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T effects of chronic exposure to sublethal doses of mercuric chloride (0.310, 0.155 and 0.078 mg L") on growth rate, physiological parameters, chemical composition of tilapia species and properties of ponds water were studied for 180 days. Tilapia species (Oreockromis niloticus, Saroiherodon galilaeus and Oreochromis aureits) with an average initial size of 13 cm for total length and 50 g for body weight were stocked in fiberglass tanks (6 m area of each) at a rate of 15 fish m* (1:1:1) and fed on artificial diet containing 30 % protein. The physicochemical properties of ponds water were slightly varied with different mercury levels. The plankton organisms (phyto and zooplankton) were gradually decreased with increasing the mercury concentration in fish ponds. The final body weight, daily weight gain and production of tilapia fish were significantly decreased with increasing the mercury level. The lowest values of these parameters (17L5g, 0.67g, 1.518 ton feddan" for O. niloticus; 144g, 0.52g, L053 ton feddan" for 5. galilaeus and 153.5 g, 0.56 g and 0.995 ton feddan * for O. aureus) were observed in the pond containing highest mercury level (0.310 mg/L). The other growth performance (specific growth rate, percentage weight gain and normalized biomass index), feed utilization (feed conversion ratio, protein efficiency ratio and protein productive value) and survival efficiency of tilapia were also affected with sublethal levels of mercury. The net returns and profitability of the reared fish were also decreased with increasing mercury concentration in ponds water. The lowest hematocrit (28.5, 29.0 166 Mohamed A, Sweilum and 30 %) and hemoglobin (5.5, 5.0 and 52 g/100 ml blood) of the three fish species were obtained from the pond containing 0.310 mg/L HgCh. At the same mercury levels, the serum protein of the three fishes decreased to 7.0, 6.6 and 6-5 g/100 ml serum, while serum glucose increased to 8L0, 79-0 and 78.5 mg/100 ml serum. The biochemical composition of fish muscles was also influenced with different sublethal levels of mercury, where the muscle protein and glycogen were slightly decreased and lipid increased with increasing the mercury level. The mercury residues in liver, gills and muscles of the reared fish were increased with increasing the mercury concentration in ponds water and its value in liver was higher than that in gills and muscles. INTRODUCTION Recently, attention has been focused on fish farming, since it has a great potential for achieving new sources of fish production and since most natural resources are overexploited. So, the interest in aquaculture has been directed mainly towards the development of pond culture (Ishak, 1985). Tilapia species are successfully cultured in the Egyptian fish farms, since they have high growth rate, can be fed on artificial diets and have high tolerance to different water quality (Bayoumi, 1987). Fish can also be a source of threat to human health by transporting the toxic materials directly to the consumers. Since heavy metals are widely distributed in aquatic habitats due to the industrial effluents and the wide use of chemicals in agriculture, the aquatic habitats with their water quality are considered the main factor controlling the state of health and disease of fish. The toxicity of heavy metals and their effects on physiological and biochemical parametes of freshwater fish have been reported by many investigators (Ghazaly & Said, 1995; Iqbal et al, 1997; EI-Nagar et ar/.s 2001; Mahmoud 2002). Mercury, like other heavy metals, may find its way to the water system and affects aquatic life, particularly fish, through its interference with their metabolic processes (Kumari and Banerjee, 1993). Mazhar et al (1987) exposed the Nile catfish (Clarias gariepinus) to sublethal levels of mercuric chloride and noticed a progressive fall in RBCs count, hematocrit and an increase in WBCs count. Similarly, Misra and Behera (1992) found the decline of erythrocyte count, hematocrit and hemoglobin in blood of Channa punctatus on exposure to sublethal concentration of mercuric chloride. Same observations were also mentioned by Shakoori et al (1994) for grass carp (C idella) and Marie (1999) for Nile tilapia (O. niloticus). The EFFECT OF EXPOSURE OF MERCURY ON 167 PHYSIOLOGICAL FUNCTIONS OF TILAPIA FISH present study was conducted to determine the effects of sublethal doses of mercury on growth rate, physiological properties and economical efficiency of three tilapia species (0. niloticus, £ galilaeus and O. aureus), in addition to its effects on water quality and plankton fertility in fish rearing ponds. MATERIALS AND METHODS Site of work and fish used The present work was carried out in rectangular fiber glass tanks (3 x 2 m) with 0.85 m depth. These tanks were filled with freshwater and supplied with sublethal doses of mercuric chloride as follows; the control one was maintained without mercury, the other tanks contained 0.310, 0.155 and 0.078 mg L'\ while the tanks were duplicated with the same experimental treatments. The water in ponds was continuously aerated by mean of an electric compressor, faeces and other remains were removed daily by sucking. The fish used were Oreochromis niloticus, Sarotherodon galilaeus and Oreochromis aitreus with an average starting body weight of 50g/fish and 13 cm for total length. The fish were stocked in tanks at a rate of 15 fish m' (90 fish pond"), 30 0. niloticus, 305. galilaeus and 30 O. aureus. The supplementary feed was pelleted diets containing 30 % crude protein and formulated from fish meal, soybean, rice bran, yellow com and sunflower oil (commercial diets). The fish fed six days a week, once per day at a level of 3 % of the average body weight, Physico-chemical and hydro-biological aspects Water temperature, dissolved oxygen and pH value were daily determined (9.00 am and 2.00 pm) by using a simple thermometer, oxygenometer (Cole Partner Model 5946) and pH meter (Orion Digital Model 210). Total alkalinity, ammonia, nitrate and phosphate were monthly estimated, using methods described by Arnold et al (1980). Samples of plankton were taken biweekly from 10 liters of tanks water. The phytoplankton organisms were collected by a fine nylon net of 20 micron mesh size, then identified and counted by Sedwick Rafter Counting Cell, through a research microscope (200 X). The zooplankton individuals were collected, using a nylon net of 50 micron mesh size and counted by Tray Counting Cell through microscope (100 X). Growth and feed utilization The total length (cm) and body weight (g) of reared fish (15 fish from each species in each treatment) were measured biweekly. Specific growth rate (SGR), percentage weight gain (PWG), normalized biomass 168 Mohamed ASweilum index (NBI), feed conversion ratio (FCR), protein efficiency ratio (PER) and protein productive value (PPV) were calculated according to the following equations; SGR = (Ln final weight Ln initial weight) x 100 / Period (days) PWG = (Final weight Initial weight) x 100 / Initial weight NBI = Final weight Initial weight / 100 FCR = Food consumed (g)/Weight gain(g) PER = Weight gain (g) / Protein consumed (g) PPV = Increasing of body protein (g) / Protein consumed (g) x 100 Meanwhile, the total survival rate (TSR) and condition factor (k) of experimental fish in each treatment were estimated by applying these formulae; TSR ̂ Survived fish number x 100 / Total fish number ■ K = Body weight x 100 / (Total length). Production and economical efficiency The total fish production and economical efficiency (net returns and profitability) of rearing ponds were determined in different treatments at the end the experiment (180 days) according to the following equations: Fish production (ton feddan") = 4200 [Weight gain (g) x Number, of survived fish/10] Net returns (LE feddan") = Price of produced fish.(LE) Total costs (LE) Profitability (percent) = Price of produced fish (LE) / Total costs (LE)xl00 (feddan = 4200 m, hectar = 10000 m, hectar = 2.38 feddans) Blood and biochemical composition The blood samples (10 fish were examined from £ach species in each treatment) were taken over heparinized vials by severing the caudal peduncle of fish (Dabrowska et al, 1989). The hematocrit percent was measured by drawing the blood directly from fish into a heparinized hemopipette, then centrifuged at 3000 r.b.m. (Hesser, 1960). Hemoglobin content was estimated by Van Kampen & Zijfstra method (1961)* Serum glucose was determined by using Boehringer Manneim Kits as described by Trinder (1969) and serum lipid by Zollner & Kirsch method (1962). Protein in serum and muscles were determined according to Gomall et al (1949). Glycogen in fish muscles was measured by using the anthrone reagent (Handel, 1965), while total lipid in muscles was extracted by EFFECT OF EXPOSURE OF MERCURY ON 169 PHYSIOLOGICAL FUNCTIONS OF TILAPIA FISH mixture of chloroform with methanol (2:1) and estimated using Bligh & Dyer (1959) method. Mercury residues in fish tissues The examined tissues of fish (liver, muscles and gills) were completely dried at 75 °C for about 48 hours. Half gram of dry weight of each sample was digested with the digestion solution (65% HNO3 and 30 % H2O2) using the digestor (Milestone, MLS-I200 mega) and microwave digestion system with MDR technology. Concentration of mercury (Hg) was determined by the atomic spectrophotometer (Perkin-Elmer, 2380, Germany). Statistical analysis The differences between means of experimental and control values in different treatments were considered unsignificant if P > 0.05, significant if P < 0.05 and highly significant if P < 0.01 using t-test of significance as described by Berlly & Lindgren (1990) with applying the following equations; Calculated value = Xx X2 /^(SEf{ +{SE)2 Standard error (SE) = Standard deviation (SD) l4rt where, Xi = Mean of the first group. X2 = Mean of the second group. SEi = Standard error of data in first group. SE2 = Standard error

egyptian journal of aquatic biology and fisheries

EFFECT OF CHRONIC EXPOSURE TO SUBLETHAL LEVELS OF MERCURY ON TOTAL PRODUCTION, PHYSIOLOGICAL FUNCTIONS AND ECONOMICAL EFFICIENCY OF TILAPIA FISH, WITH REGARD TO PROPERTIES OF PONDS WATER