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 fish 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.
Table 1
Composition of two foods tested
Components | Food A | Food B |
Crud protein | 47% | 48% |
Crud fat | 18% | 22% |
Crud cellulose | 1,33% | 2,2% |
Crud ash | 8,75% | 8,3% |
Total phosphorus | 1,32% | 0,8% |
Calcium | 0,80% | ------ |
Sodium | 0,62% | ------ |
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 fish 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% = (Pm f (g) - Pm i (g)):
With, Pm f = Average weight at the end of the breeding period and Pm 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:
SR (%) = (Final number of fish x 100) / Initial number of fish
2–4 Determination of the quantity of phosphorus and nitrogen discharge.
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 fish receive these two elements in their diet. P and N are involved in several metabolic processes in fish. 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 fish diet and on the Conversion Index (CI) which quantifies the performance of fish 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 fish 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 fish 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 filtration 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 fish farm varies according to several methods. The models developed for the assessment of fish 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.
For nitrogen releases : kg N = (A x C Na) - (Pr x CN p), with, A : Quantity of food distributed (kg), C Na: Percentage by weight of nitrogen in the food (% protein + 6.25), Pr: Weight gain achieved by the fish (kg), and C Np: Percentage by weight of nitrogen in the fish (3%).
For phosphorus discharges: kg P = (A x C Pa) - (Pr x C Pp) with, A: Quantity of food distributed (kg), C Pa: Percentage by weight of phosphorus in the food, Pr: Weight gain made by fish (kg) and CPp: Percentage by weight of phosphorus in fish (0.4%).