1 Introduction

Freshwater aquaculture in West Bengal, India depends mainly on Indian major carps (IMCs) that have proved sustainable at different levels of production over the years. West Bengal is one of the leading producers of carps and the largest producer of carp seeds in India (Anon, 2015). Fish growth rates vary with season, age and nutritional status. A number of factors such as condition factor (Carlander, 1969), organosomatic indices (Goede and Barton, 1990), superoxides (Sahoo et al., 2005), insulin-like growth factor (IGF)-1, and IGF binding proteins (Picha et al., 2012) have been proposed to assess the growth potential and health of culturable species. Most of these regulatory events are mediated by the growth hormone (GH) and IGF axis (Pérez-Sánchez et al., 2002; Shimizu et al., 2006). The number of insulin and IGF receptors in fish is regulated by the nutritional status so that it can be altered according to the physiological need (Planas et al., 2000). Reports are available on the levels of serum IGF-1 in different fish species such as chinook salmon Oncorhynchus tshawytscha (Beckman et al., 2004a; b), coho salmon O. kisutch (Pierce et al., 2004; Beckman et al., 2004a; b), Atlantic salmon Salmo salar (Dyer et al., 2004; Shimizu et al., 2006), tilapia Oreochromis mossambicus (Uchida et al., 2003), gilthead seabream Sparus aurata (Pérez-Sánchez et al., 1995; Mingarro et al., 2002; Gómez-Requeni et al., 2004; 2005), channel catfish Ictalurus punctatus (Silverstein et al., 2000; Li et al., 2004) and Atlantic cod Gadus morhua (Davie et al., 2007). Li et al. (2014) used transgenic crucian carp, Carassius auratus as the first teleost model to study the IGF-1 over expression in-vivo. Most likely no information is available on the levels of IGF-1 in IMCs. Since Kolkata, India is known for sewage-fed fish farming, the present study was undertaken to quantify the levels of circulating IGF-1 as growth indicator in IMCs, viz., Labeo rohita, Catla catla and Cirrhinus mrigala reared under varied conditions such as captive, normal and sewage-fed ponds.

2 Materials and Methods

2.1 Experimental feed

Commercial pellet feed (CP 9931, Charoen Pokphand Pvt. Ltd., India) containing 30% protein, 3% lipid, 8% fibre, 12% of moisture, and 47% ash and nitrogen-free extract was used.

2.2 Experimental fish and design

2.2.1 Captive rearing of IMCs

Apparently healthy sub-adult IMCs, viz., Labeo rohita (348.50±9.53 g), Catla catla (350.89±8.53 g) and Cirrhinus mrigala (345.25±9.93 g) were collected from a commercial grow-out pond in Naihati (Lat. 22°54’04” N; Long. 88°25’38’’ E), North 24 Parganas district, West Bengal, India. The fish, on receipt, were disinfected with 5 ppm potassium permanganate solution and conditioned for 21 days in circular fibreglass reinforced plastic (FRP) tanks of 500 L capacity containing 300 L bore-well water with continuous aeration. Manual water exchange (20-25%) was done on alternate days after syphoning out the uneaten feed and excreta. Each of the three species was stocked separately at 15 fish/tank, in triplicate and fed with commercial pellet feed at 3% of the body weight twice daily.

2.2.2 Culture of IMCs in net cages in normal and sewage-fed ponds

Experiments were simultaneously conducted in a normal pond of size 0.33 ha situated at Chakgaria, Kolkata, India (Lat. 22°82’ N; Long. 88°20’ E) and also in a sewage-fed pond (locally called bhery) of size 18.67 ha situated at Haripota, South 24 Parganas district, West Bengal, India (Lat. 22°29’ N; Long. 88°29’ E). Apparently healthy juvenile to sub-adult C. catla (150.61±16.38 g), L. rohita (101.40±16.60 g) and C. mrigala (121.64±24.82 g) for rearing in normal pond were procured from Gangajoara (Lat. 22°27’ N; Long. 88°26’ E), South 24 Parganas district, transported in double layer oxygen packed polyethylene bags and acclimatised as above. Likewise, healthy juvenile to sub-adult C. catla (148.61±12.38 g), L. rohita (111.80±14.60 g) and C. mrigala (132.34±18.92 g) for rearing in the sewage-fed pond were harvested from the same farm complex. The details of culture and management practices followed in normal and sewage-fed farms are presented in Table 1.

The experiments were conducted in a 2 (farm) × 3 (species) factorial design in triplicate. Six numbers of net cages, each with the dimension of 2.43 m × 1.37 m × 1.37 m, were erected by the support of bamboo poles in each pond. The net cage is a fixed and fine-meshed net enclosure, similar to an inverted mosquito net made out of nylon netting with joints in nylon thread, double stitched to prevent splitting. Individual net cage was fixed in such a way that one-third portion of the net cage remained above the water surface to allow the surfacing of fish. Healthy and active C. catla, L. rohita and C. mrigala were stocked in each net cage @ 18 fish/cage, 6 individuals of each species in the ratio 1:1:1. An additional cage was also maintained with few individuals of each species in both ponds for the emergency requirement. Fish were conditioned for 21 days in the net cage. During the period of acclimatisation, the dead ones were replaced immediately with respective fish species of the uniform size. The fish were fed with pellet feed at 3% of the body weight twice daily.

2.3 Determination of water quality parameters

The temperature of the water was recorded by centigrade thermometer at the site. The pH of water samples was estimated by pH meter (Eutech Instruments Pte Ltd). Total dissolved solids (TDS) were measured by TDS meter (HiMedia, India) and expressed as mg/L. The biological oxygen demand (BOD), dissolved oxygen, free carbon dioxide, phosphate-phosphorous, ammonia-nitrogen and total hardness were determined by APHA/AWWA/WEF (2005) methods and expressed as mg/L (Table 2).

2.4 Blood collection and determination of serum insulin-like growth factor- 1 (IGF-1)

The sterile disposable syringe of 2 ml capacity with 23G needle (Hindustan Syringes and Medical Devices Ltd, India) was used for each fish for the collection of blood. On day 21, the blood (min: 0.5 ml and max: 1.5 ml/fish) was collected by the caudal venous puncture from five fish of each replicate for each species of the normal pond, sewage-fed pond and FRP tanks. The blood was allowed to clot overnight at 4ºC by keeping the syringes at 15º angle. Serum from the syringes was then carefully poured-out into Eppendorf tubes and centrifuged in a microfuge (Tarsons, India) at 2500 × g for 10 min. The serum samples of five fish from each of the three replicate were pooled separately (0.5-1.0 ml serum/tube), labelled and stored in a deep freezer (Blue Star India, Ltd) at -20ºC until use.

The serum IGF-1 was determined by the fish IGF-1 ELISA kit (Catalogue Number: MBS700712; MyBiosource, San Diego, CA) as per the manufacturer’s instructions (www.mybiosource.com/images/tds/protocol_manuals/000000-799999/MBS700712.pdf.). In brief, 50 µl each of the test serum or standard was taken in wells in triplicate for each sample. To which added was 50 µl of HRP-conjugate, mixed well and incubated for one hour at 37°C. The blank well was also set without any solution and HRP-conjugate. The contents of the wells were aspirated and washed. The process was repeated two times for a total of three washes. Washing was done by filling each well with 200 µl wash buffer using a squirt bottle and let it stand for 10 sec. After the last wash, any remaining wash buffer was removed by decanting. The plate was inverted and blotted it against clean paper towels. Then 50 µl each of substrate A and substrate B were added to each well, mixed well and incubated for 15 min at 37°C. Finally, 50 µl of stop solution was added to each well and gently tapped the plate to ensure thorough mixing. The optical density of each well was measured within 10 min using a visible microplate reader (HALO-MPR-96, Dynamica, Australia) set to 450 nm.

2.5 Statistical analysis

One-way analysis of variance (ANOVA) and Tukey’s comparison of means was used to test the significance of differences, at probability levels of 0.05, among the fish species and treatment groups. Statistical analyses were performed using SPSS 16.0 for Windows software (SPSS Inc., Chicago, IL, USA).

3 Results

3.1 Water quality parameters

The water quality parameters in captive FRP tanks, normal and sewage-fed farms, and the characteristics of sewage during the study period are presented in Table 2. The water in captive FRP tanks had high levels of free carbon dioxide, hardness and total dissolved solids. In both normal and sewage-fed pond water, the levels of free carbon dioxide, hardness, ammonia and total dissolved solids were higher than the respective optimal levels.

3.2 Serum IGF-1 levels in IMCs

3.2.1 Under captive condition

As shown in Table 3, C. catla recorded the highest serum IGF-1 (2.10±0.19 ng/ml) compared to L. rohita (2.04±0.08 ng/ml) and C. mrigala (1.90±0.09 ng/ml) under captive condition. The differences in IGF-1 levels were, however, insignificant (P>0.05).

3.2.2 Under normal pond condition in net cages

Labeo rohita of normal pond recorded the highest serum IGF-1 level (2.10±0.14 ng/ml) followed by C. catla (1.99±0.17 ng/ml) and C. mrigala (1.82±0.12 ng/ml). The differences in IGF-1 levels (Table 3) were, however, insignificant (P>0.05).

3.2.3 Under sewage-fed pond condition in net cages

Sewage-fed pond reared C. catla recorded the highest serum IGF-1 (2.34±0.36 ng/ml) followed by L. rohita (2.06 ±0.03 ng/ml) and C. mrigala (1.07±0.06 ng/ml). There existed significant differences (P<0.05) in the IGF-1 levels of L. rohita and C. mrigala as well as C. catla and C. mrigala (Table 3).

Cirrhinus mrigala recorded the lowest serum IGF-1 levels under all culture conditions during the 21 days of confinement. Significant differences (P<0.05) existed in the IGF-1 levels of net-caged C. mrigala of normal and sewage-fed ponds as well as captive reared and sewage-fed pond. The difference in the IGF-1 levels of C. mrigala of normal farm and captive-reared was insignificant (Table 3).

4 Discussions

Among the various factors available for assessing the growth, serum IGF-1 is the most promising candidate as a measure of instantaneous growth in fish (Picha et al., 2012). As there were no reports on the levels of serum IGF-1 in IMCs, quantification of IGF-1 levels of IMCs cultured under varied conditions was attempted in this study. The serum IGF-1 levels of IMCs varied with culture conditions despite the pellet feeding of uniform nutritional status. Variations in serum IGF-1 levels were noticed between the species and within the species analysed, possibly due to the feed availability, and physiological and nutritional status of fish, which regulate the IGF receptor (Planas et al., 2000). In the present study, C. catla recorded the highest serum IGF-1 levels under captive and sewage-fed pond conditions. Though the level was slightly higher in the sewage-fed pond than in captive condition, the difference was insignificant (P>0.05). While in normal pond, L. rohita recorded the highest serum IGF-1 level. The serum IGF-1 levels of L. rohita were almost constant ranging from 2.04±0.08 ng/ml in captive to 2.10±0.19 ng/ml in normal pond condition. Likewise, the serum IGF-1 levels in C. catla ranged from 1.99±0.17 ng/ml in normal pond to 2.34±0.36 ng/ml in sewage-fed pond, but the differences were insignificant (P>0.05). Also, the serum IGF-1 levels of sewage-fed pond grown C. catla and L. rohita differed insignificantly (P>0.05), which indicated that the growth of C. catla and L. rohita is unaffected despite growing in sewage-fed pond and the adverse environmental conditions such as high ammonia level (0.48±0.21 mg/L) and other parameters (Table 2). In composite fish culture and/or polyculture systems, IMCs are considered due to their compatibility for habitat preference and food to utilize ecological niches of culture system. Catla catla, L. rohita and C. mrigala feed on the foods available in the surface, column and bottom layers of the pond, respectively (ICAR, 2006). In fact, sewage-fed pond condition favoured the growth of C. catla, possibly because of the abundant availability of natural foods, particularly zooplankters in the surface layer of the water body. Contrarily, the growth rate of L. rohita was reportedly higher than that of C. mrigala in polyculture practice (FAO, 2012) and C. catla in a culture condition that received organic manure, inorganic fertilizer and supplementary feed (Abbas et al., 2010).

Picha et al. (2012) suggested that IGF-1 could provide an accurate indication of the long-term effects of stress on growth so also the results of the present study. Cirrhinus mrigala, a bottom dweller and detritus feeder, recorded the lowest serum IGF-1 levels under all culture conditions. The observed significant differences in the IGF-1 levels of bottom dwelling C. mrigala of normal farm and sewage-fed farm, as well as captive reared and sewage-fed farm indicated that the growth of C. mrigala is adversely affected by sewage, possibly due to the long-term exposure, and prevailing water quality and unfavourable pond bottom conditions. The results further corroborate the observations of Sahoo et al. (2005), who recorded better health status of L. rohita and C. catla than C. mrigala in aquaculture conditions on the basis of respiratory activity. Hayat et al. (2007) reported significantly lower weight increments in C. mrigala among IMCs exposed to sub-lethal concentrations of heavy metals, viz., Fe, Zn, Pb, Ni, Mn and their mixture for 90 days culture. The observed high serum IGF-1 levels in L. rohita and/or C. catla than C. mrigala might have contributed to their tolerance in captive and sewage-fed pond condition.

Further, the serum IGF-1 levels of IMCs were markedly lower than those recorded in channel catfish, 4-12 ng/ml (Silverstein et al., 2000), in transgenic coho salmon, 35-400 ng/ml (Devlin et al., 2004), coho salmon, 17 ng/ml and fasted coho salmon, 7-12.5 ng/ml (Shimizu et al., 2009), male lingcod Ophiodon elongates, 3.8–34.7 ng/ml and female lingcod, 3.8–35.3 ng/ml (Andrews et al., 2011), pink salmon O. gorbuscha, 28 ng/ml; chum salmon, O. keta 33 ng/ml; chinook salmon O. tshawytscha, 48 ng/ml; sockeye salmon O. nerka, 84 ng/ml; coho salmon O. kisutch, 120 ng/ml (Myers et al., 1998). The total plasma IGF-1 levels (5-32 ng/ml) of Atlantic cod Gadus morhua (Davie et al., 2007) and of tilapia O. mossambicus (140 ng/ml; Kajimura et al., 2003) were also higher than those of the IMCs. The magnitude of difference in IGF-1 levels of carps and other fish species was in the order of 2-200. Available evidence suggested that serum IGF-1 levels in teleosts positively correlate with growth rates of individuals and could be used to evaluate the growth performance for aquaculture and stock assessment (Silverstein et al., 2000; Beckman et al., 2004a; Dyer et al., 2004; Li et al., 2004; Picha et al., 2009; 2012). The results on serum IGF-1 levels of IMCs also provide supportive evidence to the above earlier reports on several other species as indicator of fish growth.

In general, the present study documented the serum IGF-1 levels in IMCs, the most preferred fish of Indian freshwater aquaculture. It recorded insignificant variations in the serum IGF-1 levels of C. catla and L. rohita reared in captive, normal and sewage-fed pond conditions. The results suggested that both C. catla and L. rohita are the ideal species for culture under the sewage-fed aquaculture system. The culture of bottom dwelling C. mrigala must be discouraged in sewage-fed farm as revealed by the serum IGF-1 levels. The results further suggested that serum IGF-1 levels can serve as growth indicator of carps. It may also be an effective indicator of the differences in growth of certain carps.

Conflict of interest

The authors declare that there is no conflict of interest.

Acknowledgements

The research work was supported by the Indian Council of Agricultural Research, Government of India, New Delhi under the Niche Area of Excellence program. The authors thank the Vice-Chancellor, West Bengal University of Animal and Fishery Sciences, Kolkata for providing necessary infrastructure facility to carry out the work.

Authors’ contributions

TJA contributed to conception and design, analysis and interpretation of results, and write-up of the manuscript. FH and AD contributed to sample collection and analysis as well as acquisition of data. Statistical analysis and interpretation of results were done by TSN. All the authors read and approved the final manuscript.

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