FISH FEED AND NUTRITION
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.
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.
Pelletized fish feed
There are a number of different sizes of fish feed pellets, ranging from
2 to 10 mm. 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.
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. 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. The picture below 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.
Periodical weight measurements will give the average growth rate of the
fish, which will be obtained by subtracting the average fish weight,
calculated above, over two periods. The FCR is obtained by dividing the
total feed consumed by the fish by the total growth during a given
period, with both values expressed in the same weight unit
(i.e. kilogram, gram). Total feed / Total growth = FCR The total feed
can be obtained by summing all the recorded amount of feed consumed each
day. The total growth can be calculated by simply multiplying the
average growth rate by the number of the fish stocked in the tank. At
the grow-out stage, the feeding rate for most cultured fish (as
discussed in this publication) is 1–2 percent of their body weight per
day. On average, a 100 gram fish eats 1–2 grams of pelletized fish feed
per day. Monitor this feeding rate at the same time as the FCR to
determine growth rates and fish appetite and to help maintain overall
system balance.