Evaluation of the environmental impact of two types of food in intensive farming of rainbow trout fry (Onchoryncus mykiss. Walbaum, 1792).

The environmental impact of two types of food distributed during the rst phase of intensive breeding of rainbow trout (Oncorhynchus mykiss) fry at the station of the National Center for Hydrobiology and Pisciculture constitutes the main objective of this study. the method used for the evaluation of the impact of these two foods is the mass balance method based on the calculation of the quantities of nitrogen and phosphorus excreted according to the quantities of food ingested and the composition of the carcasses. The results obtained show that the weight growth of fry and the rate of nitrogen and phosphorus rejection during the experimental period are very different and vary depending on the type of food received.


1-Introduction
Food requirements for animal protein continue to increase due to global population growth. Fish farming, which constitutes the main branch of aquaculture, can contribute on the one hand to the increased demand for protein, on the other hand, to the reduction of the impact on the natural resources of the marine environment. The last two decades have seen signi cant sh production; in 2014, the contribution of sh farming to sh intended for human consumption exceeded that of the shing sector for the rst time [1]. This increase and intensi cation of world production, especially in freshwater environments, could have some impacts on the environment. Typical changes in the quality of water after its use for sh farming, among others the decrease in oxygen, the variation of the Hydrogen potential (pH), the increase in suspended matter [2] and especially phosphorus and nitrogen inputs.
These last elements constitute potential sources of food for phytoplankton and consequently the increase in the productivity of the aquatic ecosystem causing the phenomenon of eutrophication which is directly associated with discharges of solids and metabolic waste and therefore depends on the quality and the biological valorization of food [3,4,5,6].
The main objective of this study is to assess the impact of sh discharges from the rainbow trout (Oncorhynchus mykiss, Walbaum, 1792) fry fed two types of food at the National Center sh farm. Hydrobiology and sh farming (Morocco). In order to reach our objective, we followed the weight growth which varies according to the composition of the food, their digestibility and the rate of food conversion. These various parameters largely condition the level of discards from sh farming [7], especially for phosphorus and nitrogen which play a crucial role in the phenomenon of eutrophication which leads to the imbalance of species aquatic. [8]. In addition, the presence of nitrogenous forms (ammonia nitrogen) in water can cause poisoning in sh farming [9].

− 1 Description of the experimental station
The experiment was carried out in an incubation and nursery room at the National Center of Hydrobilogical and Pisciculture (NCHP) salmon aquaculture station in speci c rectangular troughs in parallel with a suitable volume of the order of 0.16 m3 and circular tanks. The water supply was made by taps with a ow rate of 0.97 m3 / h whose origin is a source. The troughs are equipped with a ventilation system by diffuser allowing the concentration of dissolved oxygen to be kept close to saturation if necessary. During the experimental period (nursery phase), physicochemical parameters (pH, dissolved oxygen and temperature) were monitored by calibrated devices of the Oreon type.

2-2 Biological material and food ration
After hatching, 2000 fry from the same batch of eggs were distributed randomly and equally in 4 troughs (A1, A2, A3 and A4). The fry of the different troughs are subject to the same nursery conditions. Each week, and in order to determine the food ration for all the biomass according to the rationing table provided by the manufacturer, the weight of 30 sh per trough was determined after anesthesia. The fry in groups 1 and 2 are fed on food A and those in groups 3 and 4 are fed on food B ( Table 1). The quantity of food was distributed in four meals from 9 a.m. to 5 p.m. during the whole experimental period.
The food ration is determined according to the biomass of the different trout by the following formula: TN = (Biomass × Feeding rate) / 100 [10].

2-3 Zootechnical parameters
Monitoring of weight growth is an important parameter in species biology regardless of the type of breeding. The evaluation of the quality of a feed in an intensive sh farm and therefore the rate of phosphorus and nitrogen rejection and their impact on the environment can be determined from the combination of several parameters including the weight gain during the experimentation period.
According to [11], the weight gain is calculated as follows: G.P% = (P m f (g) -P m i (g)): With, P m f = Average weight at the end of the breeding period and P m i = Initial average weight at the start of the breeding.
In addition to the weight growth parameter, we carried out the survival rate (SR %) of the fry during the nursery period according to the following formula: To grow, organisms need fundamental nutritious food, among which phosphorus (P) and nitrogen (N).
Plants assimilate dissolved forms of P and N in water (ortho phosphates and nitrates) in synergy with other nutrients [4].
In contrast, intensively farmed sh receive these two elements in their diet. P and N are involved in several metabolic processes in sh. Unfortunately, the quantities provided by food are not completely ingested and digested [12], and therefore they are released into the aquatic environment in different forms.
The quantity of P and N discharges into the aquatic environment closely depends on the quantity of protein in the sh diet and on the Conversion Index (CI) which quanti es the performance of sh farming [13]. For rainbow trout, 40 to 60% of N contained in the proteins received is excreted in dissolved form through the urine and the gills, 10 to 25% is found in the fecal matter, only the remaining 35% are used in sh growth.
For P, the retention rate is between 20% and 55% [14,15,16,17]. These quantities vary depending on the food, its digestibility rate and environmental conditions. According to these different authors, for rainbow trout, 60 to 80% of the phosphorus is rejected in particulate form (the phosphorus not ingested is eliminated by the faeces), and between 40 to 20% is eliminated in dissolved form through the urine and gills. On average, only 40% of phosphorus in food proteins is used by sh for growth. With respect to environmental impact, the dissolved part of phosphorus and nitrogen is the major problem compared to the solid part that undergoes treatments that improve over time through decanter or ltration systems [16,18]. In our case, the removal of the solid part was carried out by the siphoning technique during the whole period of breeding. The evaluation of the quantity of phosphorus and nitrogen discharges into the environmental medium in a sh farm varies according to several methods. The models developed for the assessment of sh discharges present different results [14,19]. In this study, we used mass balances to calculate nitrogen and phosphorus excretion based on the amounts ingested and the composition of the carcasses [20,21,22]. This nutritional method was developed to overcome the heaviness, costs and biases caused by other methods. The quantities of nitrogen and phosphorus released to water were evaluated as follows.

3-1Physicichemical parameters of water
Monitoring of basic physico-chemical parameters, temperature (° C), dissolved oxygen (mg / L) and hydrogen potential (pH) during the experimental period shows that the averages of the determined parameters ( Table 2) are in accordance the needs of the nursery stage in salmonids [23]. The results obtained [24] corroborate those obtained by [25].

2-3 Zootechnical parameters
The study of the zootechnical parameters during the test period per week (S1, S2, ... S13) shows a big difference between the nal weight and the weight gain of each fry according to the type of food received. Table 2 shows the average of the results (G1 + G2 and G3 + G4) of the weight growth of the fry according to the composition of the food tested. The fry fed with food A, show a signi cant growth, their average weight passed from 2,58 g (t0: rst day of the experiment) to an average weight of 50 g at the thirteenth week (S13) of breeding against only 9.28 g for fry fed on food B. The average weight gain (g) and the survival rate (%) are respectively 47.42 g and 99.8% for the rst batch of fry (G1 + G2) against only 6.7 g and 25.3% for the batch (G3 + G4) (

3-3 Nitrogen and phosphorus discharges from tested foods
The quantities of nitrogen and phosphorus released to water were evaluated according to the following formulas: Nitrogen releases: kg N = (A x C Na) -(Pr x C Np), Phosphorus releases: kg P = (A x C Pa) -(Pr x C Pp).
The calculation of the quantity of feed distributed per week is determined according to the total biomass of the fry in the troughs and according to the water temperature (14 ° C) and the ration table.
The biomass of the fry at time t0 and that of the fry at the end of the experiment S13 on the one hand, and the quantity of feed distributed during the whole nursery period on the other hand (Table 5 ) make it possible to evaluate the pollution rate generated by each food and therefore its impact on the environment.  The amount of nitrogen and phosphorus generated by the two foods tested are listed in Table 6.
During the nursery period, the total biomass obtained is 50 kg for the fry fed by food A and 9.3 Kg for the fry fed by food B. The quantity of feed distributed is respectively 5, 4 kg for the rst batch of fry and 3.2Kg for the second batch. In terms of zootechnical parameters, which are the basis in sh farming, the results clearly show the e ciency of food A compared to that of food B. This big difference is explained in particular by the rate of survival which is 99.8% for the fry fed on food A, against only 25.3% for the fry fed on food B.
The quantities of nitrogen and phosphorus released into the natural environment during the nursery period are respectively around 0.254 kg of N and 0.097 kg of P for food A; and 0.034 kg of N and 0.026 kg of P for food B. These results clearly show that food B has more negative impact on the environment compared to food A since, the amount of 'food B (3.2Kg) distributed generates more nitrogen released into the natural environment (1.5KgN) than the amount of nitrogen (1.4Kg) eliminated by the use of 5.4Kg of food A. These results can be explained by the fact that the digestibility of food B is not important, this corroborates with the weight gain which is very low for food B compared to food A. For phosphorus, the impact on the natural environment of food A is of the order of 0.12 kg / 107 d for a quantity of food distributed equal to 5.4 kg, for food B, the quantity of P released is 0.015Kg / 107J for 3.2Kg of food distributed. This difference in results can be explained on the one hand, by the quantity of food distributed (5.4 and 3.2Kg / 107J), on the other hand, the quantity of phosphorus contained in the two foods which is respectively 1, 32% P for food A, against only 0.8% for food B.

Conclusion
Foods used in the eld of breeding in general, and in sh farming mainly, must meet some criteria. On the one hand, the yield, the quality of the sh pulp, availability at a lower price and the welfare of the sh and on the other hand respect for the environment. By way of this experimental study, the main objective of which is the evaluation of the impact of these two foods on water resources, it can be concluded that food A meets the needs of the farmer vis-à-vis the yield since the survival rate of fry is close to 100% with a biomass which far exceeds that of food B. In terms of impact on the environment, food A, shows good results for discards in nitrogen since the quantity eliminated is not signi cant compared to the quantity of food distributed. For phosphorus, the impact of the two foods on water resources is high for food A. Declarations