was 4.8%, the amount of protein would be 4.8% * 6.25% = 30%.
Finally, to calculate the total grams of nitrogen lost in solids per the amount of feed we provide to the tank, we need to multiply the amount of feed (120 g) by the percentage of feed that is lost in solids (faeces and feed not eaten), and the percentage of nitrogen in the solids (4.8%). Say that the percentage of feed lost in solids is 10%, the nitrogen lost in solids in that case would be: 120g * 10% * 4.8% = 0.576 g of nitrogen in the feed is lost as solids. Again, this is only an example, and that percentage can vary depending on the system and other conditions.
Nitrogen dissolved in water as ammonia
Next we can use the above calculations to quantify the nitrogen dissolved in the water, which is essentially lost as ammonia waste. First we add the nitrogen absorbed by the fish and lost in faeces, and then subtract it from the nitrogen applied via the feed. The remaining nitrogen is the amount lost or dissolved in the water. In the case above, 6.72 – (1.90 + 0.576) = 4.24 g NH3. That is, 63.1% (4.24/6.72) of the nitrogen from the feed is converted into NH3. It is excreted by the branchia as NH3 but, depending on the water pH, it is converted into NH4. The term TAN denotes total ammonia nitrogen, or the combination of NH3 + NH4. In Figure 6 we provide an example of results from our lab where the total nitrogen was calculated in feed, and then measured in the fish, faeces, and water.
Figure 5: Example of a nitrogen cycle analysis in tilapia using four different feeds based on different protein sources (fish meal, soy, corn gluten, and pea concentrate)
Copyright © Partners of the Aqu@teach Project. Aqu@teach is an Erasmus+ Strategic Partnership in Higher Education (2017-2020) led by the University of Greenwich, in collaboration with the Zurich University of Applied Sciences (Switzerland), the Technical University of Madrid (Spain), the University of Ljubljana and the Biotechnical Centre Naklo (Slovenia).