The performance and success of a fish production rely mainly on the quality of the feeding. In many technical models, in particular in raceways and cages where little, if any, natural food is available the fish farmer must provide a complete diet. These are usually diets rich in protein and vitamins, and they are therefore quite expensive.
The costs of feed in intensive culture systems may reach 50% or more of total costs. Consequently, adequate nutritional practices have an ever-increasing determinant role, not only in terms of economic optimization, but also in terms of maintenance of good health, improvement of reproduction or growth performance.
The protein nutritional requirements of fish are definitely higher than those of the mammals or the birds. The content of protein in food must be of 40–45% (dry weight) for salmon, 32–36% for the American catfish, 31-43% for carps, 44% for eel, 30–40% for tilapias, and using expensive fishmeal generally provides it.
However, if it is true that fish have characteristics which explain why their metabolism depends more on the contribution of food proteins than the one of any other animal group (weak participation of the body proteins to the pool of amino acid precursors, importance of direct catabolism through the oxidative pathway of absorbed amino acids), the repercussion of these characteristics on the need in essential amino acids is hardly significant.
On the whole, the latter is rather similar from one species to the other and is comparable with that of superior vertebrates, if one excluding the case of arginine, which can be synthesized by the cycle of urea. Under these conditions, the myth of the strict protein demanding fish must be abandoned.
Moreover, even for such a long food chain fish as salmon, the percentage of retained protein from feed to edible food is as high as 30%, compared with 18 and 13% in chicken and s wine respectively.
Tilapias are known to be much more herbivorous than catfish or carp. For that reason, several attempts were made to substitute fishmeal by plant protein in order to limit food cost. Replacement of fish meal by increasing proportions of algal meal showed decreased growth when more than 5% algae were included, as imbalances of amino acids or minerals weren’t compensated.
However, under practical culture conditions in Israel, replacement of half the fishmeal by soybean has been found nutritionally equivalent, without any supplementation. The lipid nutrition is one of the best-studied sectors in aquatic nutrition research. The main steps of the lipid metabolism are known. In spite of great similarities, they differ from those of superior vertebrates from various points of view.
In particular, the aquatic environment is characterized by great amounts of polyunsaturated fatty acids in particular those with long chain (>20 atoms of carbon). Those of the series (n-3) are the ones for which fish have the highest needs, as opposed to the terrestrial superiors vertebrates. In the case of trout, the polyunsaturated fatty acids (n-3) must account for at least 10% of the food lipids and at least 14% in the case of carp. Moreover, the lipids can ensure a supply of energy.
As the majority of fish badly digests the complex carbohydrates but catabolises the proteins for meeting their energy requirements, this characteristic makes it possible to save proteins and to reduce the cost of the feeding. Thus, the growth of trout can be improved without increasing the content of food protein by raising the lipid level. For this reason, lipids are nowadays employed more and more to manufacture very powerful feed.
The food ration must contain lipids in significant but variable quantity according to the species considered. Salmon require 18–20% of lipid in their ration, whereas cyprinids need 7–18%. For tilapias, their requirements are lo wer than 10%. For most fish, the digestion is very high, since it is of 90%.
The third major component of fish nutrition is carbohydrates, which constitute a cheap source of energy. Generally speaking, fish do not have as large a capacity to use the carbohydrates as do the superiors vertebrates, although omnivorous and herbivorous fish like catfish, carps, and tilapia utilize it better than salmon. For the latter, carbohydrate excess can even entail an elevated mortality.
The carbohydrates account for about 30% of the salmon ration and more than 50% of cyprinids. The digestibility of small carbohydrates (glucose, saccharine) is much higher (near 100%) than that of starch (70–80% generally, but sometimes < 50%), but the latter is the only carbohydrate that can be economically incorporated into feed.
In fact, its digestibility increases with temperature, so that it is generally higher for tropical species than for temperate ones. Moreover, it is possible to improve it by cooking. Thus, the quantities of starch that are incorporated today largely exceed the limits, which were asserted two decades ago. Retained energy from feed to edible food in fish is 27% for salmon, compared with 12 and 16% for chicken and swine respectively.
Carbohydrates also contain more or less complex and weakly hydrolysable polysaccharides. The food role of these compounds is still badly known (stimulation of the digestive transit by some fibers or delayed-action of stomach draining by others, exfoliation of the intestinal cells, increased elimination of cholesterol and minerals).
Their food value is certainly negligible, even if the digestion of cellulose seems to be possible in the intestine of some herbivorous fish, due to the presence of bacteria.
In tropical zone, many species have adaptations that enable them to better digest these compounds, in particular the detritus matter. Indeed, for the major part of fish, the organic detritus constitutes a temporary nutritive resource, when traditional food is not available.
The slimming that often accompanies this qualitative change in feeding proves that these species are not physiologically able to exploit a detritus diet. But this general rule does not seem to apply in many tropical ecosystems where the species, in particular the Nile tilapia Oreochromis niloticus, are detritivorous.
The differences bet ween fish and higher vertebrates are very tiny with regard to the requirements in vitamins and minerals. The A2 vitamin, another form of the retinol, is specific to them but it is quite as effective as vitamin A, cholin, and inositol are necessary, whereas calciferol and K vitamin have a limited role.
Conversely, the E and C vitamins have a greater importance than that of superior vertebrates. Others food compounds can be added, in particular for the pigmentation of the flesh (salmon). These are the carotenoids. The effectiveness of their retention in flesh depends on many factors that vary according to the species and its age. If the pigmentation and transformation of carotenoids into vitamin A are well studied, the other nutritional aspects remain little known.
From this knowledge, it is possible to define formulas like those presented in Table 8.
The calculation of the rations encounters three difficulties: the effective ingestion of distributed food, the pollution of the environment by unconsumed or undigested food and, the temporal variability of the fish nutritional needs, which requires a permanent adjustment of the distributed rations. The feeding ad libitum is difficult to estimate and is likely to cause losses.
It can also be confronted with problems of energetic content. Thus, with low energy components, the ingested food can be limited by the volume of the stomach and in some cases, be insufficient to ensure a good growth performance. Conversely, with highly energetic food, the ingested ration can exceed the fish needs. The surplus is then stored into fat, which creates losses at the time of evisceration.
The rationing of animals must thus take into account parameters related to the fish (size, mass, and growth expected), to the food and to the environment (temperature). It is defined using the food conversion ratio (FCR, ratio between the quantity of distributed food and the weight gain of fish) and the gro wth rate.
Food ration is calculated using former rearing results and indicative values of FCR given in rationing tables delivered with commercial feeds. The distribution can be carried out in a manual, automatic way or upon request.
The manual distribution is by far the most frequent and most flexible method as it makes it possible to adjust the quantity of distributed food to the behavior of fish. It is however expensive in labor and constraining, even if the feeding is only practiced once a day.
The automatic feeding allows the fractionation of the distribution, and even the uninterrupted distribution, making it possible to improve the food conversion ratio. Lastly, with the distribution upon request, fish come to take the desired quantity of food, using a suitable mechanical or electronic device (tactile stem).
The disadvantage of this device is that it frequently exacerbates the competition among fish and thus increases the disparities of growth. Obviously, it is possible to combine the various modes of feeding, in order to benefit from the advantages of each one.
Authors:
Lionel Dabbadie and Jerome Lazard
