Soybean Meal Incorporation in Diet Improves the Growth and Survival Performances of Juveniles of Nile Tilapia (Oreochromis niloticus) in Rural Area of Senegal

This study was conducted to investigate if the soybean meal incorporation in locally made diet will improves the growth and survival performances of fry of Nile tilapia in rural area of Senegal. A completely randomized design with four treatments of soybean meal incorporation (0, 15, 30 and 32%) was utilized in this study. The formulated diets had 35% crude protein content. The fingerlings were fed to satiation two times a day at 9 AM and 4 PM for 56 days. The analysis of variance showed final weight, specific growth rate and percent weight gain of fish increased significantly (P < 0.05) as soybean meal was added up to 30% level. The feed conversion ratio values were high and decreased significantly (P < 0.05) with the increase of soybean meal in the diet, ranging from 2.17 to 2.90. There was also a significant difference (P < 0.05) in survival among treatment, survival ranged from 62.22% to 91.11%. Except temperature, the water quality parameters were in the optimal range of O. niloticus. In conclusion, the result of this study shows that soybean meal can be added to up to 30% in the O. niloticus diet, without affecting growth performances.


Introduction
The fish-farming sector has not yet reached a viable economic dimension in Africa, either in terms of volume or in terms of the place of this activity in other production systems [1]. Oreochromis niloticus, is an important fish for aquaculture production in the north of Senegal. This species is able to tolerate environmental stress, reproduce easily, grow at a fast rate coupled with a high market demand [2]. Fish feed is the key component of aquaculture production and it's expensive cost caused failure in many farms. Fish meal is considered as the most nutritious animal ingredients in fish feed formulation but his cost makes many researches to turn on soybean meal which is the most nutritious plants ingredients in term of protein and it's widely used diet [3]. As in most aquaculture programs, reducing feeds cost is a permanent concern due to its direct impact on the cost of production.
In order to reduce the feed cost, the use of fish meal has to be reduced. This can be achieved by using alternative protein sources in fish diets mainly plant protein ones. [4][5] reported a wide range of agro-industrial by products used in fish feed. Moreover, the nutritional requirement of O. niloticus is very well documented, to achieve good growth. Normally, protein requirements of O. niloticus differ with each life stage of the fish, for fry dietary protein levels ranging from 36-50% have shown to produce maximum growth, for juveniles 29-40% has been determined to produce optimal growth and young adult fish 27-35% protein appears to be optimal [6].
Also, the successful use of plant ingredients for O. niloticus has been reported by several authors [6,7]. There are differences in the maximum dietary levels of soybean meal tolerance in different species [8][9][10][11][12][13]. This study considers using small amount of fish meal and different plant ingredients such as corn flour, peanut cake and soybean meal in mixture will improve growth and reduce the cost of the feed in rural area. Therefore, the soybean meal was used gradually in this study. The aim of this study was to evaluate if the soybean meal incorporation in farm made diet will improves the growth and survival performances of fry of Nile tilapia in rural area of Senegal.

Experimental Design
The research experiment was conducted at the 3FT integrated farm at the North of Senegal, between November to Mars 2020. Fish were obtained from the Aquaculture department of the Gaston Berger University, Saint-Louis, Senegal. The fish were acclimated in one tank for one week. At the beginning of the experiment, fingerlings were selected randomly and stocked 12 experimental unit at a stocking density of 15 fish per unit of 0.67 m 3 . The experimental units are made of fiberglass tank, with meshed hole, where the water was circulating from one unit to another one. The water was changed up to 50% each week. The experimental fish were fed to satiation two times a day at 9h: 00 and 16h: 00 for 8 weeks.

Experimental Diet
Four isonitrogenous diets of 35% crude protein with elevated soybean meal, at 0, 15, 30 and 60%, were prepared for the experiment. Fishmeal corn flour, peanut cake and soybean meal were finely grounded and poured through a sieve of 425µm mesh. Vitamin and mineral premix were mixed separately with the cellulose and the binder before being added to the main ingredient mixture. Diets were supplemented with fish oil after the addition of water. The semi-moist mixture was then pressure pelleted in the food grinder using a 2 mm die, dried at 35°C for two days, cut to desired sizes, packaged into plastic bag and stored frozen until its usage. The diets were screened prior to feeding in order to remove the fines.

Measurements
Temperature (°C), dissolved oxygen (O 2 , mg l -1 ), and pH were measured using a hand-held multi-probe water meter (YSI 556 MPS). These measurements were taken once every week. Water sample for ammonia measured in mg l -1 , were weekly collected and measured using a Palintest kit (Photometer 7100).
The fish were weighted every two weeks and their weight gain, specific growth rate, feed conversion rate and survival rate was calculated as follow: Where ln= natural logarithm, Wt = final body weight (g), Wi = initial body weight (g), and t = experiment duration time (days), FN = total number of fish at the end of the experiment, FI = total feed consumed (g), WI = weight increment (g) and IN = total number of fish stocked.

Statistical Analysis
All results are presented as mean ± standard error of the mean (SEM). Data were analyzed by one-way analysis of variance (ANOVA) to test the effect of the experimental diets. Where significant differences were found at (P < 0.05), a Turkey's test was used to determine which treatment means were significantly different from each other. The statistical analyses were made using XLSTAT (Version 2020: 5.1)

Water Quality Parameters
Water quality parameters measurements are presented in Table 1. Except the temperature, no water quality parameter (dissolved oxygen, pH and Ammonia excretion) varied significantly (p > 0.05) during the experimental period. The water temperature varied significantly ((p < 0.05) during the all period of the experimentation. The lowest water temperature was recorded at the last two weeks of the study.

Growth Performance
Initial weight, Final weight, per cent weight gain, specific growth rate, feed conversion ratio and survival values are summarized in Table 2. Final weight, specific growth rate and percent weight gain of fish increased significantly (P < 0.05) as soybean meal was added up to 30% level. The feed conversion ratio values were high and decreased significantly (P < 0.05) with the increase of soybean meal in the diet, ranging from 2.17 to 2.90. There was also a significant difference (P < 0.05) in survival among treatment, survival ranged from 62.22% to 91.11%. (Table 2).
Regarding the specific growth rate and the weight gain, the best growth performance is observed in fish fed the R2 diet containing 30% soybean meal, followed by the R3 diet containing 60% soybean meal.

Discussion
The protein requirement of tilapia ranges from 25 to 35% [3]. The crude protein content of the formulated diets in this study reached 35%, therefore fulfilling this requirement.
Water quality parameters except the temperature were within the range of the culture conditions of O. niloticus. In [2] it was recommended an optimal temperature range of 20 to 35°C for fish farming. However  suggested that when considering this optimum for growth comparisons, it is also important to refer to the optimal O. niloticus-specific temperature of 24-32°C.
The temperature was not controlled in our study due to the open system used and ranged from 17.05 to 25.86°C, which was lower than the optimum for the cultured species. Temperature is the most critical water quality variable which affects a number of biological processes in fish culture systems [15]. Water temperature affects the physiology of fish, low water temperature leads to poor fish feeding, starvation, reduced growth and increased fish mortality, directly or indirectly. The decrease of growth was also reported by [16]. Who obtained a negative growth in low temperature (average of 15.6°C) which they considered below the optimum threshold for this species.
Temperatures between 20 and 36°C have been reported by various researchers as being suitable for tilapia culture. According to [17], for instance, the preferred temperature range for optimum tilapia growth in ponds is between 25 and 27°C. The preferred temperature ranges for tilapia have been reported to be between 31 and 36°C [18], while [19] gave a range between 20 and 35°C as ideal for tilapia culture. Water temperature is a key water quality variable because it influences all other water quality parameters such as dissolved oxygen concentrations.
The average dissolved oxygen ranged from 6.09 to 7.30 mg l -1 during the entire study period of 60 days. According to [19], the preferred DO for optimum growth of tilapia is above 5 mg/L.
Ammonia levels determined in this study ranged from 0.03 to 0.14 mg l-1 and were less than 0.5 mg l-1, which is considered tolerable for most farmed fish species [20,21].
The pH level ranged from 6.91 to 7.93 and was within the recommended range (6.5 -9.0) of cultured species [19]. It can then be assumed that the observed mortalities are not related to water quality because of the low pH and low ammonia concentration. Prolonged exposure to unionized ammonia exceeding 0.2 mg L-1 is highly toxic to fish at pH levels above 9 and can lead to mass mortalities [22].
The SGR attained at the end of the study (after 56 days), ranged from 0.44-0.97%/day. Regarding the specific growth rate and the weight gain, the best growth performance is observed in fish fed the R2 diet containing 30% soybean meal, followed by the R3 diet containing 60% soybean meal. The SGR value for the tilapia obtained in this study was similar with those obtained by [23] in their study, but lower than of Karina et a [3] where the SGR value was 1.36%.
The effects of increasing soybean meal ratio in diet on growth and specific growth rate have been assessed in several studies and generally show that increased soybean meal lead to increased growth up to a certain level, beyond which growth rates level off, or even begin to decline [13,9,24]. The present study shows that for Nile tilapia, increasing soybean meal in diet leads to increased body weight gain and specific growth rates up to 30% inclusion, but also that no further improvement in growth rate and specific growth rate are achieved from 30 to 60% inclusion, indicating that 30% inclusion could represent a break point. The growth retardation could be attributed to unfavorable temperatures that did not support the optimal growth of O. niloticus. In addition, The poor growth observed in all treatments may be related to the presence of anti-nutritional factors present in the plant ingredients. The presence of anti-nutritional factors in materials of vegetable origin affects directly (trypsin inhibitors, phytic acids that complex phosphorus and make it indispensable) or indirectly (oligosaccharides, tannins, glucosinolates whose metabolites affect appetite and disrupt the synthesis of thyroid hormones), the digestive capacity of fish [25]. It is also known that soybean meal, even though it is considered the best source of vegetable protein in terms of amino acid profile, may not contain enough methionine to meet the essential amino acid requirements of fish [26].
The present study shows that FCR increases as soybean addition increases from 2.17 to 2.9. The high level of FCR observed in this study could be explained by the texture of homemade food pellets that lost nutrients through leaching into the water and worsened the feed conversion rate, which is contrary to the conclusion of [27], whose pellet texture improved the growth rate of O. niloticus. In addition, it is important to note that the amount of feed included in the FCR calculation can greatly exceed the amount actually consumed by the fish because, we assumed that the amount of feed distributed was totally consumed by the fish. It could be assumed that the poor performance could be due to the low degree of convertibility (by fish) of the ingredients of agricultural by-products used in food formulation.
Survival rate is usually influenced by food and environmental conditions. Bad food and environmental conditions will negatively affect the fish health and reduce the survival rate. The present study showed that the survival rate varies between 62.22 and 91.11% and that the best survival was observed with the commercial food. Apart from the control treatment, these mortalities can be considered high, as the survival rate generally accepted in fish farming is 90%. Few mortalities was observed after the first weighing of control, the other was observed by the time and seems not to be due to the diets. Some mortalities were observed after the first control weigh, others were observed over time and do not appear to be diet-related.

Conclusion
In conclusion, dissolved oxygen, temperature, pH and ammonia in the experimental system were within the optimum range for growth of tilapia, while the temperature was not. The growth retardation was attributed to unfavorable temperatures that did not support the optimal growth of fish. The soybean meal may be added to up to 30% in the O. niloticus diet, without affecting growth performances.