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Fish feed is the driving force behind the aquaponic system. Fish feed is primarily made up of protein, carbohydrate, and fat, with other ingredients like fiber, vitamins, minerals, and binders in smaller quantities. The nutrient components of these ingredients, whether pre-digested by the fish or simply broken down in the water, become the nutrient source for the plants in the system. However, for better or worse, these are the only nutrients available for plant crop growth, so fish feed input requires careful management. Table 5 outlines the feeding rate for fish based on body weight. To calculate the amount of feed needed to support plant and fish growth, Feed Conversion Ratio (FCR) must be calculated. The
FCR is a ratio of fish diet fed in relation to fish flesh gained. The ideal FCR is 1, or 1 pound of diet fed to 1 pound of fish growth, but a more realistic number is closer to 1.4-1.8. FCR is calculated using the following formula:
$$FCR = total\ feed\ input\ (g) ÷ total\ weight\ gain\ (g)$$
Protein is the limiting factor in fish growth but is also the most expensive dietary component. For these reasons, it is important to choose the appropriate diet for fish. Inadequate protein will reduce growth and too much protein is cost prohibitive and can lead to water quality issues. Figure 15 details protein requirements for commonly cultured fish species.
Before feed is purchased for production, practical considerations include fish age, feed size, protein/ carbohydrate content, floating vs. sinking pellets, length of time in storage, and feed storage area. The rule of thumb when choosing pellet size is that the pellet should be as big as the fish's mouth. As fish grow, so should the size of the pellet. Pellets are classified as float, slow sink, or sink, and the right choice depends on the species being fed. Feed manufacturers are able to give directions on the right type of feed for each production stage. Storing feed is actually a big consideration, as nutrient quality begins to decay after production. Feed storage in a dark, chilled or frozen environment is preferable, as it delays nutrient quality degradation but can introduce moisture resulting in moldy pellets. Molded feed must be thrown out, or composted into a garden, but must never be fed to fish, as it may contain toxins produced by the mold. Only enough feed should be purchased that can be fed in six months of straight production. When not used, feed should be kept in a cold, dry place with low relative humidity.
Table 5: Recommended feed chart for tank culture of Tilapia.
Length (cm)Average Weight (g)Standard Feed SizeRange of Feeding Rate (% biomass/day)Feeding Frequency< 2.5< 0.5#00, #0, #1 Crumble20 – 154x per day2.5 - 6.40.5 – 5#2 Crumble15 – 104x per day6.4 - 10.25 – 18#3 Crumble10 – 54x per day10.2 – 15.218 – 751 mm5 – 33x per day15.2 – 20.375 – 1501/8 inch (3 mm)3 – 1.53x per day20.3 – 33150 – 4503/16 inch (4 mm)3 – 1.52x per day33+> 4503/16 inch (4 mm)11-2x per day
*Reproduced and adapted from DeLong et al. (2009) and Sawyer, J.D. (2019).
Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.