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Apart from using adequate feeds, we need to ensure that the pellets provided are the right size for the mouth of the fish. For small fish this usually means a fine powder and for larger fish a round pellet that can be several mm in diameter. For example, Aquaponics USA suggests using powder for tilapia from hatching to 3 weeks old, and then a fingerling crumble (1/32 inch or 0.9 mm) until they grow to about 2 cm in length, fingerling pellet (1/16 inch or 1.6 mm) until about 4 cm in length, and grow out pellet (3/16 inch or 4.8 mm) after about 6 cm in length.
It is also necessary to distribute the feed adequately. Normally feed is thrown onto the surface of the tank and personnel perceive how the fish react – whether they move to the surface and begin to eat (generally a good sign), or whether they remain on the bottom of the tank (generally a bad sign). However, in neither case is it obvious whether they are eating properly, how much ends up in their mouths, and how much is wasted. Due to these problems it is quite easy to overfeed.
In general, feed is distributed to the fish according to feeding tables that are prepared by the feed producer in terms of water temperature and growth stage. But the perception of the feeder, the personnel giving out the food, is very important since he/she can tell how hungry the fish are, and that is related to health and welfare. More and more efforts are being made to automate the process, and systems have improved considerably, but we cannot underestimate the importance of observing the fish, which is probably the best and most direct method of understanding their status. While much research has been performed to optimize feeding for maximum growth, it is obvious that if we provide less feed than they need, they will grow less, and the producer will lose money.
In order to understand the feeding process we need to define some concepts, based on Figure 2, which was developed by Skretting, an important feed company. We need to define the concept of maximum ration, which is the theoretically ideal ration to be given to the fish. However, it is specific to each farm since it depends on external conditions such as water quality and temperature, as well as tank design. The main concepts and indices used commercially include the following:
Feed conversion rate (FCR): this is the ratio between the amount of feed ingested (in kilograms or grams) divided by the live weight increase (in kg or g). On a commercial level we sometimes use an ‘industrial FCR’ which is an approximate figure based on all the feed provided over a period of time divided by the tonnes of fish produced during that same period. In that case, if there was mortality, we do not subtract the feed consumed by the fish before their death. This industrial FCR provides an idea of the real production costs. Another similar index is the biological conversion factor (BCF), which is the kg of feed really consumed by the fish divided by kg gained. It is harder to calculate the BCF at an industrial level since the fish have to be handled and the feed put down their throats, but is useful when we want to know the maximum efficiency of newly developed feeds. FCR describes the amount of feed needed for one kg weight gain by the fish:
This ratio reflects the nutritional and economic value of a feed. An FCR of 1 means that you have a live weight gain of 1 kg if you feed 1 kg of feed. The higher the FCR is, the higher your feed expenses are. Young fishes have a lower FCR (between 0.4 – 0.8), while adult fishes have an FCR between 0.9 – 2. The FCR depends on fish species and feed manufacturer. Sometimes you get more economic value with high quality food and the related better growth of the fish, in comparison to cheaper feed with a lower FCR.
Specific growth rate (SGR): this represents the percentage daily growth of fish. It is specific for each species and related to fish size and water temperature. Like the FCR it is dimensionless (no units) and is useful for comparing data between farms or species. The SGR shows the daily average growth of a fish in percentage of its bodyweight:
where W1 and W2 denote the weight of fish at the beginning and at the end of the growth period, respectively, and (T2-T1) denote the duration of growth period in days.
The daily feed rate (DFR): the percentage of feed provided expressed as a percentage of fish weight (% fish weight per day). Normally this percentage is higher for younger fish (around 10%) and lower for older fish (around 1-2%).
Ration consumed: the ration really consumed by the fish.
Maintenance ration: the precise ration needed in order to maintain the fish at a constant weight without growth.
Maximum ration: the ration needed in order to obtain the maximum possible growth.
In Figure 2 we can visualize the concept of the maximum ration, which provides maximum growth of the species under culture. This maximum ration will be specific to each farm, and depends on local conditions. As we get nearer to the maximum ration, growth will increase, but if we go over the limit, we are wasting feed. However, in general terms it is advisable to feed small fish more than the maximum ration, since the waste will be small due to the small existing biomass, and we will tend to maximize growth. But in the case of final growth, we tend to be more prudent, since there is a large biomass in the water, and any extra feed that is lost will be costly and will increase the negative environmental impact, making it necessary to clean it up.
Following Figure 2, with a small ration the fish will use all the energy for their daily activities and may even lose weight (where the FCR will be infinite). If we increase the ration, the fish will improve their growth as well as the FCR. At the point of maximum growth, any feed provided in excess will be an economic and environmental problem, with no benefits for production. For that reason, we have to adjust the feed ration to the growth of the fish to a point that is close to the maximal ration, but being careful not to go past that point.
Figure 2: Evolution of specific growth rate (SGR), feed conversion rate (FCR) and ration of feed provided to the fish in terms of the percentage of feed per live-weight of the fish per day
As mentioned above, the control of biological processes involved in aquaculture requires supervision in order to anticipate possible problems. It is important to be able to fix problems as far in advance as possible, which implies detecting very mild symptoms at the outset. All that will help reduce production costs and improve efficiency. As a result, the aquaculture sector understands that it needs to train personnel adequately and continuously, especially those in charge of feeding.
Even in modernised aquaculture systems such as RAS, which are increasingly computerized and automated, personnel need to be aware of the sophisticated biological processes occurring within the unit. Technological developments are increasing but should be accompanied by adequate training in the use of available techniques to improve production on all levels. Those concepts are a foundation for success. Indeed, the continuous training of personnel involved in feeding is a very important tool in farm operations. The supervisor of feeding determines, to a large degree, the profitability of the farm, since he/she provides the energy for the fish to grow. Any changes in feeding habits, however small, can be a symptom of problems in the system which, if uncorrected, can become serious sanitary problems.
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