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When research began on fish feeds more than 50 years ago, scientists first analysed the natural diets of the species in question. Trout, as an example of a carnivorous fish, had a natural diet that consisted of 50% protein, 15% fat, 8% fibre, and 10% ash, which is high in protein compared to terrestrial mammals. Ever since then researchers have been trying to find the right balance of protein, carbohydrates, fats, fibre, vitamins and minerals for fish used in aquaculture (Bhilave et al. 2014).
One of the most important components of any fish feed is protein. All proteins are composed of amino acids in different proportions. Thus, modern nutritionists tend to look at protein requirements in terms of amino acid requirements and aim to identify the ideal levels of the most important ones. This makes the whole system more efficient since fish are not getting any extra amino acids (that are then wasted), and have enough of the essential amino acids to grow healthily. Usually the level of protein is the first and most important question to ask when designing a diet. This is also a key issue in aquaponics since the protein in feed is the source of all the nitrogen waste that will later be used by plants (see Chapter 5).
Carbohydrates are composed of glucose, the main energy source for animals. In fish feed the most commonly found carbohydrate is starch, which helps to hold feed pellets together and provides an inexpensive source of energy. Although typically found in low amounts in fish feed, recent developments have led to an increase in its use. Now, in an effort to spare protein, that is, to reduce the amount of amino acids that are broken down to make energy, fish nutritionists are supplying more carbohydrates, with the advantage that the latter are also cheaper than protein (e.g.,
Lazzarotto et al. 2018). The only drawback is that this approach effectively makes many carnivorous fish more herbivorous, or vegetarian, since the extra carbohydrates are mostly of plant origin. Many studies in the past 5 years have been analysing how this can affect fish growth and welfare, and the results are promising.
Fats are made up of triglycerides or fatty acids which, like carbohydrates, provide energy to fish and, unlike carbohydrates, can be stored in different organs. Many fish, especially from colder waters, rely on high levels of fat in their diet (less than 15%), including omega-3 and omega-6 fatty acids. Fatty acids are also needed to transport fat-soluble vitamins. The relatively high levels of fat in most fish diets means that anti-oxidants are required to maintain their stability, avoiding degradation during processing and storage of the feed (Harper & Wolf 2009).
Crude fibre is the indigestible or difficult to digest part of feed that helps to promote gut motility (peristalsis). Ash represents the minerals in feed, such as potassium, phosphorus, copper and zinc. Exceeding the minerals that can be assimilated by the fish means that the extra minerals will be dissolved in the water. This is also important in aquaponics since we can design feeds that provide excess minerals that will end up being excreted by the fish and will therefore be available for the plants. However, it is usually a good idea to optimize feed for fish first.
An important concept in fish nutrition is the digestible protein to digestible energy ratio, often abbreviated as DP/DE. If the diet given to the fish is healthy and balanced, they will stop eating when they ‘feel’ their energy budget is reached. Energy can come from fat, carbohydrates or protein. As seen above, the most accessible source of energy is carbohydrate, followed by fat, and lastly protein. If the diet is high in protein compared to easily accessible energy (a high DP/DE), fish will have to eat more protein than they need to grow. Thus, that extra protein will not turn into muscle but will be broken down and used for other metabolic purposes, or simply wasted. On the other hand, if the DP/DE is low, then the fish will stop eating before then have enough to grow properly, and will be debilitated (Oliva-Teles 2012).
Table 2: Summary of feed composition (as percentage of dry weight) for a carnivore (trout) and a herbivore (tilapia). The remaining 10% includes ash with vitamins and minerals
In summary, Table 2 provides the general composition of a diet for adult trout (carnivore) and adult tilapia (herbivore), the latter being the most commonly used fish in aquaponics. The amount of vitamins and minerals is low compared to the other main components, and depends on the vitamin/mineral mix used by the feed producer. For example, the aquaponic system at the Arizona State University that is used to grow tilapia uses feed with 5 mg/kg of folic acid and 66 mg/kg of vitamin E in terms of vitamins, and 7 mg/kg of phosphorus and 0.5 mg/kg of magnesium in terms of minerals (see Fitzimmons 2018), among others.
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