7.2 FISH FEED AND NUTRITION

7.2.1 Components and nutrition of fish feed

Fish require the correct balance of proteins, carbohydrates, fats, vitamins and minerals to grow and be healthy. This type of feed is considered a whole feed. Commercially available fish feed pellets are highly recommended for small-scale aquaponics, especially at the beginning. It is possible to create fish feed in locations that have limited access to manufactured feeds. However, these home-made feeds need special attention because they are often not whole feeds and may lack in essential nutritional components. More on homemade feeds can be found in Section 9.11 and Appendix 5.
Protein is the most important component for building fish mass. In their grow-out stage, omnivorous fish such as tilapia and common carp need 25-35 percent protein in their diet, while carnivorous fish need up to 45 percent protein in order to grow at optimal levels. In general, younger fish (fry and fingerlings) require a diet richer in protein than during the grow-out stage. Proteins are the basis of structure and enzymes in all living organisms. Proteins consist of amino acids, some of which are synthesized by the fishes' bodies, but others which have to be obtained from the food. These are called essential amino acids. Of the ten essential amino acids, methionine and lysine are often limiting factors, and these need to be supplemented in some vegetable-based feeds.

Lipids are fats, which are high-energy molecules necessary to a fish's diet. Fish oil is a common component of fish feeds. Fish oil is high in two special types of fats, omega-3 and omega-6, that have health benefits for humans. The amount of these healthy lipids in farmed fish depends on the feed used.
Carbohydrates consist of starches and sugars. This component of the feed is an inexpensive ingredient that increases the energy value of the feed. The starch and sugars also help to bind the feed together to make a pellet. However, fish do not digest and metabolize carbohydrates very well, and much of this energy can be lost.
Vitamins and minerals are necessary for fish health and growth. Vitamins are organic molecules, synthesized by plants or through manufacturing, that are important for development and immune system function. Minerals are inorganic elements. These minerals are necessary for the fish to synthesis their own body components (bone), vitamins and cellular structures. Some minerals are also involved in osmotic regulation. 7.2.2

Pelletized fish feed

There are a number of different sizes of fish feed pellets, ranging from 2 to 10 mm (Figure 7.4). The recommended size of these pellets depends on the size of the fish. Fry and fingerlings have small mouths and cannot ingest large pellets, while large fish waste energy if the pellets are too small. If possible, the feed should be purchased for each stage of the lifecycle of the fish. Alternatively, large pellets can be crushed with a mortar and pestle to create powder for fry and crumbles for fingerlings. Another method is to always use medium-sized pellets (2-4 mm). This way, fish are able to eat the same-sized pellet from the fingerling stage right up to maturity.
Fish feed pellets are also designed to either float on the surface or sink to the bottom of the tank, depending on the feeding habits of the fish. It is important to know the eating behaviour of the specific fish and supply the correct type of pellet. Floating pellets are advantageous because it is easy to identify how much the fish are eating. It is often possible to train fish to feed according to the food pellets available; however, some fish will not change their feeding culture.
Feed should be stored in dark, dry, cool and secure conditions. Warm wet fish feed can rot, being decomposed by bacteria and fungi. These micro-organisms can release toxins that are dangerous to fish; spoiled feed should never be fed to fish. Fish feed should not be stored for too long and should be purchased fresh and used immediately to conserve the nutritional qualities, wherever possible.

Avoid overfeeding

Uneaten food waste should never be left in the aquaponic system. Feed waste from overfeeding is consumed by heterotrophic bacteria, which consume substantial amounts of oxygen. In addition, decomposing food can increase the amount of ammonia and nitrite to toxic levels in a relatively short period. Finally, the uneaten pellets can clog the mechanical filters, leading to decreased water flow and anoxic areas. In general, fish eat all they need to eat in a 30 minute period. After this length of time, remove any food. If uneaten food is found, lower the amount of feed given the next time. Further feeding strategies are discussed in Section 8.4.

 

FIGURE 7.4
Example of fish feed in pellets and powder used for various size classes of fish

7.2.3 Feed conversion ratio for fish and feeding rate

The FCR describes how efficiently an animal turns its food into growth. It answers the question of how many units of feed are required to grow one unit of animal - FCRS exist for every animal and offer a convenient way to measure the efficiency and costs of raising that animal. Fish, in general, have one of the best FCRs of all livestock. In good conditions, tilapias have an FCR of 1.4-1.8, meaning that to grow a 1.0 kg tilapia, 1.4-1.8 kg of food is required.
Tracking FCR is not essential in small-scale aquaponics, but it can be useful to do in some circumstances. When changing feeds, it is worth considering how well the fish grow in regard to any cost differences between the feeds. Moreover, when considering starting a small commercial system, it is necessary to calculate the FCR as part of the business plan and/or financial analysis. Even if not concerned about the FCR, it is good practice to periodically weigh a sample of the fish to make sure they are growing well and to understand the balance of the system (Figure 7.5). This also provides a more accurate growth rate expectation for harvest timing and production. As with all fish handling, weighing is easier in darkness to avoid stressing the fish. Box 3 lists simple steps for weighing fish. Weighing fish of the same age growing in the same tank is in general more preferable than heterogeneous cohorts of fish because the measurement should provide more reliable averages.

BOX 3

Simple steps for weighing fish
1) Fill a small bucket (10 litres) with water from the aquaponic system.
2) Weigh the bucket and water using a weighing scale and record the weight (tare).
3) Scoop 5 average size fish with a landing-net, drain the landing-net from excess of water for a few seconds and place the fish into the bucket.
4) Weigh again and record the gross weight.
5) Calculate the total weight of the fish by subtracting the tare from the gross weight.
6) Divide this figure by 5 to retrieve an average weight for each fish.
7) Repeat steps 1-6 as appropriate. Try to measure 10-20 percent of the fish (preferably no duplicates) for an accurate average.

FIGURE 7.5
Weighing a sample of fish using a weighing scale