Duckweed and/or freshwater crayfish for diet nile tilapia fish

 ^ABSTRACT

 The aim of this study was to evaluate the effect of some unconventional diets on diets composition, growth performance, blood picture, whole fish and fish muscles composition, and feed utilization by Nile tilapia fingerlings (7-8g).  Twenty six glass aquaria (60 x 35 x 40 cm) connected with electric pumps for water aeration were used.  Dechlorinated tap water was used to renew third of aquaria water, daily. The basal diet contained 25% crude protein.  The diets were offered daily (at two meals) at 3% of fish body weight. Fish were stocked at a rate of 7 fingerlings per aquarium.  The experimental dies were nearly isocaloric and isonitrogenus.  The 1^st diet was a control, diets No. 2 - 5 are the control diet but their fishmeal was substituted by 25, 50, 75 and 100%, respectively with duckweed meal (DW), diets No. 6 - 9 included crayfish meal (C_rF_i) at the same previous replacement rates, and diets No. 10 - 13 included a mixture of DW + C_rF_i (1:1) as a substitute for fishmeal at the same rates. The obtained results included that DW contained higher crude protein and ether extract percentages as well as cadmium level than C_rF_i. C_rF_i contained more nitrogen free extract, ash, lead and silica than DW. There were significant differences among the experimental diets in their dry matter, crude protein, ether extract, crude fiber and ash contents. Diet No. 13 included the highest crude protein percentage. The increased DW substitution rate up to 75% and C_rF_i up to 50% led to increase in the dietary crude protein. The increased C_rF_i level from 25 to 100% gradually decreased the ether extract % in diets No. 6 - 9. The increased dietary inclusion of DW from 25 to 100% (diets No. 2 - 5) led to increase in dietary crude fiber %. There were significant differences among dietary treatments in growth performance parameters including final body weight, body weight gain, daily body weight gain and relative growth rates. The highest values of these criteria were realized with diets No. 11 and 6, respectively. The dietary treatments significantly affected feed intake, feed conversion, protein intake, protein productive value, protein efficiency ratio, and energy retention. The highest feed and protein intakes were found with diet No. 6, but the lowest were recorded for diet No. 12. The best feed conversion was calculated for diet No. 11 (the best treatment in fish bodyweight gain). The best protein utilization (protein productive value and protein efficiency ratio) was calculated for diet No. 13 although the superiority of diet No. 5 in energy retention. Diet No. 6 reflected the highest values of hemoglobin, hematocrit, total protein, and globulin. Transaminases activity increased with diet No. 5 and decreased with diet No. 7 referring to affected hepatic function. The 5^th diet also increased blood levels of urea, uric acid and creatinine, referring to affected kidney function. Carcass and muscle composition of the tested fish significantly affected by dietary treatments. The highest dry matter was determined in the 5^th group. Diet No. 6 was responsible for the highest protein % and lowest ether extract %, respectively. There were positive relationships between moisture and protein as well as between moisture and ash contents, but negative relationships between moisture and ether extract as well as between protein and ether extract % in whole fish body and fish muscles.  Decreased silica level of the 2^nd diet led to low silica content of fish body of this group. The lowest lead content in diet and fish was of the 2^nd treatment. The 13^th group reflected the highest cadmium levels in diet and fish.

Key words: Nile tilapia - Duckweed - Freshwater crayfish - Performance - Feed utilization - Blood - Composition.

Tilapia culture is one of the fastest growing farming activities, with an average annual growth rate of 13.4%, during 1970-2002. Tilapia is widely cultured in about 100 countries in the tropical and subtropical regions. As a result, the production of farmed tilapia has increased from 383,654 mt in 1990 to 1,505,804 mt in 2002, representing about 6% of total farmed finfish. Nile tilapia is, by far, the most important farmed tilapia species in the world. The  production of farmed Nile tilapia reached 1,217,055 mt representing about 81% of total production of farmed tilapia in 2002 (FAO, 2004). Tilapias are currently raised in different types of production systems ranging from pond, tank, cage, flowing water and intensive water reuse culture systems. (El Sayed et al., 2005). Commercial fish feeds utilized in aquaculture often contain fishmeal, which can comprise up to 65% of the diet. Nile tilapia is herbivorous by nature, consuming mainly phytoplankton (Moriarty, 1973), but can as well consume a variety of other natural food organisms found in ponds. To increase fish production, supplementary or artificial feeds may be added. However, supplementary feeds can take up to 60% of fish production costs (Green, 1992) making them unaffordable for most farmers in developing countries (Liti et al., 2005). Recently, many areas of the world have dietary protein shortage and this situation is most severing in Africa. Fishmeal has been the major source of animal protein in fish diets, its high cost and dwindling availability has called for the search for alternative sources of protein (Adeparusi and Ajayi, 2000). As long as protein component represents 55-75% of the total diet cost, protein alternatives have the first priority in formulating diet of tilapia as alternatives for the high cost of fish meal (Hanley, 2000). The utilization of the cheaper sources such as freshwater crayfish meal or aquatic plants meal is promising and need further investigations (El Sayed et al., 2005). So, the objective of this study was the evaluation of replacing dietary fish   meal protein by plant and animal protein sources in tilapia fish diets and to investigate its effects on growth performance of tilapia fingerlings.

Feeding experiment was conducted to evaluate some no conventional diets concerning growth performance, carcass and muscles composition and feed utilization of Nile tilapia, Oreochromios niloticus, fingerlings for 16 weeks. The experimental system consisted of 26 glass aquaria (60×35×40cm), each aquarium was continuously supplied with a compressed air from an electric compressor (Shenzehe Company BS410). Dechlorinated tap water was used to change one third of the water in each aquarium every day. Water was aerated before be used for about 24 hours to remove chlorine. Experimental Fish: A group of Nile tilapia O. niloticus with an average initial body weigh of 7 - 8 g were obtained from a private farm at AL Hamoul, Kafr El-Sheikh Governorate, Egypt and transported to the aquaria Fish were maintained in these aquaria for 2 weeks before the beginning of the experiment for acclimatization purpose. The fish were fed during the acclimatization period on the basal diet (25% crude protein) at a rate of 3% of the body weight daily, at 2 times daily. The experimental treatments were tested at two aquaria (replicates) for each. Fish were stoked at a density of 7 fish / aquarium. Experimental Diet: Partial or complete replacement of fishmeal (0, 25, 50, 75, and 100%) by whole crayfish meal and / or duck weeds meal in Nile tilapia fish diets was carried out. All feedstuffs used in the experimental dies were purchased from the local market. Grayfish was sun-dried, ground and sieved (21 mashes).  Duckweed was collected from a drainage then sun-dried and ground.  Diets were formulated by hand mixing the ground ingredients with little water through meat mincer to pellets (3 mm), and then air dried. The basal diet No.1 was considered as a control. Composition and chemical analysis of the basal and experimental diets are presented in Tables (1 - 3). The composition of the vitamins and minerals mixture is presented in Table (4). Experimental Procedure: The experiment continued for 16 weeks. During the experimental period, the fish were fed the experimental diets at a rate of 3% of the live body weight daily. The diet was introduced twice daily, at 8 a.m. and 2 p.m. The amount of food was adjusted weekly based on the actual body weight changes. Samples of water were taken daily before changing the water and after adding the diets weekly from each aquarium to determine water quality parameters. Light was controlled by a timer to provide a 14h light: 10h dark as a daily photoperiod. Analytical Methods: Samples of water from each aquarium were taken to determine the water temperature, pH value, and dissolved oxygen concentrations according to Abdelhamid (1996). Water temperature in degree centigrade was recorded every day by using a thermometer. The pH value of water was measured daily using an electric digital Jenway Ltd, model 350-pH meter. Dissolved oxygen concentration was determined weekly using an oxygen meter model (d-5509). Determinations of DM, CP, EE, CF, ash and silica in the ingredients diets and in fish body at the start and at the end of the experiment for different groups were carried out according to the method of A.OA.C. (1990). At the end of the experiment, three fish were derived from each group (aquarium) for drying at 60ºC for 48 hours and then milled through electrical mill and kept at 4^oC until analysis. Heavy metals determination was carried out using Atomic Absorption Spectrophotometry (Germany Company).

 

*GE (kcal/100 g DM) = CP x 5.64 + EE x 9.44 + NFE x 4.11   calculated according to (Macdonald et al., 1973)

**ME (kcal/100g DM) = Metabolically energy was calculated by using factors 3.49, 8.1 and 4.5 kcal/g for carbohydrates, fat and protein, respectively according to Pantha (1982).

 

*GE (kcal/100 g DM) = CP x 5.64 + EE x 9.44 + NFE x 4.11   calculated according to (Macdonald et al., 1973)

**ME (kcal/100g DM) = Metabolizable energy was calculated by using factors 3.49, 8.1 and 4.5 kcal/g for carbohydrates, fat and protein, respectively according to Pantha (1982). 

 

*GE (kcal/100 g DM) = CP x 5.64 + EE x 9.44 + NFE x 4.11   calculated according to (Macdonald et al., 1973)

**ME (kcal/100g DM) = Metabolizable energy was calculated by using factors 3.49, 8.1 and 4.5 kcal/g for carbohydrates, fat and protein, respectively according to Pantha (1982).

 

 Growth Performance and Efficiency of Feed and Protein Utilization: The growth performance and feed utilization parameters were calculated according to the following equations: Average weight gain (AWG):  AWG ( g/fish ) = Average final weight (g)-Average initial weight (g). Average daily gain (ADG): ADG (g/fish) = Average final weight (g)-Average initial weight (g) / Time (days). Survival rate (SR %): SR= Total number of fish at the end of the experiment×100/total number of fish at the start of the experiment. Relative growth rate (RGR): RGR = Average weight gain (g) / Average initial weight (g). Specific growth rate (SGR, % / day): SGR = 100 [ln wt₁- ln wt_o/T]. Where: ln: Natural log. Wt_o: Initial weight (g), Wt₁: Final weight (g), and T: Time in days. Feed conversion ratio (FCR): FCR = Total feed consumption (g)/ Weight gain (g). Protein efficiency ratio (PER): PER = Body weight gain (g)/protein intake (g). Protein productive value (PPV %): PPV% = 100 [Retained protein (g)/protein intake (g)]. Energy retention (ER %): ER % = 100 [Retained energy (Kcal) / Energy intake (Kcal). Blood Analysis: Blood samples were withdrawn for hematological and biochemical determination in a pathological Lab. Microbiological Examination: For microbiological test, samples were taken in sterilized test tubes then prepared for total bacterial count according to AOAC (1990) and kindly examined by Dr. Manal I. EL-Barbary from the Aquatic Pathology Lab., National Institute of Oceanography and fisheries. Statistical Analysis: The data were statistically analyzed by using general linear models procedure adapted by SAS (1996) for users guide. Means were separated using Duncan's multiple range test (Duncan, 1955).

Chemical Composition of the Experimental Diets: The chemical analysis of both tested substitute unconventional materials crayfish (C_rF_i) and duckweed (DW) is illustrated in Table (5), from which it is obvious that DW contained more crude protein (CP), crude fiber (CF), ether extract (EE) and cadmium (Cd) contents, but lower nitrogen free extract (NFE), ash, lead and silica contents than C_rF_i, on dry matter (DM) basis.  In this respect, Hassan and Edwards (1992) reported that DW (Lemna perpusilla) contained 25.3% CP, 4.5% crude fat, .6% CF and 17.6% ash on dry matter basis.  Therefore, DW is used as a protein feedstuff in fish diets (Schneider et al., 2004 and BMO, 2009). Also, Tharwat (2000) analyzed the entire body of C_rF_i (Procambarus clarkii) and found that it contained 62.2% CP, 6.1% EE and 27.0 ash which is more proteinious than shrimp meal and local fish meal.  Moreover, Eweedah et al. (2006) and Abd El-Rahman and Badrawy (2007) reported the chemical composition of the freshwater crayfish meal on DM basis as 32.1% CP, 1.9% EE and 33.9 ash.  So, C_rF_i was evaluated as a protein source in fish diets. Habib (2004) cited that crayfish contain lead and cadmium as 1.82 - 2.41 and 0.06 - 0.70 mg/g dry weight of the external skeleton but 0.28 - 0.49 and 0.02 - 0.03 mg/g dry weight of the muscles when their rearing water contains 0.08 - 0.10 ppm Pb and 0.01 - 0.40 ppm Cd.  However, freshwater crayfish included 1.95 mg/g Pb and 6.37 mg/g Cd.

Table (6) presents data of chemical analysis of the tested diets.  Their analysis of variance (ANOVA) reflects significant differences among the experimental diets concerning DM, CP, EE, CF and ash contents.  Diet No. 13 (100% replacement of fishmeal with DW + C_rF_i, 1:1) contained the highest CP%.  Increasing DW replacement level up to 75% or C_rF_i up to 50% led to significantly higher CP content than the other replacement levels (except diet No. 13) and the control (diet No. 1).  Increasing C_rF_i replacement level from 25 to 100% gradually decreased EE% of the diets No. 6 - 9.  Increasing DW from 25 to 100% replacement in diets No. 2 - 5 increased their CF%.  Diets No. 6 - 9 had the highest ash % in gradual increase in proportion to the C_rF_i inclusion level.  These variations are mainly due to the variations between C_rF_i and DW analyses (Table 5).

 

*Means (in the same column) superscripted with different letters significantly  (P≤0.05) differ.

Physico - Chemical Parameters of Water Quality: Fish rearing water was analyzed periodically for some water quality criteria (Table 7) including temperature, pH and dissolved oxygen (DO).  However, there was no effect on these parameters of different experimental diets, whether of substitute commodities or their replacement levels.  However, the ranges of these criteria (25.5 - 26.3^oC, 7.12 - 7.70 pH value, and 5.90 - 6.10 mg/l DO) are suitable for rearing Nile tilapia fish (Abdelhamid, 1994, 1996 and 2009).

 

 

 

Fish Growth Performance: The world cultured fish production had been increased from < 1 million ton in the 1950s to 51.7 million tons (47% of the total world fish production) in 2006.  Egypt produced 1.08 million ton, from which 65% are coming from aquaculture. Kafr El-Sheikh governorate alone produces ca. 40% of the local fish production and 5% of the world tilapia production (Al-Shoraky, 2009).  So, Kafr El-Sheikh governorate became a leader in the field of exporting tilapia to the Arab and African countries (Ibrahim, 2009). Many Egyptian fish producers lost money and decided to leave the fish production sector for the increasing prices of the aqua feeds throughout the last 2 years (Radwan, 2009).  Yet, Kafr El-Sheikh has many potentialities for fish production (Al-Seretty and Abdel-Hafez, 2009).  To increase and sustain fish production, nutritionists may give concern to offer alternative feed stuffs, thus the present study deal with this concept. Although there were no significant differences for initial bodyweight (IW) among the experimental fish groups (Table 8); yet, the other growth performance parameters including final bodyweight (FW), average weight gain (AWG) and average daily gain (ADW) show significant variations due to the dietary treatments (Table 8).  Since the heaviest FW, AWG and ADG were realized by diet No. 11 (50% replacement by 1:1 DW + C_rF_i) followed by diet No. 6 (25% C_rF_i).  This may be due to the chemical composition of both commodities used herein as novel protein sources to replace fish meal in the control diet No. 1, i.e. high CF, EE, and Cd levels in DW as well as high inclusion levels of ash, lead (Pb) and silica of C_rF_i.  These may affect feed intake (Table 10) as well as nutrients digestibility which reflected also on nutrients utilization in form of protein efficiency ratio (PER, Table 10). Specific growth rates (SGR) and survival rates (SR) did not differ significantly by the dietary treatments (Table 9); yet, relative growth rate (RGR) differed significantly among fish groups (Table 9), being the highest for diets No. 11 and 6, respectively.

 

However, Hassan and Edwards (1992) working on Nile tilapia found that the optimal daily feeding rates of Lemna were 5, 4 and 3% of the total fish body weight on a duckweed - dry - weight basis for fish of 25 - 44 g, 45 - 74 g and 75 - 105 g in weight, respectively.  Since DW has potential as fish food in the development of low-cost aquaculture systems in the tropics; yet, it must be fed not more than 4% of the fish body weight daily to avoid its negative effects on the fish weight gain and survival.  Moreover, Eid et al. (1995) reported that 2.5% inclusion level of DW of the Nile tilapia fish diet showed the highest body weight gain, absolute growth rate, and SGR.  Additionally, Fasakin et al. (1999) did not find differences (P ³ 0.05) in growth performance of Nile tilapia fish fed on diets containing up to 20% duckweed inclusion and the control. It was noticed that 30% fermented Lemna leaf meal incorporated in the diet resulted in the best growth performance of the fish superior to those fed diets containing raw leaf meal (Bairagi et al., 2002). However, El-Shafai et al. (2004b) and Schneider et al. (2004) found that more than 20% DW in the diet resulted in lower growth, although tilapia fish have the potency to digest and metabolize green food (Bakeer, 2006), but it may be due to its inclusion of high levels of trace metals, since such aquatic plants are able to significantly reduce the pollution load of the aquaculture wastewater by accumulating it in their tissues (Snow and Ghaly, 2008). Generally, developing alternate protein sources for fish feeds which support rapid fish growth but do not increase pollution from aquaculture will require the combined efforts of all of the major scientific disciplines that collectively constitute aquaculture (Hardy, 1999).  Feeding tilapia fish on diets supplemented with chitin and chitosan depresses tilapia growth regardless of the supplementation level (Shiau and Yu, 1999).  This may interpretate the C_rF_i effect on fish performance recorded herein.  Recently, Eweedah et al. (2006) reported that including crayfish meal in the diet caused a decrease in growth parameters but crayfish silage at a level of 33% did not decrease AWG, ADG or SGR.

*Means (in the same column) superscripted with different letters significantly (P≤0.05) differ.

 

*Means (in the same column) superscripted with different letters significantly  (P≤0.05) differ.

Feed and Nutrients Utilizations: Table (10) present data of feed intake (FI), feed conversion ratio (FCR), dietary protein intake (PI), protein productive value (PPV), protein efficiency ratio (PER), and energy retention (ER).  The ANOVA of these data presented significant effects of the dietary treatments on FI, FCR, PI, PPV, PER and ER.  The diet No. 6 (25% C_rF_i replacement) was responsible for significantly the highest FI and PI while the worst was the diet No. 12 (75% replacement 1:1 DW + C_rF_i).  The best feed conversion was obtained with mixed DW + C_rF_i at 50% replacement level (diet No. 11, which led to best AWG and ADW (Table 8)), while the worst was the diet No. 3 (50% DW replacement, led to the lowest AWG and ADW as shown from Table 10).  The best protein utilization expressed as PPV and PER was realized with the diet No. 13 (100% replacement by mixed DW + C_rF_i, 1:1, due to its highest CP content, Table 7); yet, diet No. 5 (100% DW replacement) was the best in ER.  This may be due to high energy content or low ash of DW (Table 1).  In these concerns, Hassan and Edwards (1992) registered lower feed conversion by increasing DW level for in Nile tilapia diets.  However, Eid et al. (1995) reported best digestion by fish fed on 2.5% DW level.  Also, Fasakin et al. (1999) did not find significant differences in nutrient utilization of fish fed on diets containing up to 20% DW inclusion and the control.  Yet, Bairagi et al. (2002) reported that feed utilization efficiencies of fish fed fermented leaf meal containing diets were superior to those fed diets containing raw leaf meal. It is worth mentioned that unionized ammonia nitrogen (UIAN) concentration must be maintained below 0.1 mg/l to avoid chronic ammonia toxicity to duckweed-fed tilapia, which increased FCR and reduced PER.  Since DW grown on domestic sewage increases water UIAN (El-Shafai et al., 2004a).  Moreover, ammonia excretion rate increases with a decline in protein quality (Eid and Matty, 1989).  Yet, Ruenglertpanyakul et al. (2004) mentioned that DW could efficiently remove nutrients in the effluent, especially ammonia, which seemed to be the preferred nitrogen sauce of the plant.  However, Abdel-Aziz and El-Shafai (2004) concluded that DW could be used in intensive tilapia culture either as partial substitute of fishmeal or complete substitute of some plant ingredients.  DW provided good values for FCR (0.98 - 1.1), PER (2.49 - 2.78), CP digestibility (78 - 92%) and energy digestibility (78.1 - 90.7%). Anyhow, Nile tilapia fish reflect digestibility of energy and protein in duckweed as 7.81 - 10.7% and 88.4 - 93.% (El-Shafai et al., 2004 a and b) and an crayfish meal being 88.8 and 68.4% (Boscolo et al., 2004), respectively.  DW reflected a high N-retention (Schneider et al., 2004).  Tilapias are capable to digest and metabolize algae (Bakeer, 2006). Eweedah et al. (2006) reported that C_rF_i can be used successfully up to 33% replacement of fishmeal of the Nile tilapia diets to reduce the feeding cost without a significant decrease in growth performance.  Also, 50% crayfish meal diet reflected comparable FCR (Abd El-Rahman and Badrawy, 2007).  However, Snow and Ghaly (2008) found that aquatic plants did not contain sufficient amounts of protein and fat to meet the dietary requirements of fish.  They also contain high minerals concentrate, which can lead to reduce feed intake, weight gain and growth rate in fish.

 

*Means (in the same column) superscripted with different letters significantly (P≤0.05) differ.

 

Blood Analysis: Data of blood hematological and biochemical analyses in tilapia fish as affected by the dietary treatments are given in Table (11).  Diet No. 6 reflected the highest Hb and PCV values as well as total protein and globulin.  While, diet No. 4 gave the lowest values of Hb, PCV, total protein, albumin and globulin.  Hepatic function in terms of AST and ALT activities was highest with diet No. 5 and lowest with diet No. 7.  Also, kidney function in terms of urea, uric acid and creatinine concentrations was highest with diet No. 5.  This means that 75% duckweed replacing fish meal in tilapia diets reduced the hematological parameters measured as well as blood total protein and globulin and 100% DW diet led to dysfunctions of both liver and kidney.  Yet, 25% C_rF_i improved (duplicated) the hematological parameters and total protein (and globulin) to be better than the control.  Moreover, 50% C_rF_i also led to hepatic dysfunction through lowering the activity of both enzymes AST and ALT to be half that of the control.  The negative effects of both substitutes may be attributed to their high content of Pb, Cd, and silica (Tables 15 and 16), as well as the level of CF and/or ash of these diets (Tables 1 - 3).  The blood parameters are harmonized also with growth performance parameters (Tables 8 and 9) and feed and nutrients utilization by the fish (Table 10). However, disturbing liver function be caused by hepatitis and/or liver injure, and kidney dysfunction may be occurred by kidney disease, hemolysis, and/or hypoproteinemia (Merck, 1974).  Moreover, the low Hb concentration is an anemia symptom (Merck, 1976).  Increases in blood urea may occur in a number of diseases in addition to those in which the kidneys are primarily involved.  Also, the plasma creatinine increases in rend disease.  Yet, the evidence regarding the blood creatine in nephritis is conflicting.  Uric acid belongs to, and is the end product of the metabolism of, the group of substances known as the purines, some of which are present in nucleic acid found in the molecule of the nucleoproteins.  It is accordingly formed endogenously from nucleoprotein metabolism, and exogenously from the metabolism of purines taken in the food.  Uric acid estimations give little information of value to the clinician except in cases of gout.  A reduction in the total protein is one of the causes of edema.  In all conditions, there is a negative nitrogen balance due to increased protein breakdown, and increase in blood urea may be found.  A low serum albumin is found in severe liver disease and may be a factor in causing edema in liver disease. An insufficient amount of dietary protein may also lead to a low plasma albumin, with a low total protein also.  Increase in globulin occurs most commonly in advanced liver disease.  Increases in both transaminases are a common finding in liver diseases (Varley, 1978).

 

 

Hb: hemoglobin, PCV: packed cell value, P: protein, A: albumin, G: globulin ,U: urea , UA: uric acid, C: creatinine, AST: aspartate amino transaminase, and ALT: alanine amino transaminase. 

 

Chemical Composition of Fish: Chemical composition of the whole fish body (Tables 12 and 13) revealed that DM, CP, and EE increased from the start to the end of the experimental period.  However, the dietary treatments affected significantly all fractions of the proximate analysis of the carcass (Table 13) as well as of the muscles (Table 14).  The highest DM% was recorded in both carcass and muscles with 100% DW diet but the lowest with 25% C_rF_i diet (Tables 13 and 14); yet, the highest CP% in both carcass and muscles was found with 25% C_rF_i diet, but the lowest with 75% DW.  Ether extract percentages were highest and lowest with 100% DW and 25% C_rF_i, respectively in the carcass and muscles, too.  The diets containing 75 - 100% DW reflected the lowest ash percentages and the diets containing 50% C_rF_i gave the highest values in both carcass and muscles. Hassan and Edwards (1992) found that increasing Lemma rates in the diet (up to 3%) increased DM, CP and EE of the fish, but higher levels led to the opposite trend.  Shiau and Yu (1999) reported that body lipid content of the fish reflects the general pattern of the lipid digestibility.  Moreover, Eweedah et al. (2006) registered decreases in DM and CP of Nile tilapia carcasses at the supplementing levels (33.3 - 100%) of either C_rF_i meal or silage comparing with the control. Tables 13 and 14 reflect positive relationships between EE and EC of the carcass and between moisture and protein contents as well as moisture and ash contents of both carcass and muscles, and negative relationship between moisture and EE as well as between protein and EE percentages.  These relations were confirmed before by Abdelhamid et al. (1995 a&b, 1997 b, 1998, 2002 and 2007 a & b) and El-Ebiary et al. (2004).

 

 

 

 

*Means (in the same column) superscripted with different letters significantly (P≤0.05) differ.

*Means (in the same column) superscripted with different letters significantly  (P≤0.05) differ.

Silica contents (mg/kg) of diets and fish are presented in Table (15).  Diet No. 2 contained the lowest content, thus led to the lowest silica content in the fish carcass.  Yet, the highest dietary silica content and fish carcass content were found in diet No. 9 and with diet No. 13, respectively. Table (16) presents lead (Pb) and cadmium (Cd) contents of both diets and fish carcasses.  The highest dietary Pb content was found in diet No. 9, and the lowest in diets No. 2 and 10.  Yet, the lowest Pb content of fish was registered with diet No. 2, but the highest with diets No. 7 and 13.  However, the highest Cd level was found in diets No. 8, 9, 11 and 13, but the lowest was in the control diet No. 1; yet, the highest fish Cd level was given with diet No. 13, and the lowest with diets No. 1, 2, 5, 6 and 7. However, many heavy metals are frequently occurred in Egyptian water, earth, plants and animals including fish, whether of freshwater or saltwater, and differ in their concentrations from species to another species of fish, and from season to other as well as from location to other (Abdelhamid 1988 and 1999; Abdelahmid et al., 1992, 1997 a, 2000, 2006 a & 2007 a and El-Gawady, 2002). Recently, Abd El-Rahman and Badrawy (2007) reported the lowest Pb and Cd levels in Nile tilapia fed on 50% replacement of fish meal with crayfish meal comparing with the control. Moreover, they reported that the concentrations of Cd and Pb in edible muscle of different freshwater fish of Egypt are often above the maximum permissible limits according to FAO standards.  The child intake of fish with these levels is generally above the maximum allowable concentrations, which means human health risk with current Egyptian dietary intakes of fish.

 

 

 

Microbiological test: Throughout the course of the experiment, the 25 and 50% DW diets led to changes in water quality, i.e. dark silver color with off smell.  This led to carrying out the total count of microorganisms (Table 17).  Yet, there no critical counts, whether in water, diet and fish tissues.  Also, there were no relations among the counts in water, diets, muscles and/or liver of the tested fish. El-Gawady (2002) counted up to 1.9 x 10⁶ - 2.4 x 10⁶ CFU/ml total aerobic bacterial in water of fish farm in winter and summer, respectively.  However, the gut and skin of O. niloticus microbial count reached 4.3 x 10⁸ CFU/g. However, Ampofo and Clerk (2003) identified 25 species of bacteria as associated with the fish culture systems.  They also added that manuring causes organic enrichment, it may also hasten the deterioration of the water quality making the aquatic environment favorable for the growth and multiplication of human pathogenic bacteria. Moreover, Abdelhamid et al. (2006b and 2007a) registered the presence of pathogenic bacteria (1.3 - 2.0 x 10⁵) in samples of water, feed, sediments and fish, mainly in summer season.  There was no difference between fish of natural resources and those of aquaculture concerning bacterial contamination.  Also, Abdelhamid et al. (2007b and 2008) found pathogenic bacteria (X 106 CFU) in samples of water, feed, spleen, intestinal, and liver of Nile tilapia up to 113.0, 38.7, 28.33, 23.33, and 25.00, respectively.   Shaltout et al. (2009) counted the total microbial count including fungi, yeasts and highest total bacterial counts (TBCs) as indicators for faecal pollution in Kafr El-Zayat industrial area.

Cfu: colony forming unit

Conclusion: From the foregoing results, it would be clear that the 6^th diet (25% freshwater crayfish meal as partial replacer of dietary fish meal) was significantly the best concerning fish bodyweight gain, relative growth rate, feed and protein intakes; as well as blood hemoglobin, hemotocrit, total protein, and globulin besides highest protein and lowest fat in whole fish and fish muscles.  This was followed by the 11^th diet (50% substitution with mixture (1/1) of duckweed meal and freshwater crayfish meal), which was responsible for highest final body weight, bodyweight gain, daily body weight gain, and feed conversion, which may be reflected the economical diet by decreasing feed costs to produce one Kg fish bodyweight gain.  This leads to recommend the partial replacement of fish meal in Nile tilapia diets with 25% crayfish meal or 50% mixture of crayfish meal plus duckweed meal (1/1).  These diets were responsible for better results than control and it is to expect that they will reduce the costs of fish feeding and production for the lower prices of either duckweed meal or freshwater crayfish meal comparing with the very expensive price of fish meal.

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