The minimum content of nutrients in the blend for fish is determined by scientific standards (1). The needs in amino acids vary depending on the species, growing technology, and the water temperature (2). Lysine, methionine, tryptophan, and arginine are the limiting essential amino acids which determine the growth and development of carp. Balanced content of these amino acids and the content of protein normally ensure sufficient quantity and the rest of the ration (3). Fishmeal contains three major nutrients; protein, fat and minerals. The protein in fishmeal is an excellent source of the aforementioned amino acids lysine, methionine and tryptophan in finfish feeds (4). In recent years, replacement of fish meal with cheaper ingredients of either vegetable or animal origin in fish feed is necessary, because of the rising cost of raw materials and uncertain availability of fish meal (5). Nowadays, the research has focused on utilizing less expensive and readily available resources to replace fish meal, without reducing the nutritional quality of feed (6, 7, 8, 9). There are several protein sources that have the potential of replacing fishmeal in aquaculture feeds without affecting the growth performance of fish (10). Products like soybean, canola meals, fish silage and microalga, in particular, are widely used in animal nutrition because of their high protein content (40-50%) low cost and relative availability. The nutritive value of a raw material depends on its chemical composition, the content of essential amino acids, carbohydrates, minerals and their availability to fish during the metabolic process (11) and water temperature (12).

The experiment tried to evaluate the replacement of fish meal by low cost waste nonstandard fish with wheat bran filler according to the method described by Ilieva et al (13, 14) for carp in recirculation system.

Group of 5 fish, randomly selected from each tank, were weighed every 45 days. The results of the growth parameters, feed conversion and protein efficiency ratios are shown in Table 3. Experimental fish utilized the two feeds at varying levels, which brought no significant variations (p>0.05) to growth parameters. At the start of the trial the Initial Body Weight (IBW) for the fish from the control group were 0.047 ± 6.52 and 0.046 ± 8.04 respectively for the fish from the experimental variant (Table 3). We should point out that even though it was not significant, the Final Body Weight of carp fed with FM was higher, but the differences were not statistically proven. Similar tendency was observed for PER, FCR and SGR. All three parameters were lower for the experimental treatment (fish fed with the Ribotricin supplement diet), compared with the values of these parameters received for the fish fed with the control diet (fish meal (FM). The obtained data for these performance traits was not significant (p>0.05).

After 90 days of rearing, all fish were harvested from the recirculation system. The survival rate for all the fish was found to be 100% throughout the experimental period.

The measured temperature and dissolved oxygen were optimal for carp, because the optimal temperature must be in a range between 22 and 24ºC and the oxygen level should not fall below 8 mg/l (2). The used experimental diet did not significantly change the pH of the water in the tanks.

Fishmeal is generally added to animal diets to increase feed efficiency and growth through better feed palatability; it enhances nutrient uptake, digestion, and absorption (17). However, the inconsistency of supply, the growing demand and price, among other problems, are limiting the use of fishmeal and putting greater pressure on the feed industry to find an economical alternative source of protein (18). The possibility of replacing fishmeal with fishery by-products, such as waste nonstandard fish, shrimp head meal and fish silage have been tested in various aquaculture feeds (19). Although not significant, the results received from the trial showed that fish fed with control diet consisting of fish meal exhibited higher growth rate (Final body weight, Feed conversion ratio, Protein efficiency ratio and Specific growth rate) when compared to carp fed with experimental diet. Our results were in accordance with the data obtained by Toledo et al. (20, 21). They replaced fishmeal with shrimp head meal in Israel tilapia (Oreochromis aureus) feed without any adverse effects on feed and growth efficiency. Similar data was received in research conducted to investigate the replacement of fish meal with fish silage in Nile tilapia (Oreochromis niloticus) (22, 23, 24, 25) and Mozambique Tilapia (Oreochromis mossambicus) (26). Carp species are omnivorous species and because of that they have the ability to assimilate alternative protein like tilapia. The results of the experiment showed that feeding fish on compound feeds containing Ribotricin had no negative effect on growth parameters. Based on the obtained data, the authors assumed that the Ribotricin is an adequate feed for carp and could be successfully used for substitution of fish meal.