EFFECTS OF POPULATION DENSITY ON GROWTH AND PRODUCTION OF TILAPIA IN MONOCULTURE

The experiment was conducted to determine the effect of population density on growth and production of Tilapia (monosex GIFT tilapia) in monoculture system for a period of 110 days. The experiment was carried out in six earthen ponds, which were situated in the Bangladesh Agricultural University Campus, Mymensingh. The experiment was carried out under three treatments each with two replications. Fish population density was 200 fish per decimal under treatment-I, 400 fish per decimal under treatment-II and 600 fish per decimal under treatment-III. In the ponds supplementary feed of wheat bran and rice bran mixture were used daily at the rate of 46 g, 92 g and 136 g per decimal under treatment-I, II, and III, respectively. The average initial length and weight of tilapia were 4.63 cm and 2.82 g respectively. The ponds were fertilized fortnightly with urea and TSP at the rates of 60 g and 90 g, respectively. During experimental period, the ranges of water temperature (25.82 to 29.80 oC), transparency (28.00 to 38.00 cm), dissolved oxygen (5.50 to 8.30 mg/L), pH (7.00 to 7.90), total alkalinity (130.00 to 200.00 mg/L), free CO2 (2.00 to 6.00 mg/L), phosphate-phosphorus (1.20 to 2.30 mg/L), and nitrate-nitrogen (3.20 to 4.00 mg/L) were within the productive range and more or less similar in all the ponds under three treatments. There were 25 genera of phytoplankton under five major groups and 10 genera of zooplankton under three major groups in the experimental ponds. Mean survival rate of fish under treatment-I, treatment-II, and treatment-III were 82.75%, 77.12% and 74.33% respectively. The calculated net fish production under treatment-I was 6.75 ton/ha/yr and that of the ponds under treatment-II was 10.26 ton/ha/yr and that of the ponds under treatment-III was 12.15 ton/ha/yr. The net fish production under treatment-II and treatment-III were 152% and 180% higher than that of treatment-I, taking net fish production under treatment-I for 100%. According to profit-cost analysis, the ratios of net profit under treatments I, II, and III were 1:0.44, 1:0.27, and 1:0.09. According to specific growth rate and survival, treatment-I is the best, and according to profit-cost analysis treatment-I (ratio 1:0.44) also the best. So, the population density of 200 fish per decimal (under treatment-I) might be considered the best among the three treatments.


Introduction
Bangladesh is situated in a zone where climatic conditions and geographical position are very much favorable to fisheries resources.Fisheries sector plays a major role in the development of socio-economic situation, employment generation, poverty alleviation, nutrition of large number of population, and foreign currency earnings in the economy of Bangladesh.Fish is considered as one of the most delicious and an essential food all over the world including Bangladesh.Capture fisheries, although still the major source of supply of fish are in decline due to over-fishing and environmental degradation, and it is now considered that aquaculture has the greatest potential to meet the growing demand for fish from the increasing population.Currently aquaculture production accounts for about onethird of the total fish production in Bangladesh (DoF, 2011).Aquaculture is rapidly gaining attention with increase in human population and reduced natural fisheries resources.Pond aquaculture is growing fast in many Asian countries.However, fish culture on a small-scale basis has often failed due to inappropriate knowledge regarding optimum stocking density and feeding regime of fish.Stocking density is generally defined how many fishes are released per unit area of the pond.It is considered one of the main reasons that affect fish growth, feed utilization and gross fish yield.The full utilization of space for maximum fish production through intensive culture system can increase the profitability of the fish farm.Projections from the Food and Agriculture Organization (FAO) of the United Nations, estimate the animal protein requirement of 2050 to be about 889 million metric tons per year or 3 times the consumption in 2010 in China (FAO/UNDP, 2011).Other estimates show that we may need up to six times the levels produced in 2010.This means that all sectors of agriculture will require expanding to meet the increased demand.If aquaculture is to fulfill its potential and contribute to the improvement of world protein production then we must improve culture technologies, feed formulations and genetics of our culture species.Tilapias are an integral component of subsistence fisheries for thousands of years but have gained popularity in recent years in areas where they are not endemic.Introductions of better performing tilapia species/strains and development of techniques to manage unwanted reproduction have spurred significant developments that became a huge success in tilapia farming.Tilapia is a important food fish in many tropical and subtropical countries and one of the first fish species that was cultured in the world.It is considered suitable for culture because of high tolerance to hostile environmental conditions, relatively fast growth and the ease with which they can be bred.It provides one of the most important sources of animal protein and income throughout the world .Tilapia can tolerate dissolved oxygen concentration of 1 mg/L and can survive by using atmospheric oxygen when at dawn DO concentration dropped to less than 1 mg/L.It has also the ability to survive under extremely low dissolved oxygenated surface water layer by reducing activity .Tilapia is the common name applied to three genera of fish in the family Cichlidae: Oreochromis, Sarotherodon, and Tilapia which are widely distributed in many countries of the world.Now it can be found in more than 100 countries (Ballarin and Haller, 1982).The development of Genetically Improved Farmed Tilapia (GIFT) technology that is based on traditional selective breeding and is meant to improve commercially important traits of tropical farmed fish is a big milestone in the history of tilapia culture.Modern fish culture is usually improvement of culture practices through adopting different measures such as proper doses of fertilizer application, regular feeding, optimum stocking density, maintenance of physicochemical factors, disease prevention and various control measures (Ballarin and Haller, 1982).The stocking density is the major concern for monoculture.Sometimes excellent fish fry do not perform satisfactory growth unless correct stocking practices (Sanches and Hayashi, 1999).So considering the importance the present experiment has been undertaken to find out the effects of three different population densities on the growth and production of Tilapia in monoculture.

Experimental duration and location
The duration of the experiment was 4months from 29 June to 29 October in six ponds.The ponds were situated at the southeast corner of the Faculty of Fisheries Buildings, Bangladesh Agricultural University, Mymensingh.

Experimental ponds
Six earthen ponds of similar size about 40 m 2 (1 decimal) area each and rectangular in shape, were used for the experiment.All ponds were situated side by side.Each pond have an average depth of 0.90 m.All ponds were free from aquatic animals, unwanted fishes and aquatic higher vegetation.The source of water of these ponds was rainfall and provision for water supply from a deep tube-well using flexible plastic pipes whenever needed.The embankment was well protected and covered with grass.All the experimental ponds were arbitrarily numbered as pond no. 1 (P1), pond no. 2 (P2)…..and pond no.6 (P6).For the convenience of the research work pond 1 and 6 were considered as treatment no.I, pond 3 and 4 as treatment no.II and pond 2 and 5 as treatment no.III.

Experimental design
The experimental layout has been given in the Table 1 below

Pond drying, dyke repairing and liming
Before starting the experiment, the ponds were dried, aquatic higher vegetation's and unwanted aquatic animals were removed manually.Pond dykes were repaired and renovated.Liming (CaO) was done in all the ponds at rate of 1 kg/decimal before 7 days of fertilization.

Water supply
Ponds were supplied with water after 7 days of liming from a deep tube-well water supply system; rainfall was also a source of water supply to the ponds.

Fertilization of the ponds
Fertilization of ponds was done fortnightly with the application of urea (60 g/decimal) and TSP (90 g/decimal).TSP was dissolved in water for 24 hours in a plastic bucket and then applied by spreading over the pond surface by a mug.Urea was also dissolved in the same plastic bucket before spreading on the water surface of the ponds.

Stocking of fish
Fingerlings of monosex GIFT tilapia (O.niloticus) was stocked in the ponds.All the fish fry were collected from BFRI, Freshwater station, Mymensingh.Transportation of fry was done in well oxygenated polythene bags.Stocking was done in the morning and care was taken to acclimatize the fish gradually to pond condition.The initial average weight of O. niloticus was 2.82 g and initial average length of O. niloticus was 4.63 cm.

Study of water quality parameters
Different types of water quality parameters were estimated and recorded fortnightly throughout the experimental period.Water quality measurement and sample collection were made between 8.00 am to 12.00 noon.Physical parameter such as water temperature (ºC), air temperature (ºC), transparency (cm), and water depth (m) were measured at the pond site on every sampling day.Chemical parameters such as pH, dissolved oxygen (mg/L), free carbon dioxide (mg/L), PO4-P (mg/L), NO3-N (mg/L) and biological parameters such as phytoplankton density (cells/L), zooplankton density (cells/L) were determined in the Limnology Laboratory of the Department of Fisheries Management.Water samples were collected and transported in black plastic bottles having a volume of 250 ml each marked with pond number to the laboratory for chemical analysis and for biological analysis, plankton samples were collected using plankton net of 55 µ mesh-size.Plankton samples were preserved in 5% formalin and plankton samples were studied under a compound microscope.

Methods for study of physical and chemical parameters
The physical and chemical factors of experimental ponds were studied followings standard methods and using different digital meters.

Collection and preservation of plankton sample
Water samples in a 500 ml bottle were randomly collected for quantitative and qualitative study of phytoplankton and zooplankton of water from different locations of each of the ponds and passed through a plankton net (mesh-size 55 µ) and finally concentrated to 100 ml.Then concentrated samples were preserved in small plastic bottles in 5% formalin for study under a compound microscope.

Counting of plankton
Counting of both phytoplankton and zooplankton were done with the help of Sedgwick-Rafter Counting Cell (S-R cell).The S-R cell is 500 mm long and 20 mm wide and 1 mm deep.The volume of the chamber is equally divided into 1000 fields of 0.001 ml each.From the concentrated plankton samples, 1 ml was taken by a dropper and then put in the S-R cell.Before counting plankton sample was left to stand for about 10 minutes to allow the plankton settle down and then it was studied under a compound microscope and planktons were counted in 10 squares of the S-R cell chosen randomly.

Calculation of plankton
The plankton population was determined by Sedgwick Rafter Counting Cell (S-R Cell) using the following formula (Rahman, 1992).

Harvesting of fish
At the end of the experiment the water of the ponds were pumped out and all the fish were harvested.Then the final growth gained by the fish was recorded by measuring the length (cm) and weight (g) of the harvested fish by using a measuring scale and a balance respectively.

Statistical analysis
T-test of net fish production of the ponds under three treatments was done by a computer using SPSS package programme.

Physico-chemical parameters
The results of the different physico-chemical parameters of the experimental ponds have been presented in the Table 2.All physico-chemical parameters were found to be within the acceptable ranges for fish culture in all treatments.

Biological parameters
The results of biological parameters such as phytoplankton density (cells/L) and zooplankton density (cells/L), generic status of phytoplankton and zooplankton, and growth and production of fish have been presented in Tables 3 to 5 and Figs. 1 to 2.

Zooplankton (cells/L)
During the experimental period a total of 10 genera of zooplankton belonging to 3 group of Crustacea (Cladocera and Copepoda) and Rotifera were found in the experimental ponds (Table 3).Fortnightly means of zooplankton density of the experimental ponds under treatments I, II, and III have been presented in Table 8.The mean values of zooplankton density of the pond nos. 1 and 6 under treatment-I were 10.71±1.80 and 10.50±0.95(×10 3 ) cells/L, respectively and those of the pond nos. 3 and 4 under treatment-II were 10.00±1.41 and 11.00±1.73(× 10 3 ) cells/L, respectively and those of the pond nos. 2 and 5 under treatment-III were 10.57±1.51 and 11.14±1.68(×10 3 ) cells/L, respectively .The average density of zooplankton of the pond under treatment-I was 10.61±0.99 (×10 3 ) cells/L and that of the ponds under treatment-II was 10.50±1.44 (×10 3 ) cells/L and that of the ponds under treatment-III was 10.85±0.90(×10 3 ) cells/L (Table 3).Generic status of phytoplankton and zooplankton found in the culture pond.
Table 3. Generic status of plankton found in the culture ponds under treatments-I,II and III.

Survival rate
The survival rates (%) of fish were different in different treatments.The survival rates in treatment-I was 82.75% and in treatment-II was 77.1% and in treatment-III was 74.33%.The survival rate in treatment-I is significantly higher than those in treatment-II and treatment-III (Table 5).

Specific growth rate (% per day)
The specific growth rates (SGR % per day) of fish in different treatments were different.In treatment-I SGR value recorded was 2.0% per day and in treatment-II SGR value recorded was 1.94% per day and in treatment-III SGR value recorded was 1.80% per day.SGR value in treatment-I was higher than those in treatment-II and III (Table 4).

Production of fish
The productions of fish were different in different treatments.The gross and net productions of fish of the ponds under treatment I, II, and III have been presented in the Tables 4 and 5 and Figs. 1  and 2. The calculated gross production of fish of the ponds under treatments I, II, and III were 7.68, 12.06, and 14.94 ton/ha/yr respectively (Table 4).The net productions of fish of the ponds under treatments I, II, and III were 6.75, 10.26, and 12.15 ton/ha/yr respectively (Table 4).The gross and net productions of treatment-III were higher than those of other two treatments.In the present experiment the calculated gross and net fish productions of the ponds under treatment-I were 7.68 ton/ha/yr and 6.75 ton/ha/yr and those of the ponds under treatment-II were 12.06 ton/ha/yr and 10.26 ton/ha/yr and those of the ponds under treatment-III were 14.94 ton/ha/yr and 12.15 ton/ha/yr, respectively.
Considering the above discussion, it is evident that treatment-I (fish population density 200 per decimal) is the best in respect of survival of fish . of plankton cells per liter of original water A = Total no. of plankton counted C = Volume of final concentrate of the sample in ml V = Volume of a field = 1 mm³ F = No. of the fields counted L = Volume of original water in liter The number of phytoplankton and zooplankton were expressed as cells/L Effects of population density on growth and production of tilapia Int.J. Agril.Res.Innov.& Tech.7 (2): 49-56, December, 2017 body weight (g) at time T1 W2= Final live body weight (g) at time T2 (iii) Calculation of gross fish production (ton/ha/yr) = (iv) Calculation of net fish production (ton/ha/yr) =

Fig. 1 .
Fig. 1.Gross production of fish under treatments I, II, and III.

Table 1 .
: The layout of the experiment.

Table 2 .
n=3)of the ponds during the experimental period.

Table 4 .
Gross and net production of fish of the ponds under treatments I, II, and III.Percent increase of net productions of treatment-II and treatment-III, over treatment-I which has been taken for 100%. *

Table 5 .
Total survival rate, growth and production (gross and net) of fishes under treatments I, II, and III.