Growth performance of silver carp ( Hypophthalmichthys molitrix ) in cage stocked at different densities

ARTICLE INFO An experiment was conducted to evaluate the growth performance of silver carp (Hypophthalmichthys molitrix) in cage stocking at different densities. It was expected that silver carp could grow feeding on plankton without external feeding. In this experiment, silver carps in nine cages were stocked in three different stocking densities such as 20, 30 and 40 fishes/m designated as treatments T1, T2 and T3, which were located in the south west corner of the L-shaped pond in Field Laboratory Complex, Faculty of Fisheries, Bangladesh Agricultural University (BAU), Mymensingh consisting of three treatments with three replications of each for a period of 90 days from 18 August to 17 November, 2015. Each cage size was 1m x 1m x1m (length x width x height) and silver carp was cultured in the cage with natural feed which filtered phytoplankton mainly. Twenty five genera of phytoplankton were identified in pond and the average phytoplankton production was around 8.44x10cells/liter. Water quality parameters such as temperature, dissolved oxygen, pH and transparency were recorded fortnightly. At the completion of the experiment, weight showed significant difference (p>0.05) among three treatments although length didn’t vary significantly. The highest length gain (18.61+0.369 cm) and the highest weight gain (62.52+2.249 g) were found in T1. The highest SGR% for silver carp was found in T1 and the highest total production was obtained in T3 (5.00+ 0.015 kg/m). Individual growth performance of silver carp was satisfactory. Further studies are needed with different stocking densities and duration to verify th Article history: Received: 11 February 2018 Accepted: 11 August 2018


Introduction
Different types of fish species are cultured in Bangladesh like Indian major carps, Chinese carps, indigenous catfish, Thai pangus, koi, tilapia etc.Among them, silver carp (Hypophthalmichthys molitrix) has become one of the most important commercial culturable species and it is a freshwater cyprinid fish originated from Eurasia (FRSS, 2017).Silver carp is generally considered as a planktivorous fish (Cremer and Smitherman, 1980;Spataru et al., 1983).The effect of filter-feeding fish on the plankton communities of aquatic ecosystem have been studied by Zhang et al. (2006); Milstein et al. (2008); Yan et al. (2009) and Wahab et al., 2011).The culture of silver carp in cage has shown promising production without external feeding as this species living the cage can filter plankton horizontally from the water body (Roy, 2015).The culture of fish in cages is a promising aquaculture technology that has already proven itself in many Asian countries.However, cage culture is a relatively novel concept in Bangladesh.Cage culture is a viable alternative to traditional techniques of rearing, due to its practicability and mainly low costs (Beveridge, 1996).Cages are easy to manage, produce fish of high quality and utilize existing water bodies (Beveridge, 1996).Therefore, cage aquaculture offers a viable alternative for the landless and poor farmers in Bangladesh.Cage culture of fish consists of raising a group of fish from the juvenile stage to the table size in a volume of water enclosed with all sides, including the bottom, while permitting the free circulation of water through the cage culture.It can be practiced intensively in low productive water bodies.It permits manipulation of harvest th very profitable for poor farmers (CARE, 2000).
In Bangladesh, intensive monoculture is increasingly being practiced.In this culture system, farmers stock the fish at high densities and offer large quantity of feed for getting higher profit within short time, which resulted in excessive accumulation of metabolic and feed wastes in the pond bottom.The decomposition of these metabolic and feed wastes make the pond water nutrient rich that favor the excessive growth of phytoplankton (Haque and Sarkar, 2006).Silver carp mainly phytoplanktivore and it has short food chain and higher efficiency at converting primary production into fish protein than that of carnivorous fish.Cage culture of silver carp has the potential to use phytoplankton from the ponds where primary productivity is higher and ponds have multiownership problems to bring into aquaculture practices.Considering above facts, the present study was carried out to asse p d and Methods sheries, U, Mymensingh.(Fig. 1) It is a L-shaped pond and the depth was 1 meter where the cages were set.

Study site
The experiment was conducted in the pond at south-west side of Field Laboratory Complex, Faculty of Fi BA Google earth map showing c 1.
age location in the L-Laboratory Complex, Faculty of ymensingh to 17 ovember, 2015.Fish fingerlings were collected from BFRI).

Study on phytoplankton
Both qualitative and quantitative observations were done to examine planktons.Phytoplankton was identified using a light microscope (Primo Star, USA).uantitative study of phytoplankton was done by Sedge en set on an electric icroscope.10 fields from the total 1000 fields were oplankton, the following formula was 1992).

Plankton collection and sampling procedure
Plankton samples were collected from the experimental pond where the cages were set.Five liters of water were passed through the plankton net to get a 50 ml sample.The sample was preserved immediately with 5% formalin in a bottle.Before counting the S-R cell was allowed to stand for at least 15 minutes to settle the planktons.The cell was th m randomly chosen for counting.

Average daily gain (ADG)
Average daily gain was computed by su In the experimental period, the average phytoplankton production was around 8.44×10 5 cells/litre.Among different phytoplankton groups, Chlorophyceae was found dominating with 52% of the total count (Table 3 and Fig. 2).

Production of phytoplankton
As the cages were set in the same pond, during the sampling the abundance of phytoplankton groups in three treatments were more less same.Five major groups and twenty five genera of phytoplankton were identified of which included Bacillariophyceae (6 genera), Chlorophyceae (12 genera), Cyanophyceae (4 genera), Euglenophyceae (2 genera) and Rodophyceae (1 genus) (Table 2).

Water quality parameters
The mean values of water quality parameters are presented in the Table 4 that did not show any significant difference among the treatments.

Growth performance and survival rate
Length gain (cm): During the study period, length gain (cm) of silver carp was higher in T 1 in comparison with T 2 and T 3 (Table 5).Statistically, there is no significant difference (p>0.05)among the treatments for length gain in silver carp (Table 5).Weight gain (g): During the study period, weight gain (g) of silver carp was higher in T 1 in comparison with T 2 and T 3 (Table 5).Similarly, the highest percent (%) weight gain of silver carp was found in T 1 compared to T 2 and T 3 (Table 5).There were significant differences (p<0.05)among the three treatments for weight gain and % weight gain in silver carp.

Specific growth rate (SGR %) of silver carp:
The specific growth rate (SGR %) of silver carp were significantly varied (p<0.05)among the three treatments (Table 5 and Fig. 3).

Individual growth performance of weight (g) of silver carp:
The weight gain of silver carp was recorded highest in T 1 (52.21+2.249g) and lowest in T 3 (39.18+0.930g).

Survival rate:
The survival rate of silver carp in cage was highest in T 1 (79.23+1.118)and lowest in T 3 (70.36+0.575)(Table 5).

Total production
The net production of fish in treatments T 1 , T 2 and T 3 were 2.16+0.01kg/m 3 , 3.90+0.02kg/m 3 and 5.00+0.015kg/m 3 , respectively.The highest total production was found 5.00+0.015kg/m 3 in T 3 due to higher stocking density and the lowest 2.16+0.01kg/m 3 was found in T 1 due to lower stocking density ( Fig. 4).

Discussion
Plankton production Phytoplankton production of the experimental pond was found to be consisted of five major phytoplanktonic groups, which included Bacillariophyceae, Chlorophyceae, Cyanophyceae, Euglenophyceae and Rodophyceae.The most dominant group was Chlorophyceae.More or less similar results were obtained by Haque et al. (1998), Paul (1998), Chowdhury (1999) and Yeamin (2000).
Twenty five genera of phytoplankton were found from the experimental pond of the study.Nirod (1997) studied plankton population in nine experimental ponds of Fisheries Field Laboratory, BAU, Mymensingh and found 25 genera of phytoplankton belonging to Bacillariophyceae (3), Chlorophyceae (15), Euglenophyceae.Similar results were found by Paul (1998), Yeamin (2000), Raihan (2001), andKohinoor (2000).These sorts of phytoplankton are good food of indigenous and exotic carps such as Labeo rohita, Cirrhinus cirrhosus, Hypophthalmichthys molitrix, and Oreochromis niloticus.The presence of such types of phytoplankton in the pond reflected higher growth of silver carp (Haque, 2007).

Water quality parameters
Environmental parameters exert an immense influence on the maintenance of a healthy aquatic environment and production of food organism.Fish growth, metabolism and other biological activities are mostly dependent on water temperature so temperature is one of the vital water quality parameters for fish culture.The mean values of water temperature were 30.50±0.61 o C, Treatments T 1 (20 fish) T 2 (30 fish) T 3 (40 fish) 30.34±0.44 o C and 31.46±0.51o C in T 1 , T 2 and T 3 , respectively.The maximum temperature (32 o C) was recorded in T 2 on 18 th August due to high intensity of sunlight and absent of cloud in the sky.The lowest temperature (18 o C) was recorded in T 3 on 17 th November due to low intensity of sunlight.Hasan (2007) measured water temperature in ponds of BAU campus, Mymensingh and found the range from 21 to 32.8 o C which was more or less similar to this study.Another vital water quality parameter is dissolved oxygen on which fish depend on to live.Shortage of dissolved oxygen in water, fish start gulping.Sustainable fish culture depends on the optimum level of dissolved oxygen concentration.In this study the mean values of dissolved oxygen concentration were 6.50±0.49ppm in T 1 , 6.00±0.47ppm in T 2 and 6.33±0.58ppm in T 3 .Bhatnagar and Devi (2013) reported that the desirable dissolved oxygen of 5 ppm which was slightly lower than the present study.The pH is another productivity index of water body.The circum-neutral pH or slightly alkaline pH is most suitable for fish culture.For pond fish culture, the suitable range of pH is 6.5 to 8.5 (Byod, 2008).During the experimental period the mean values of pH were 7.04±0.27,8.27±0.03and 8.50±0.06 in T 1 , T 2 and T 3 , respectively.Sarker (2000) and Maghna (2012) measured pH in ponds of BAU campus, Mymensingh and found to vary between 6.8 to 8.3 and 7.6 to 8.3, respectively which were relevant in the present study.Water transparency is a gross measure of pond productivity.When the abundance of phytoplankton in a water body, the secchi disk reading becomes lower and vice-versa.Boyd (1982) suggested a transparency (cm) ranges 15-40 cm is good for fish culture.The concentration of transparency in the present study varied from the 31±0.53cm, 32±0.62 cm and 32±0.67 cm in T 1 , T 2 and T 3 , respectively that ranged corresponds to the study of Boyd (1982).

Growth parameters
Weight gain: In the present study, the length gain of silver carp were 8.25+0.369cm, 7.43+0.336cm and 7.38+0.717cm for T 1 , T 2 and T 3 , respectively.
The weight gain of silver carp was 52.21+2.249g, 41.44+1.596g and 39.18+0.930g for T 1 , T 2 and T 3 , respectively in cages which was slightly lower than the findings of Nazish (2011) who recorded total body weight gain was 86.2 to 124.3 g in different ponds.The total growth performance of silver carp was relatively low because in cage silver carp was grown up only with natural feed.Diana et al. (2004) reported that sex reversed Nile tilapia stocked in ponds at a low density showed better growth than at a higher stocking density.
Similarly in cages of silver carp we found the higher weight gain at lower stocking densities.In T 1 (20 fish/m 3 ) weight gain was higher than in T 2 (30 fish/m 3 ) and T 3 (40 fish/m 3 ), because the competition of phytoplankton which they feed was low in lower stocking density.

Specific growth rate (SGR %):
The values of specific growth rate of silver carp were observed at 0.020±0.000,0.018±0.000and 0.017±0.000 in T 1 , T 2 and T 3 , respectively.The highest specific growth rate (0.020) was observed in T 1 and the lowest specific growth rate (0.017) was observed in T 3 .In the present study, the values were lower than the finding of Ali et al. (2009) who recorded SGR of silver carp ranged from 0.52 to 0.53 in different treatments.
Survival rate and total production: The survivability of silver carp was observed as 79.23% in T 1 , 77.67% in T 2 and 70.36% in T 3 in first two weeks.This indicates the fundamental relationship between the stocking density and survival rate in cage in ponds.The variation of production of fish was found in three treatments.Among them, the higher production of fish was observed in T 3 where stocking density was 40 fish/m 3 .The lowest production was observed in T 1 where stocking density was 20 fish/m 3 .Singh et al. (2013) obtained 0.214 kg/ m 3 production for different three stocking densities (2.5, 3.0, 4.0 fish/ m 3 ) in carp polyculture during 120 days culture period.The fish production recorded in the present study was 2.16+0.01kg/m 3 , 3.90+0.02kg/m 3 and 5.00+0.015kg/m 3 in treatments T 1 ,T 2 and T 3 , respectively which was greater than the study of Singh et al. (2013).This was possibly due to following higher stocking density of silver carp in the present experiment.
Relationship between primary productivity and fish growth: Smith (1988) and Radke and Kahl (2002) reported that silver carps are effective in reducing largesized phytoplankton biomass in aquaculture ponds.In this experiment, it was found around 8.44x10 4 cells/liter plankton in the pond.Silver carp was grown up with filtered planktons from water body.In the cage water set in pond, the abundance of Chlorophyceae and other groups of phytoplankton indicates sufficient amount of food organisms available in the pond water for growth and production of silver carp.In this kind of water body abundant with phytoplankton, silver carp is an ellicient fish species that can grow up filtering plankton.Final weight of silver carp with natural feeding (plankton) was satisfactory level in a short culture period.In fact, if it is implemented at the farmer level, this type of technique can ensure the cheaper production of fish.

Conclusion
Cage culture is one of the fastest growing methods of cultured fish which requires small amount of capital investment and make profit within some months.In this method, landless people can grow fish in cages.In cage aquaculture, the owners of the cages are the owners of the fish within the pond and this is one of the advantages where there is multiple ownership problems in traditional pond management.In cage, silver carp survived with phytoplankton and grown up.In the present study, the growth and production of silver carp was satisfactory in three different densities with natural feed phytoplankton.From the above discussion, it was found that cage culture may have impacts in providing extra aquaculture opportunities in water body which is more beneficial for farmer as the additional feeding cost can be saved by this technique.
of samples in ml eter = Number of the fields counted in liter perature (°C), dissolved oxygen pm), pH, transparency were recorded by using were growth and production of the fishes.ength gain (cm) = Mean final length (cm) -Mean eight gain (g) = Mean final weight (g) -Mean initial Percent (%) weight grain = Q wick-Rafter cell (S-R cell).

=
. of phytoplankton cells per liter A = Total No. phytoplankton counted C = Volume of final concentration V = Volume of a field in cubic millim F L = Volume of original waterWater quality parametersThe water quality parameters were recorded during sampling.Water tem (p portable instruments.Growth performanceSampling was done 15 days interval by scoop net.In every sampling, 3 fishes were collected randomly from each treatment and their individual length and body weight were recorded.After harvesting of fishes final weight were recorded and following parameters used to Time at end of the experiment riment btracting the initial body weigh from the final body weight and then dividing by the number of days of culture.Time at start of the expe

Fig. 2 .
Fig. 2. Percent distribution of different phytoplankton groups in cage water

Fig. 4 .
Fig. 4. Production of silver carp among three treatments in different sampling stage

Table 3 . Phytoplankton content of water in cage set in the pond Total production
The production of silver carp was determined by multiplying the average gain in weight (kg) by the total number of fishes survived at the end of the experiment.The data found during the investigation were entered into MS Excel 2010.The significant differences among three treatments for silver carp in cages were tested using oneway ANOVA.All the statistical tests were done by SPSS (Statistical Package for the Social Sciences) software version 16.